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Hydrogen is a chemical element; it has the symbol H and atomic number 1. It is the lightest and most abundant chemical element in the universe, constituting about 75% of all normal matter. Under standard conditions, hydrogen is a gas of diatomic molecules with the formula H
2, called dihydrogen, or sometimes hydrogen gas, molecular hydrogen, or simply hydrogen. Dihydrogen is colorless, odorless, non-toxic, and highly combustible. Stars, including the Sun, mainly consist of hydrogen in a plasma state, while on Earth, hydrogen is found as the gas H
2 (dihydrogen) and in molecules, such as in water and organic compounds. The most common isotope of hydrogen, H, consists of one proton, one electron, and no neutrons.

Hydrogen gas was first produced artificially in the 17th century by the reaction of acids with metals. Henry Cavendish, in 1766–1781, identified hydrogen gas as a distinct substance and discovered its property of producing water when burned: this is the origin of hydrogen's name, which means (from Ancient Greek: ὕδωρ, romanized: húdōr, lit. 'water', and γεννάω, gennáō, 'I bring forth'). Understanding the colors of light absorbed and emitted by hydrogen was a crucial part of the development of quantum mechanics.

Hydrogen, typically nonmetallic except under extreme pressure, readily forms covalent bonds with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial in acid–base reactions, which mainly involve proton exchange among soluble molecules. In ionic compounds, hydrogen can take the form of either a negatively-charged anion, where it is known as hydride, or as a positively charged cation, H+
, hydron. Although tightly bonded to water molecules, hydrons strongly affect the behavior of aqueous solutions, as reflected in the importance of pH. Hydride, on the other hand, is rarely observed because it tends to deprotonate solvents, yielding H
2.^[1]

In the early universe, neutral hydrogen atoms formed about 370,000 years after the Big Bang as the universe expanded and plasma had cooled enough for electrons to remain bound to protons. After stars began to form, most of the hydrogen in the intergalactic medium was re-ionized.

Nearly all hydrogen production is done by transforming fossil fuels, particularly steam reforming of natural gas. It can also be produced from water or saline by electrolysis, but this process is more expensive. Its main industrial uses include fossil fuel processing and ammonia production for fertilizer. Emerging uses for hydrogen include the use of fuel cells to generate electricity.

Properties

Atomic hydrogen

Electron energy levels

The ground state energy level of the electron in a hydrogen atom is −13.6 electronvolts (eV),^[2] equivalent to an ultraviolet photon of roughly 91 nanometers wavelength.^[3] The energy levels of hydrogen are referred to by consecutive quantum numbers, with $n = 1$ being the ground state. The hydrogen spectral series corresponds to emission of light due to transitions from higher to lower energy levels.^[4]^: 105 Each energy level is further split by spin interactions between the electron and proton into four hyperfine levels.^[5]

High-precision values for the hydrogen atom energy levels are required for definitions of physical constants. Quantum calculations have identified nine contributions to the energy levels. The eigenvalue from the Dirac equation is the largest contribution. Other terms include relativistic recoil, the self-energy, and the vacuum polarization terms.^[6]

Nomenclature

The standards organization for chemical names, IUPAC, gives general names when the context assumes natural isotope abundance or ignores the isotope. These general names are hydrogen for the neutral atom, hydron for the cation, H⁺, hydride for the anion, and H^-. The name proton is often used for the positively charged cation, but this is strictly correct only for the cation of the dominant isotope .^[7]

Isotopes

Hydrogen has three naturally-occurring isotopes, denoted , and . Other, highly-unstable nuclides ( to ) have been synthesized in laboratories but not observed in nature.^[8]^[9]

' is the most common hydrogen isotope, with an abundance of >99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium.^[10] It is the only stable isotope with no neutrons ().Cite error: Invalid <ref> tag; refs with no name must have content

', the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in the nucleus. Nearly all deuterium nuclei in the universe are thought to have been produced in Big Bang nucleosynthesis, and have endured since then.^[11]^: 24.2 Deuterium is not radioactive, and is not a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for -NMR spectroscopy.^[12] Heavy water is used as a neutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial nuclear fusion.^[13]

' is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium-3 through beta decay with a half-life of 12.32 years.^[14] It is radioactive enough to be used in luminous paint to enhance the visibility of data displays, such as for painting the hands and dial-markers of watches. The watch glass prevents the small amount of radiation from escaping the case.^[15] Small amounts of tritium are produced naturally by cosmic rays striking atmospheric gases; tritium has also been released in nuclear weapons tests.^[16] It is used in nuclear fusion,^[17] as a tracer in isotope geochemistry,^[18] and in specialized self-powered lighting devices.^[19] Tritium has also been used in chemical and biological labeling experiments as a radiolabel.^[20]

Unique among the elements, distinct names are assigned to hydrogen's isotopes in common use. During the early study of radioactivity, heavy radioisotopes were given their own names, but these are mostly no longer used. The symbols D and T (instead of and ) are sometimes used for deuterium and tritium, but the symbol P was already used for phosphorus and thus was not available for protium.^[21] In its nomenclatural guidelines, the International Union of Pure and Applied Chemistry (IUPAC) allows any of D, T, , and to be used, though and are preferred.^[22]

Antihydrogen () is the antimatter counterpart to hydrogen. It consists of an with a positron.^[23]^[24] The exotic atom muonium (symbol Mu), composed of an antimuon and an electron, is the analogue of hydrogen; nomenclature incorporates such hypothetical compounds as muonium chloride (MuCl) and sodium muonide (NaMu), analogous to hydrogen chloride and sodium hydride respectively.^[25]

Dihydrogen

Under standard conditions, hydrogen is a gas of diatomic molecules with the formula H
2, officially called "dihydrogen",^[26]^: 308 but also called "molecular hydrogen",^[27] or simply hydrogen. Dihydrogen is a colorless, odorless, flammable gas.^[27]

Combustion

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Hydrogen gas is highly flammable, reacting with oxygen in air to produce liquid water:

The amount of heat released per mole of hydrogen is −286 kilojoules (kJ), or 141.9 megajoules (MJ) for a 1-kilogram (2.2 lb)^* mass (based on the higher heating value measurement).^[28]

Hydrogen gas forms explosive mixtures with air in concentrations from 4%–74%^[29] and with chlorine at 5%–95%. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).^[30] In a high-pressure hydrogen leak, the shock wave from the leak itself can heat air to the autoignition temperature, leading to flaming and possibly explosion.^[31]

Hydrogen flames emit faint blue and ultraviolet light.^[32] Flame detectors are used to detect hydrogen fires as they are nearly invisible to the naked eye in daylight.^[33]^[34]

Spin isomers

Molecular H
2 exists as two nuclear isomers that differ in the spin states of their nuclei.^[35] In the ' form, the spins of the two nuclei are parallel, forming a spin triplet state having a total molecular spin $S = 1$ ; in the ' form the spins are and form a spin singlet state having spin $S = 0$ . The equilibrium ratio of ortho- to para-hydrogen depends on temperature. At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form.^[36] The ortho form is an excited state, having higher energy than the para form by 1.455 kJ/mol,^[37] and it converts to the para form over the course of several minutes when cooled to low temperature.^[38] The thermal properties of these isomers differ because each has distinct rotational quantum states.^[39]

The ortho-to-para ratio in H
2 is an important consideration in the liquefaction and storage of liquid hydrogen: the conversion from ortho to para is exothermic, and produces sufficient heat to evaporate most of the liquid if the conversion to does not occur during the cooling process.^[40] Catalysts for the ortho-para , such as ferric oxide and activated carbon compounds, are therefore used during hydrogen cooling to avoid this loss of liquid.^[41]

Phases

Liquid hydrogen can exist at temperatures below hydrogen's critical point of 33 kelvins (−240.2 °C; −400.3 °F).^[43] However, for it to be in a fully liquid state at atmospheric pressure, H₂ needs to be cooled to 20.28 K (−252.87 °C; −423.17 °F). Hydrogen was liquefied by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask.^[44]

Liquid hydrogen becomes solid hydrogen at standard pressure below hydrogen's melting point of 14.01 K (−259.14 °C; −434.45 °F). Distinct solid phases exist, known as Phase I through Phase V, each exhibiting a characteristic molecular arrangement.^[45] Liquid and solid phases can exist in combination at the triple point; this mixture is known as slush hydrogen.^[46]

Metallic hydrogen, a phase obtained at extremely high pressures (in excess of 400 billion), is an electrical conductor. It is believed to exist deep within giant planets like Jupiter.^[45]^[47]

When ionized, hydrogen becomes a plasma. This is the form in which hydrogen exists within stars.^[48]

Thermal and physical properties

Temperature (K)	Density (kg/m³)	Specific heat (kJ/kg K)	Dynamic viscosity (kg/m s)	Kinematic viscosity (m²/s)	Thermal conductivity (W/m K)	Thermal diffusivity (m²/s)	Prandtl Number
100	0.24255	11.23	4.21E-06	1.74E-05	6.70E-02	2.46E-05	0.707
150	0.16371	12.602	5.60E-06	3.42E-05	0.0981	4.75E-05	0.718
200	0.1227	13.54	6.81E-06	5.55E-05	0.1282	7.72E-05	0.719
250	0.09819	14.059	7.92E-06	8.06E-05	0.1561	1.13E-04	0.713
300	0.08185	14.314	8.96E-06	1.10E-04	0.182	1.55E-04	0.706
350	0.07016	14.436	9.95E-06	1.42E-04	0.206	2.03E-04	0.697
400	0.06135	14.491	1.09E-05	1.77E-04	0.228	2.57E-04	0.69
450	0.05462	14.499	1.18E-05	2.16E-04	0.251	3.16E-04	0.682
500	0.04918	14.507	1.26E-05	2.57E-04	0.272	3.82E-04	0.675
550	0.04469	14.532	1.35E-05	3.02E-04	0.292	4.52E-04	0.668
600	0.04085	14.537	1.43E-05	3.50E-04	0.315	5.31E-04	0.664
700	0.03492	14.574	1.59E-05	4.55E-04	0.351	6.90E-04	0.659
800	0.0306	14.675	1.74E-05	5.69E-04	0.384	8.56E-04	0.664
900	0.02723	14.821	1.88E-05	6.90E-04	0.412	1.02E-03	0.676
1000	0.02424	14.99	2.01E-05	8.30E-04	0.448	1.23E-03	0.673
1100	0.02204	15.17	2.13E-05	9.66E-04	0.488	1.46E-03	0.662
1200	0.0202	15.37	2.26E-05	1.12E-03	0.528	1.70E-03	0.659
1300	0.01865	15.59	2.39E-05	1.28E-03	0.568	1.96E-03	0.655
1400	0.01732	15.81	2.51E-05	1.45E-03	0.61	2.23E-03	0.65
1500	0.01616	16.02	2.63E-05	1.63E-03	0.655	2.53E-03	0.643
1600	0.0152	16.28	2.74E-05	1.80E-03	0.697	2.82E-03	0.639
1700	0.0143	16.58	2.85E-05	1.99E-03	0.742	3.13E-03	0.637
1800	0.0135	16.96	2.96E-05	2.19E-03	0.786	3.44E-03	0.639
1900	0.0128	17.49	3.07E-05	2.40E-03	0.835	3.73E-03	0.643
2000	0.0121	18.25	3.18E-05	2.63E-03	0.878	3.98E-03	0.661

History

18th century

In 1671, Irish scientist Robert Boyle discovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas.^[49]^[50] Boyle did not note that the gas was flammable, but hydrogen would play a key role in overturning the phlogiston theory of combustion.^[51]

In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "inflammable air". He speculated that "inflammable air" was in fact identical to the hypothetical substance "phlogiston"^[52]^[53] and further finding in 1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.^[54]^[55]

In 1783, identified the element that came to be known as hydrogen^[56] when he and Pierre-Simon Laplace reproduced Cavendish's finding that water is produced when hydrogen is burned.^[55] produced hydrogen for his experiments on mass conservation by treating metallic iron with a stream of water through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:

Fe + H
2O → FeO + H
2
2 Fe + 3 H
2O → Fe
2O
3 + 3 H
2
3 Fe + 4 H
2O → Fe
3O
4 + 4 H
2

Many metals react similarly with water, leading to the production of hydrogen.^[57] In some situations, this H₂-producing process is problematic, for instance in the case of zirconium cladding on nuclear fuel rods.^[58]

19th century

By 1806 hydrogen was used to fill balloons.^[59]

built the first  engine, an internal combustion engine powered by a mixture of hydrogen and oxygen, in 1806. Edward Daniel Clarke invented the hydrogen gas blowpipe in 1819. The 's lamp and limelight were invented in 1823. Hydrogen was liquefied for the first time by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask. He produced solid hydrogen the next year.^[55]

One of the first quantum effects to be explicitly noticed, although not understood at the time, was James Clerk Maxwell's observation that the specific heat capacity of H
2 unaccountably departs from that of a diatomic gas below room temperature, and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H
2 because of its low mass. These widely-spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.^[60]

20th century

The existence of the hydride anion was suggested by Gilbert N. Lewis in 1916 for group 1 and group 2 salt-like compounds. In 1920, electrolyzed molten lithium hydride (LiH), producing a stoichiometric quantity of hydrogen at the anode.^[61]

Hydrogen emission spectrum lines in the four visible lines of the Balmer series

Because of its simple atomic structure, consisting only of a proton and an electron, the hydrogen atom, together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of atomic structure.^[62] The energy levels of hydrogen can be calculated fairly accurately using the model of the atom, in which the electron "orbits" the proton, just as Earth orbits the Sun. However, the electron and proton are held together by electrostatic attraction, while planets and celestial objects are held by gravity. Due to the discretization of angular momentum postulated in early quantum mechanics by , the electron in the model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.^[63]

Hydrogen's unique position as the only neutral atom for which the equation can be directly solved has significantly contributed to the understanding of quantum mechanics through the exploration of its energetics.^[64] Furthermore, study of the corresponding simplicity of the hydrogen molecule and the corresponding cation, H+
2, brought understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.^[65]

Hydrogen-lifted airship

Because H
2 has only 7% the density of air, it was once widely used as a lifting gas in balloons and airships.^[66] The first hydrogen-filled balloon was invented by in 1783. Hydrogen provided the lift for the first reliable form of air-travel following the 1852 invention of the first hydrogen-lifted airship by . German count promoted the idea of rigid airships lifted by hydrogen that later were called , the first of which had its maiden flight in 1900.^[55] Regularly-scheduled flights started in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships in the form of blimps were used as observation platforms and bombers during World War II, especially on the US Eastern seaboard.^[67]

The first non-stop transatlantic crossing was made by the British airship R34 in 1919 and regular passenger service resumed in the 1920s. Hydrogen was used in the Hindenburg, which caught fire over New Jersey on 6 May 1937.^[55] The hydrogen that filled the airship was ignited, possibly by static electricity, and burst into flames.^[68] Following this disaster, commercial hydrogen airship travel ceased. Hydrogen is still used, in preference to non-flammable but more expensive helium, as a lifting gas for weather balloons.^[69]

Deuterium and tritium

Deuterium was discovered in December 1931 by Harold Urey, and tritium was prepared in 1934 by Ernest Rutherford, Mark Oliphant, and Paul Harteck.^[54] Heavy water, which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in 1932.^[55]

Chemistry

Reactions of H₂

H
2 is relatively unreactive. The thermodynamic basis of this low reactivity is the very strong , with a bond dissociation energy of 435.7 kJ/mol.Cite error: Invalid <ref> tag; refs with no name must have content It does form coordination complexes called dihydrogen complexes. These species provide insights into the early steps in the interactions of hydrogen with metal catalysts. According to neutron diffraction, the metal and two H atoms form a triangle in these complexes. The remains intact but is elongated. They are acidic.^[70]

Although exotic on Earth, the H+
3 ion is common in the universe. It is a triangular species, like the aforementioned dihydrogen complexes. It is known as protonated molecular hydrogen or the trihydrogen cation.^[71]

Hydrogen reacts with chlorine to produce HCl, and with bromine to produce HBr, via a chain reaction. The reaction requires initiation. For example, in the case of Br₂, the dibromine molecule is split apart: Br
2 + (UV light) → 2Br•. Propagating reactions consume hydrogen molecules and produce HBr, as well as Br and H atoms:

Finally the terminating reaction:

consumes the remaining atoms.^[72]^: 289

The addition of H₂ to unsaturated organic compounds, such as alkenes and alkynes, is called hydrogenation. Even if the reaction is energetically favorable, it does not occur spontaneously even at higher temperatures. In the presence of a catalyst like finely divided platinum or nickel, the reaction proceeds at room temperature.^[73]^: 477

Hydrogen-containing compounds

Hydrogen can exist in both +1 and −1 oxidation states, forming compounds through ionic and covalent bonding. The element is part of a wide range of substances, including water, hydrocarbons, and numerous other organic compounds.^[74] The H⁺ ion—commonly referred to as a proton due to its single proton and absence of electrons—is central to acid–base chemistry, although the proton does not move freely. In the –Lowry framework, acids are defined by their ability to donate H⁺ ions to bases.^[75]

Hydrogen forms a vast variety of compounds with carbon, known as hydrocarbons, and an even greater diversity with other elements (heteroatoms), giving rise to the broad class of organic compounds often associated with living organisms.^[74]

Hydrogen compounds with hydrogen in the oxidation state −1 are known as hydrides, which are usually formed between hydrogen and metals. The hydrides can be ionic (aka saline), covalent, or metallic. With heating, H₂ reacts efficiently with the alkali and alkaline earth metals to give the ionic hydrides of the formulas MH and MH₂, respectively. These salt-like crystalline compounds have high melting points and all react with water to liberate hydrogen. Covalent hydrides include boranes and polymeric aluminium hydride. Transition metals form metal hydrides via continuous dissolution of hydrogen into the metal.^[76] A well-known hydride is lithium aluminium hydride: the [AlH
4]−
anion carries hydridic centers firmly attached to the Al(III).Cite error: Invalid <ref> tag; refs with no name must have content Perhaps the most extensive series of hydrides are the boranes, compounds consisting only of boron and hydrogen.^[77]

Hydrides can bond to these electropositive elements not only as a terminal ligand but also as bridging ligands. In diborane (B
2H
6), four hydrogen atoms are terminal, while two bridge between the two boron atoms.^[14]

Hydrogen bonding

When bonded to a more electronegative element, particularly fluorine, oxygen, or nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with a lone pair like oxygen or nitrogen. This phenomenon, called hydrogen bonding, is critical to the stability of many biological molecules.^[78]^: 375^[79] Hydrogen bonding alters molecule structures, viscosity, solubility, melting and boiling points, and even protein folding dynamics.^[80]

Protons and acids

In water, hydrogen bonding plays an important role in reaction thermodynamics. A hydrogen bond can shift over to proton transfer. Under the –Lowry acid–base theory, acids are proton donors, while bases are proton acceptors.^[81]^: 28 A bare proton (H+
) essentially cannot exist in anything other than a vacuum. Otherwise it attaches to other atoms, ions, or molecules. Even chemical species as inert as methane can be protonated. The term "proton" is used loosely and metaphorically to refer to solvated hydrogen cations attached to other solvated chemical species; it is denoted "H+
" without any implication that any single protons exist freely in solution as a species. To avoid the implication of the naked proton in solution, acidic aqueous solutions are sometimes considered to contain the "hydronium ion" ([H
3O]+
), or still more accurately, [H
9O
4]+
.^[82] Other oxonium ions are found when water is in acidic solution with other solvents.^[83]

The concentration of these solvated protons determines the pH of a solution, a logarithmic scale that reflects its acidity or basicity. Lower pH values indicate higher concentrations of hydronium ions, corresponding to more acidic conditions.^[84]

Occurrence

Cosmic

Hydrogen, as atomic H, is the most abundant chemical element in the universe, making up 75% of normal matter by mass.^[85] and >90% by number of atoms.^[86] In the early universe, protons formed in the first second after the Big Bang; neutral hydrogen atoms formed about 370,000 years later during the recombination epoch as the universe expanded and plasma had cooled enough for electrons to remain bound to protons.^[87]

In astrophysics, neutral hydrogen in the interstellar medium is called H I and ionized hydrogen is called H II.^[88] Radiation from stars ionizes H I to H II, creating spheres of ionized H II around stars. In the chronology of the universe neutral hydrogen dominated until the birth of stars during the era of reionization, which then produced bubbles of ionized hydrogen that grew and merged over hundreds of millions of years.^[89] These are the source of the 21-centimeter hydrogen line, at 1420 MHz, that is detected in order to probe primordial hydrogen. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the universe up to a redshift of .^[90]

Hydrogen is found in great abundance in stars and gas giant planets. Molecular clouds of H
2 are associated with star formation. Hydrogen plays a vital role in powering stars through the proton-proton reaction in lower-mass stars, and through the CNO cycle of nuclear fusion in stars more massive than the Sun.^[91]

Protonated molecular hydrogen (H+
3) is found in the interstellar medium, where it is generated by ionization of molecular hydrogen by cosmic rays. This ion has also been observed in the upper atmosphere of Jupiter. The ion is long-lived in outer space due to the low temperature and density. H+
3 is one of the most abundant ions in the universe, and it plays a notable role in the chemistry of the interstellar medium.^[92] Neutral triatomic hydrogen H
3 can exist only in an excited form and is unstable.^[93]

Terrestrial

Hydrogen is the third most abundant element on the Earth's surface,^[94] mostly existing within chemical compounds such as hydrocarbons and water.^[14] Elemental hydrogen is normally in the form of a gas, H
2, at standard conditions. It is present in a very low concentration in Earth's atmosphere (around 0.53 parts per million on a molar basis^[95]) because of its light weight, which enables it to escape the atmosphere more rapidly than heavier gases. Despite its low concentration in the atmosphere, terrestrial hydrogen is sufficiently abundant to support the metabolism of several varieties of bacteria.^[96]

Large underground deposits of hydrogen gas have been discovered in several countries including Mali, France and Australia.^[97] As of 2024, it is uncertain how much underground hydrogen can be extracted economically.^[97]

Production and storage

Industrial routes

Nearly all of the world's current supply of hydrogen gas (H
2) is produced from fossil fuels,^[98]^[99]^: 1 with less than 1% of hydrogen being produced by low-emissions technologies in 2025.^[100] Many methods exist for producing H₂, but three dominate commercially: steam reforming often coupled to water-gas shift, partial oxidation of hydrocarbons, and water electrolysis.^[101]

Steam reforming

Hydrogen is mainly produced by steam methane reforming (SMR), the reaction of water and methane.^[102]^[103] Thus, at high temperature (1,000–1,400 K [730–1,130 °C; 1,340–2,060 °F]), steam (water vapor) reacts with methane to yield carbon monoxide and H
2.

Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide.^[104] The production of natural gas feedstock also produces emissions such as vented and fugitive methane, which further contributes to the overall carbon footprint of hydrogen.^[105]

This reaction is favored at low pressures but is nonetheless conducted at high pressures (2.0 MPa [20 atm; 590 inHg]) because high-pressure H
2 is the most marketable product, and pressure swing adsorption (PSA) purification systems work better at higher pressures. The product mixture is known as "synthesis gas" because it is often used directly for the production of methanol and many other compounds. Hydrocarbons other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly-optimized technology is the formation of coke or carbon:

Therefore, steam reforming typically employs an excess of H
2O. Additional hydrogen can be recovered from the steam by using carbon monoxide through the water gas shift reaction (WGS). This process requires an iron oxide catalyst:^[103]

Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the process for ammonia production, hydrogen is generated from natural gas.^[106]

Partial oxidation of hydrocarbons

Other methods for CO and H
2 production include partial oxidation of hydrocarbons:^[35]

Although less important commercially, coal can serve as a prelude to the above shift reaction:^[103]

Olefin production units may produce substantial quantities of byproduct hydrogen, particularly from cracking light feedstocks like ethane or propane.^[107]

Water electrolysis

Electrolysis of water is a conceptually simple method of producing hydrogen.

Commercial electrolyzers use nickel-based catalysts in strongly alkaline solution. Platinum is a better catalyst but is expensive.^[108] The hydrogen created through electrolysis using renewable energy is commonly referred to as "green hydrogen".^[109]

Electrolysis of brine to yield chlorine^[110] also produces high-purity hydrogen as a co-product, which is used for a variety of transformations such as hydrogenations.^[111]

The electrolysis process is more expensive than producing hydrogen from methane without carbon capture and storage.^[112]

Innovation in hydrogen electrolyzers could make large-scale production of hydrogen from electricity more cost-competitive.^[113]

Methane pyrolysis

Hydrogen can be produced by pyrolysis of natural gas (methane), producing hydrogen gas and solid carbon with the aid of a catalyst and 74 kJ/mol input heat:

The carbon may be sold as a manufacturing feedstock or fuel, or landfilled. This route could have a lower carbon footprint than existing hydrogen production processes, but mechanisms for removing the carbon and preventing it from reacting with the catalyst remain obstacles for industrial-scale use.^[114]^: 17^[115]

Thermochemical

Water splitting is the process by which water is decomposed into its components. Relevant to the biological scenario is this equation:

The reaction occurs in the light-dependent reactions in all photosynthetic organisms. A few organisms, including the alga Chlamydomonas reinhardtii and cyanobacteria, have evolved a second step in the dark reactions in which protons and electrons are reduced to form H
2 gas by specialized hydrogenases in the chloroplast.^[116]

Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to more efficiently generate H
2 gas even in the presence of oxygen.^[117] Efforts have also been undertaken with genetically‐modified alga in a bioreactor.^[118]

Relevant to the thermal water-splitting scenario is this simple equation:

Over 200 thermochemical cycles can be used for water splitting. Many of these cycles such as the iron oxide cycle, cerium(IV) oxide–cerium(III) oxide cycle, zinc–zinc oxide cycle, sulfur–iodine cycle, copper–chlorine cycle and hybrid sulfur cycle have been evaluated for their commercial potential to produce hydrogen and oxygen from water and heat without using electricity.^[119] A number of labs (including in France, Germany, Greece, Japan, and the United States) are developing thermochemical methods to produce hydrogen from solar energy and water.^[120]

Natural routes

Biohydrogen

H
2 is produced in organisms by enzymes called hydrogenases. This process allows the host organism to use fermentation as a source of energy.^[121] These same enzymes also can oxidize H₂, such that the host organisms can subsist by reducing oxidized substrates using electrons extracted from H₂.^[122]

Hydrogenase enzymes feature iron or iron–nickel centers at their active sites.^[123] The natural cycle of hydrogen production and consumption by organisms is called the hydrogen cycle.^[124]

Some bacteria such as Mycobacterium smegmatis can use the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking. Their hydrogenases feature small channels that exclude oxygen from the active site, permitting the reaction to occur even though the hydrogen concentration is very low and the oxygen concentration is as in normal air.^[95]^[125]

Confirming the existence of hydrogenase‐employing microbes in the human gut, H
2 occurs in human breath. The concentration in the breath of fasting people at rest is typically under 5 parts per million (ppm), but can reach 50 ppm when people with intestinal disorders consume molecules they cannot absorb during diagnostic hydrogen breath tests.^[126]

Serpentinization

Serpentinization is a geological mechanism which produces highly-reducing conditions.^[127] Under these conditions, water is capable of oxidizing ferrous () ions in fayalite, generating hydrogen gas:^[128]^[129]

Closely related to this geological process is the reaction:

This process also is relevant to the corrosion of iron and steel in oxygen-free groundwater and in reducing soils below the water table.^[130]

Laboratory syntheses

H
2 is produced in laboratory settings, such as in the small-scale electrolysis of water using metal electrodes and water containing an electrolyte, which liberates hydrogen gas at the cathode:^[84]

Hydrogen is also often a by-product of other reactions. Many metals react with water to produce H
2, but the rate of hydrogen evolution depends on the metal, the pH, and the presence of alloying agents. Most often, hydrogen evolution is induced by acids. The alkali and alkaline earth metals as well as aluminium, zinc, manganese, and iron, react readily with aqueous acids.^[84]

Many metals, such as aluminium, are slow to react with water because they form passivated oxide coatings. An alloy of aluminium and gallium, however, does react with water. In high-pH solutions, aluminium can react with H
2:^[84]

Storage

If H₂ is to be used as an energy source, its storage is important. It dissolves only poorly in solvents. For example, at room temperature and 0.1 millipascals (9.9×10⁻¹⁰ atm), 0.05 moles of hydrogen dissolve into 1 kilogram (2.2 lb)^* of diethyl ether.^[76] H₂ can be stored in compressed form, although compressing costs energy. Liquefaction is impractical given hydrogen's low critical temperature. In contrast, ammonia and many hydrocarbons can be liquified at room temperature under pressure. For these reasons, hydrogen carriers—materials that reversibly bind H₂—have attracted much attention. The key question is then the weight percent of H₂-equivalents within the carrier material. For example, hydrogen can be reversibly absorbed into many rare earths and transition metals^[131] and is soluble in both nanocrystalline and amorphous metals.^[132] Hydrogen solubility in metals is influenced by local distortions or impurities in the crystal lattice.^[133] These properties may be useful when hydrogen is purified by passage through hot palladium disks, but the gas's high solubility is also a metallurgical problem, contributing to the embrittlement of many metals,^[134] complicating the design of pipelines and storage tanks.^[135]

The most problematic aspect of metal hydrides for storage is their modest H₂ content, often on the order of 1%. For this reason, there is interest in storage of H₂ in compounds of low molecular weight. For example, ammonia borane (H
3N–BH
3) contains 19.8 weight percent of H₂. The problem with this material is that after release of H₂, the resulting boron nitride does not re-add H₂: i.e., ammonia borane is an irreversible hydrogen carrier.^[136] More attractive are hydrocarbons such as tetrahydroquinoline, which reversibly release some H₂ when heated in the presence of a catalyst:^[137]

Applications

Petrochemical industry

Large quantities of H
2 are used in the "upgrading" of fossil fuels. Key consumers of H
2 include hydrodesulfurization and hydrocracking. Many of these reactions can be classified as hydrogenolysis, i.e., the cleavage of bonds by hydrogen. Illustrative is the separation of sulfur from liquid fossil fuels:^[101]^[139]

Hydrogenation

Hydrogenation, the addition of H
2 to various substrates, is done on a large scale. Hydrogenation of N
2 produces ammonia by the process:^[139]

This process consumes a few percent of the energy budget in the entire industry and is the biggest consumer of hydrogen. The resulting ammonia is used extensively in fertilizer production; these fertilizers have become essential feedstocks in modern agriculture.^[140] Hydrogenation is also used to convert unsaturated fats and oils to saturated fats and oils. The major application is the production of margarine. Methanol is produced by hydrogenation of carbon dioxide; the mixture of hydrogen and carbon dioxide used for this process is known as syngas. It is similarly the source of hydrogen in the manufacture of hydrochloric acid. H
2 is also used as a reducing agent for the conversion of some ores to the metals.^[141]^[84]

Fuel

The potential for using hydrogen (H₂) as a fuel has been widely discussed. Hydrogen can be used in fuel cells to produce electricity,^[142] or burned to generate heat.^[143] When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapor.^[143] When burned, hydrogen produces relatively little pollution at the point of combustion, but can lead to thermal formation of harmful nitrogen oxides.^[143]

If hydrogen is produced with low or zero greenhouse gas emissions (green hydrogen), it can play a significant role in decarbonizing energy systems where there are challenges and limitations to replacing fossil fuels with direct use of electricity.^[144]^[112]

Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals, thus contributing to the decarbonization of industry alongside other technologies, such as electric arc furnaces for steelmaking.^[145] However, it is likely to play a larger role in providing industrial feedstock for cleaner production of ammonia and organic chemicals.^[144] For example, in steelmaking, hydrogen could function as a clean fuel and also as a low-carbon catalyst, replacing coal-derived coke (carbon):^[146]

Hydrogen used to decarbonize transportation is likely to find its largest applications in shipping, aviation and, to a lesser extent, heavy goods vehicles, through the use of hydrogen-derived synthetic fuels such as ammonia and methanol and fuel cell technology.^[144] For light-duty vehicles including cars, hydrogen is far behind other alternative fuel vehicles, especially compared with the rate of adoption of battery electric vehicles, and may not play a significant role in future.^[147]

Liquid hydrogen and liquid oxygen together serve as cryogenic propellants in liquid-propellant rockets, as in the Space Shuttle main engines. NASA has investigated the use of rocket propellant made from atomic hydrogen, boron or carbon that is frozen into solid molecular hydrogen particles suspended in liquid helium. Upon warming, the mixture vaporizes to allow the atomic species to recombine, heating the mixture to high temperature.^[148]

Hydrogen produced when there is a surplus of variable renewable electricity could in principle be stored and later used to generate heat or to re-generate electricity.^[149] It can be further transformed into synthetic fuels such as ammonia and methanol.^[150] Disadvantages of hydrogen fuel include high costs of storage and distribution due to hydrogen's explosivity, its large volume compared to other fuels, and its tendency to embrittle materials.^[105]

Nickel–hydrogen battery

The very long-lived, rechargeable nickel–hydrogen battery developed for satellite power systems uses pressurized gaseous H₂.^[151] The International Space Station,^[152] Mars Odyssey^[153] and the Mars Global Surveyor^[154] are equipped with nickel-hydrogen batteries. In the dark part of its orbit, the Hubble Space Telescope is also powered by nickel-hydrogen batteries, which were finally replaced in May 2009,^[155] more than 19 years after launch and 13 years beyond their design life.^[156]

Semiconductor industry

Hydrogen is employed in semiconductor manufacturing to saturate broken ("dangling") bonds of amorphous silicon and amorphous carbon, which helps in stabilizing the materials' properties.^[157] Hydrogen, introduced as an unintended side-effect of production, acts as a shallow electron donor leading to N-type semiconductor conductivity in ZnO, with important uses in transducers and phosphors.^[158]^[159] Detailed analysis of ZnO and of MgO shows evidence of four and six-fold hydrogen multicentre bonds.^[160] The doping behavior of hydrogen varies with material.^[161]^[162]

Niche and evolving uses

Beyond than the uses mentioned above, hydrogen is used in smaller scales in the following applications:

Shielding gas: Hydrogen is used as a shielding gas in welding methods such as atomic hydrogen welding.^[163]^[164]

Coolant: Hydrogen is used as a coolant in large electrical generators due to its high thermal conductivity and low density.^[165] The first hydrogen-cooled turbogenerator went into service using gaseous hydrogen as a coolant in the rotor and the stator in 1937 in Dayton, Ohio.^[166]
Cryogenic research: Liquid H
2 is used in cryogenic research, including superconductivity studies.^[167]
Food industry: Hydrogen is an authorized food additive (E949)^[168] that is used as a packaging gas,^[169] and also has antioxidant properties.^[170]
Leak detection: Pure or mixed with nitrogen (sometimes called forming gas), hydrogen is a tracer gas for detection of minute leaks. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries;^[171] it also allows for leak testing in food packaging.^{[citation needed]}

Neutron moderation: Deuterium (hydrogen-2) is used in nuclear fission applications as a moderator to slow neutrons.^{[citation needed]}
Nuclear fusion fuel: Deuterium is used in nuclear fusion reactions.^[55]
Isotopic labeling: Deuterium compounds have applications in chemistry and biology in studies of isotope effects on reaction rates.^[172]

Tritium uses: Tritium (hydrogen-3), produced in nuclear reactors, is used in the production of hydrogen bombs,^[173] as an isotopic label in the biosciences,^[20] and as a source of beta radiation in radioluminescent paint for instrument dials and emergency signage.^[15]

Safety and precautions

In hydrogen pipelines and steel storage vessels, hydrogen molecules are prone to reacting with metals, causing hydrogen embrittlement and leaks in the pipeline or storage vessel.^[174] Since it is lighter than air, hydrogen does not easily accumulate to form a combustible gas mixture.^[174] However, even without ignition sources, high-pressure hydrogen leakage may cause spontaneous combustion and detonation.^[174]

Hydrogen is flammable when mixed even in small amounts with air. Ignition can occur at a volumetric ratio of hydrogen to air as low as 4%.^[175] In approximately 70% of hydrogen ignition accidents, the ignition source cannot be determined.^[174]

Hydrogen fire, while being extremely hot, is almost invisible to the human eye, and thus can lead to accidental burns.^[34] Hydrogen is non-toxic,^[176] but like most gases it can cause asphyxiation in the absence of adequate ventilation.^[177]

Table of contents (84 articles)

Index

Composite matter

1. Materials

2. Matter

3. Plasma and fusion physics

4. Molecules

5. Nuclear matter

Sub-molecular

6. Atoms

7. Particles

8. Composite particles

9. Fields

10. Vacuum and spacetime

Full contents

1. Materials (6) Back to index

1. Physics:Quantum materials/band structure

2. Physics:Quantum materials/phonon

3. Physics:Quantum materials/superconductor

4. Physics:Quantum materials/topological phase

5. Physics:Quantum materials/crystal lattice

6. Physics:Quantum materials/solid state

2. Matter (5) Back to index

7. Physics:Quantum matter/phase

8. Physics:Quantum matter/state of matter

9. Physics:Quantum matter/thermodynamic system

10. Physics:Quantum matter/density

11. Physics:Quantum matter/temperature

3. Plasma and fusion physics (6) Back to index

12. Physics:Quantum Tokamak

13. Physics:Quantum Fusion

14. Physics:Quantum matter/plasma

15. Physics:Quantum magnetic confinement

16. Physics:Quantum Lawson criterion

17. Physics:Quantum Quark–gluon plasma

4. Molecules (6) Back to index

18. Physics:Quantum molecular structure

19. Physics:Quantum chemical bond

20. Physics:Quantum molecular orbital

21. Physics:Quantum degenerate orbitals

22. Physics:Quantum molecular spectroscopy

23. Physics:Quantum molecular vibration

5. Nuclear matter (6) Back to index

24. Physics:Quantum atomic nucleus

25. Physics:Quantum nuclear matter

26. Physics:Quantum isotope

27. Physics:Quantum deuteron

28. Physics:Quantum nuclear binding energy

29. Physics:Quantum nuclear force

6. Atoms (7) Back to index

30. Physics:Quantum atoms/electron

31. Physics:Quantum atoms/energy level

32. Physics:Quantum atoms/electron configuration

33. Physics:Quantum atoms/transition

34. Physics:Quantum atoms/ion

35. Physics:Quantum atoms/orbital

36. Physics:Quantum atoms/hydrogen

7. Particles (12) Back to index

37. Physics:Quantum particle

38. Physics:Quantum elementary particle

39. Physics:Quantum fermion

40. Physics:Quantum boson

41. Physics:Quantum lepton

42. Physics:Quantum electron

43. Physics:Quantum neutrino

44. Physics:Quantum quark

45. Physics:Quantum photon

46. Physics:Quantum gluon

47. Physics:Quantum W and Z bosons

48. Physics:Quantum Higgs boson

8. Composite particles (12) Back to index

49. Physics:Quantum hadron

50. Physics:Quantum baryon

51. Physics:Quantum meson

52. Physics:Quantum nucleon

53. Physics:Quantum proton

54. Physics:Quantum neutron

55. Physics:Quantum pion

56. Physics:Quantum kaon

57. Physics:Quantum glueball

58. Physics:Quantum tetraquark

59. Physics:Quantum pentaquark

60. Physics:Quantum exotic hadron

9. Fields (12) Back to index

61. Physics:Quantum field theory

62. Physics:Quantum electromagnetic field

63. Physics:Quantum gauge field

64. Physics:Quantum scalar field

65. Physics:Quantum vacuum field

66. Physics:Quantum wavefunction field

67. Physics:Quantum spinor field

68. Physics:Quantum matter field

69. Physics:Quantum Higgs field

70. Physics:Quantum Yang-Mills field

71. Physics:Quantum photon field

72. Physics:Quantum gluon field

10. Vacuum and spacetime (12) Back to index

73. Physics:Quantum spacetime

74. Physics:Quantum vacuum energy

75. Physics:Quantum virtual particle

76. Physics:Quantum zero-point energy

77. Physics:Quantum curved spacetime

78. Physics:Quantum quantum gravity

79. Physics:Quantum vacuum state

80. Physics:Quantum false vacuum

81. Physics:Quantum Planck scale

82. Physics:Quantum spacetime foam

83. Physics:Quantum metric field

84. Physics:Quantum field theory in curved spacetime

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External links

(Timothy Jones, Drexel University)
Hydrogen at The Periodic Table of Videos (University of Nottingham)
High temperature hydrogen phase diagram from Burkhard Militzer (University of California, Berkeley)
Wavefunction of hydrogen at HyperPhysics (Georgia State University)

Author: Harold Foppele

Source attribution: Physics:Quantum atoms/hydrogen

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@@ Line 1: / Line 1: @@
-{{Short description|Quantum Collection topic on Quantum atoms/hydrogen}}
+{{Quantum article nav|previous=Physics:Quantum atoms/orbital|previous label=Atoms/orbital|next=Physics:Quantum particle|next label=Particle}}
+{{Short description|Quantum Collection topic on Quantum atoms/hydrogen}}
 {{Quantum matter backlink|Atoms}}
@@ Line 10: / Line 14: @@
 <div style="flex:1; line-height:1.45; color:#006b45; column-count:2; column-gap:32px; column-rule:1px solid #b8d8c8;">
-'''Hydrogen''' is a [[Chemistry:Chemical element|chemical element]]; it has the [[Chemistry:Chemical symbol|symbol]]{{nbsp}}'''H''' and [[Physics:Atomic number|atomic number]]{{nbsp}}1. It is the lightest and [[Chemistry:Abundance of the chemical elements|most abundant]] chemical element in the universe, constituting about 75% of all [[Physics:Baryon|normal matter]]. Under standard conditions, hydrogen is a [[Physics:Gas|gas]] of [[Chemistry:Diatomic molecule|diatomic molecule]]s with the [[Chemistry:Chemical formula|formula]]{{nbsp}}{{chem2|H2}}, called '''dihydrogen'''<!--dihydrogen is a redirect to hydrogen; also called '''diprotium'''-->, or sometimes '''hydrogen gas''', '''molecular hydrogen''', or simply hydrogen. Dihydrogen is colorless, odorless, non-toxic, and highly combustible. [[Astronomy:Star|Star]]s, including the [[Astronomy:Sun|Sun]], mainly consist of hydrogen in a plasma state, while on Earth, hydrogen is found as the gas{{nbsp}}{{chem2|H2}} (dihydrogen) and in [[Physics:Molecule|molecule]]s, such as in [[Chemistry:Water|water]] and [[Chemistry:Organic compound|organic compound]]s. The most common isotope of hydrogen, {{sup|1}}H, consists of one [[Software:Proton|proton]], one [[Physics:Electron|electron]], and no [[Physics:Neutron|neutron]]s.
+'''Hydrogen''' is a chemical element; it has the symbol{{nbsp}}'''H''' and atomic number{{nbsp}}1. It is the lightest and most abundant chemical element in the universe, constituting about 75% of all normal matter. Under standard conditions, hydrogen is a gas of diatomic molecules with the formula{{nbsp}}{{chem2|H2}}, called '''dihydrogen'''<!--dihydrogen is a redirect to hydrogen; also called '''diprotium'''-->, or sometimes '''hydrogen gas''', '''molecular hydrogen''', or simply hydrogen. Dihydrogen is colorless, odorless, non-toxic, and highly combustible. Stars, including the Sun, mainly consist of hydrogen in a plasma state, while on Earth, hydrogen is found as the gas{{nbsp}}{{chem2|H2}} (dihydrogen) and in molecules, such as in water and organic compounds. The most common isotope of hydrogen, H, consists of one proton, one electron, and no neutrons.
-Hydrogen gas was first produced artificially in the 17th{{nbsp}}century by the [[Chemistry:Chemical reaction|reaction]] of [[Chemistry:Acid|acid]]s with metals. [[Biography:Henry Cavendish|Henry Cavendish]], in{{nbsp}}1766–1781, identified hydrogen gas as a distinct substance and discovered its property of producing water when burned: this is the origin of hydrogen's name, which means {{gloss|water-former}} (from {{langx|grc|ὕδωρ|húdōr|water}}, and {{langx|grc|γεννάω|gennáō|I bring forth|label=none}}). Understanding the [[Physics:Hydrogen spectral series|colors of light absorbed and emitted by hydrogen]] was a crucial part of the development of [[Physics:Quantum mechanics|quantum mechanics]].
+Hydrogen gas was first produced artificially in the 17th{{nbsp}}century by the reaction of acids with metals. Henry Cavendish, in{{nbsp}}1766–1781, identified hydrogen gas as a distinct substance and discovered its property of producing water when burned: this is the origin of hydrogen's name, which means  (from {{langx|grc|ὕδωρ|húdōr|water}}, and {{langx|grc|γεννάω|gennáō|I bring forth|label=none}}). Understanding the colors of light absorbed and emitted by hydrogen was a crucial part of the development of [[Physics:Quantum mechanics|quantum mechanics]].
-Hydrogen, typically nonmetallic except under extreme pressure, readily forms [[Chemistry:Covalent bond|covalent bond]]s with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial in [[Chemistry:Acid–base reaction|acid–base reaction]]s, which mainly involve proton exchange among [[Chemistry:Solubility|soluble]] molecules. In ionic compounds, hydrogen can take the form of either a negatively-charged anion, where it is known as [[Physics:Hydride|hydride]], or as a positively charged cation, {{chem2|H+}}, [[Chemistry:Hydron|hydron]]. Although tightly bonded to water molecules, hydrons strongly affect the behavior of [[Chemistry:Aqueous solution|aqueous solution]]s, as reflected in the importance of [[Chemistry:PH|pH]]. Hydride, on the other hand, is rarely observed because it tends to deprotonate solvents, yielding{{nbsp}}{{chem2|H2}}.<ref>{{Cite web |title=Element: Hydrogen |url=https://pse-info.de/en/element/H |access-date=2026-01-21 |website=Periodic table |language=en}}</ref>
+Hydrogen, typically nonmetallic except under extreme pressure, readily forms covalent bonds with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial in acid–base reactions, which mainly involve proton exchange among soluble molecules. In ionic compounds, hydrogen can take the form of either a negatively-charged anion, where it is known as hydride, or as a positively charged cation, {{chem2|H+}}, hydron. Although tightly bonded to water molecules, hydrons strongly affect the behavior of aqueous solutions, as reflected in the importance of pH. Hydride, on the other hand, is rarely observed because it tends to deprotonate solvents, yielding{{nbsp}}{{chem2|H2}}.<ref>{{Cite web |title=Element: Hydrogen |url=https://pse-info.de/en/element/H |access-date=2026-01-21 |website=Periodic table |language=en}}</ref>
-In the early universe, neutral hydrogen atoms [[Astronomy:Big Bang nucleosynthesis|formed about 370,000 years after the Big Bang]] as the universe expanded and plasma had cooled enough for electrons to remain bound to protons. After stars [[Astronomy:Star formation|began to form]], most of the hydrogen in the intergalactic medium was re-ionized.
+In the early universe, neutral hydrogen atoms formed about 370,000 years after the Big Bang as the universe expanded and plasma had cooled enough for electrons to remain bound to protons. After stars began to form, most of the hydrogen in the intergalactic medium was re-ionized.
+Nearly all hydrogen production is done by transforming fossil fuels, particularly steam reforming of natural gas. It can also be produced from water or saline by electrolysis, but this process is more expensive. Its main industrial uses include fossil fuel processing and ammonia production for fertilizer. Emerging uses for hydrogen include the use of fuel cells to generate electricity.
-Nearly all [[Chemistry:Hydrogen production|hydrogen production]] is done by transforming fossil fuels, particularly [[Chemistry:Steam reforming|steam reforming]] of [[Chemistry:Natural gas|natural gas]]. It can also be produced from water or saline by [[Chemistry:Electrolysis|electrolysis]], but this process is more expensive. Its main industrial uses include fossil fuel processing and [[Chemistry:Ammonia production|ammonia production]] for fertilizer. Emerging uses for hydrogen include the use of [[Physics:Fuel cell|fuel cell]]s to generate electricity.
-{{Toclimit}}
 </div>
@@ Line 33: / Line 37: @@
 ==== Electron energy levels ====
 {{Main|Physics:Hydrogen atom}}
-The [[Physics:Ground state|ground state]] [[Physics:Energy level|energy level]] of the electron in a hydrogen atom is −13.6{{nbsp}}electronvolts{{nbsp}}(eV),<ref>{{cite web|author1=NAAP Labs|title=Energy Levels|url=http://astro.unl.edu/naap/hydrogen/levels.html|publisher=University of Nebraska Lincoln|access-date=20 May 2015|date=2009|archive-url=https://web.archive.org/web/20150511120536/http://astro.unl.edu/naap/hydrogen/levels.html|archive-date=11 May 2015|url-status=live}}</ref> equivalent to an [[Physics:Ultraviolet|ultraviolet]] [[Physics:Photon|photon]] of roughly 91{{nbsp}}nanometers wavelength.<ref>{{cite web|url=http://www.wolframalpha.com/input/?i=photon+wavelength+13.6+ev|title=photon wavelength 13.6 eV|access-date=20 May 2015|date=20 May 2015|work=Wolfram Alpha|archive-url=https://web.archive.org/web/20160512221720/http://www.wolframalpha.com/input/?i=photon+wavelength+13.6+ev|archive-date=12 May 2016|url-status=live}}</ref> The energy levels of hydrogen are referred to by consecutive [[Physics:Quantum number|quantum number]]s, with <math>n=1</math> being the ground state. The [[Physics:Hydrogen spectral series|hydrogen spectral series]] corresponds to emission of light due to transitions from higher to lower energy levels.<ref>{{Cite book |last=Levine |first=Ira N. |title=Quantum chemistry |date=1970 |publisher=Pearson |isbn=978-0-321-89060-3 |edition=2 |series=Pearson advanced chemistry series |location=Boston}}</ref>{{rp|105}} Each energy level is further split by [[Spin|spin]] interactions between the electron and proton into four [[Physics:Hyperfine structure|hyperfine]] levels.<ref>{{Cite book |last1=Feynman |first1=Richard P.  |title=The Feynman lectures on physics |last2=Leighton |first2=Robert B. |last3=Sands |first3=Matthew L. |date=2011 |publisher=Basic Books |isbn=978-0-465-02414-8 |edition=New millennium |location=New York |chapter=The Hyperfine Splitting in Hydrogen |oclc=671704374 |chapter-url=https://www.feynmanlectures.caltech.edu/III_12.html}}</ref>
+The ground state energy level of the electron in a hydrogen atom is −13.6{{nbsp}}electronvolts{{nbsp}}(eV),<ref>{{cite web|author1=NAAP Labs|title=Energy Levels|url=http://astro.unl.edu/naap/hydrogen/levels.html|publisher=University of Nebraska Lincoln|access-date=20 May 2015|date=2009|archive-url=https://web.archive.org/web/20150511120536/http://astro.unl.edu/naap/hydrogen/levels.html|archive-date=11 May 2015|url-status=live}}</ref> equivalent to an ultraviolet photon of roughly 91{{nbsp}}nanometers wavelength.<ref>{{cite web|url=http://www.wolframalpha.com/input/?i=photon+wavelength+13.6+ev|title=photon wavelength 13.6 eV|access-date=20 May 2015|date=20 May 2015|work=Wolfram Alpha|archive-url=https://web.archive.org/web/20160512221720/http://www.wolframalpha.com/input/?i=photon+wavelength+13.6+ev|archive-date=12 May 2016|url-status=live}}</ref> The energy levels of hydrogen are referred to by consecutive [[Physics:Quantum number|quantum number]]s, with <math>n=1</math> being the ground state. The hydrogen spectral series corresponds to emission of light due to transitions from higher to lower energy levels.<ref>{{Cite book |last=Levine |first=Ira N. |title=Quantum chemistry |date=1970 |publisher=Pearson |isbn=978-0-321-89060-3 |edition=2 |series=Pearson advanced chemistry series |location=Boston}}</ref>{{rp|105}} Each energy level is further split by spin interactions between the electron and proton into four hyperfine levels.<ref>{{Cite book |last1=Feynman |first1=Richard P.  |title=The Feynman lectures on physics |last2=Leighton |first2=Robert B. |last3=Sands |first3=Matthew L. |date=2011 |publisher=Basic Books |isbn=978-0-465-02414-8 |edition=New millennium |location=New York |chapter=The Hyperfine Splitting in Hydrogen |oclc=671704374 |chapter-url=https://www.feynmanlectures.caltech.edu/III_12.html}}</ref>
-High-precision values for the hydrogen atom energy levels are required for definitions of physical constants. Quantum calculations have identified nine contributions to the energy levels. The eigenvalue from the [[Dirac equation]] is the largest contribution. Other terms include [[Physics:Relativistic quantum mechanics|relativistic]] recoil, the [[Physics:Self-energy|self-energy]], and the [[Physics:Vacuum polarization|vacuum polarization]] terms.<ref>{{Cite journal |last1=Tiesinga |first1=Eite |last2=Mohr |first2=Peter J. |last3=Newell |first3=David B. |last4=Taylor |first4=Barry N. |date=2021-09-23 |title=CODATA Recommended Values of the Fundamental Physical Constants: 2018* |journal=Journal of Physical and Chemical Reference Data |volume=50 |issue=3 |page=033105 |doi=10.1063/5.0064853 |issn=0047-2689 |pmc=9888147 |pmid=36726646|bibcode=2021JPCRD..50c3105T }}</ref>
+High-precision values for the hydrogen atom energy levels are required for definitions of physical constants. Quantum calculations have identified nine contributions to the energy levels. The eigenvalue from the Dirac equation is the largest contribution. Other terms include relativistic recoil, the self-energy, and the vacuum polarization terms.<ref>{{Cite journal |last1=Tiesinga |first1=Eite |last2=Mohr |first2=Peter J. |last3=Newell |first3=David B. |last4=Taylor |first4=Barry N. |date=2021-09-23 |title=CODATA Recommended Values of the Fundamental Physical Constants: 2018* |journal=Journal of Physical and Chemical Reference Data |volume=50 |issue=3 |page=033105 |doi=10.1063/5.0064853 |issn=0047-2689 |pmc=9888147 |pmid=36726646|bibcode=2021JPCRD..50c3105T }}</ref>
 ==== Nomenclature ====
-The standards organization for chemical names, IUPAC, gives general names when the context assumes natural isotope abundance or ignores the isotope. These general names are ''hydrogen'' for the neutral atom, ''[[Chemistry:Hydron|hydron]]'' for the cation, H<sup>+</sup>, ''[[Physics:Hydride|hydride]]'' for the anion, and H<sup>-</sup>.  The name ''proton'' is often used for the positively charged cation, but this is strictly correct only for the cation of the dominant isotope {{chem|1|H}}.<ref name="iupac1988proton">{{cite journal |title= Names for hydrogen atoms, ions, and groups, and for reactions involving them (Recommendations 1988) |last= Bunnet |first= J.F. |author2=Jones, R.A.Y. |year= 1968 |volume= 60 |issue= 7 |pages= 1115–6 |journal= Pure Appl. Chem. |url= http://www.iupac.org/publications/pac/1988/pdf/6007x1115.pdf |doi= 10.1351/pac198860071115 |quote=[T]he word <u>proton</u> is used not only for the <sup>1</sup>H<sup>+</sup> ion but commonly, and incorrectly, for H<sup>+</sup> in natural abundance. In many contexts this creates no ambiguity and it is likely that this usage will continue.}}</ref>
+The standards organization for chemical names, IUPAC, gives general names when the context assumes natural isotope abundance or ignores the isotope. These general names are ''hydrogen'' for the neutral atom, ''hydron'' for the cation, H<sup>+</sup>, ''hydride'' for the anion, and H<sup>-</sup>.  The name ''proton'' is often used for the positively charged cation, but this is strictly correct only for the cation of the dominant isotope .<ref name="iupac1988proton">{{cite journal |title= Names for hydrogen atoms, ions, and groups, and for reactions involving them (Recommendations 1988) |last= Bunnet |first= J.F. |author2=Jones, R.A.Y. |year= 1968 |volume= 60 |issue= 7 |pages= 1115–6 |journal= Pure Appl. Chem. |url= http://www.iupac.org/publications/pac/1988/pdf/6007x1115.pdf |doi= 10.1351/pac198860071115 |quote=[T]he word <u>proton</u> is used not only for the <sup>1</sup>H<sup>+</sup> ion but commonly, and incorrectly, for H<sup>+</sup> in natural abundance. In many contexts this creates no ambiguity and it is likely that this usage will continue.}}</ref>
 ==== Isotopes ====
 {{Main|Physics:Isotopes of hydrogen}}
 [[File:Blausen 0530 HydrogenIsotopes.png|thumb|The three naturally-occurring isotopes of hydrogen: hydrogen-1 (protium), hydrogen-2 (deuterium), and hydrogen-3 (tritium)|alt=Diagram showing the structure of each of Hydrogen-1 (mass number 1, 1 electron, 1 proton), Hydrogen-2 or deuterium (mass number 2, 1 electron, 1 proton, 1 neutron), and Hydrogen-3 or tritium (mass number 3, 1 electron, 1 proton, 2 neutrons)]]
-Hydrogen has three naturally-occurring isotopes, denoted {{chem|1|H}}, {{chem|2|H}} and{{nbsp}}{{chem|3|H}}. Other, highly-unstable [[Physics:Nuclide|nuclide]]s{{nbsp}}({{chem|4|H}} to {{chem|7|H}}) have been [[Physics:Nucleosynthesis|synthesized]] in laboratories but not observed in nature.<ref name="Gurov">{{cite journal
+Hydrogen has three naturally-occurring isotopes, denoted ,  and{{nbsp}}. Other, highly-unstable nuclides{{nbsp}}( to ) have been synthesized in laboratories but not observed in nature.<ref name="Gurov">{{cite journal
 |author=Gurov, Y. B.
 |author2=Aleshkin, D. V.
@@ Line 86: / Line 90: @@
 |first9=N.}}</ref>
-'''{{chem|1|H}}''' is the most common hydrogen isotope, with an abundance of >99.98%. Because the [[Physics:Atomic nucleus|nucleus]] of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name ''protium''.<ref>{{cite journal
+'''''' is the most common hydrogen isotope, with an abundance of >99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name ''protium''.<ref>{{cite journal
 |last1=Urey|first1=H. C.
 |last2=Brickwedde|first2=F. G.|last3=Murphy|first3=G. M.
@@ Line 93: / Line 97: @@
 |issue=2035|pages=602–603
 |doi=10.1126/science.78.2035.602
-|pmid=17797765|bibcode = 1933Sci....78..602U }}</ref> It is the only stable isotope with no neutrons ({{xref|see diproton for a discussion of why others do not exist}}).<ref>{{NUBASE2020}}</ref>
+|pmid=17797765|bibcode = 1933Sci....78..602U }}</ref> It is the only stable isotope with no neutrons ().<ref></ref>
-'''{{chem|2|H}}''', the other stable hydrogen isotope, is known as [[Physics:Deuterium|deuterium]] and contains one proton and one [[Physics:Neutron|neutron]] in the nucleus. Nearly all deuterium nuclei in the universe are thought to have been produced in [[Astronomy:Big Bang nucleosynthesis|Big Bang nucleosynthesis]], and have endured since then.<ref>{{Cite journal |last1=Particle Data Group |last2=Workman |first2=R L |last3=Burkert |first3=V D |last4=Crede |first4=V |last5=Klempt |first5=E |last6=Thoma |first6=U |last7=Tiator |first7=L |last8=Agashe |first8=K |last9=Aielli |first9=G |last10=Allanach |first10=B C |last11=Amsler |first11=C |last12=Antonelli |first12=M |last13=Aschenauer |first13=E C |last14=Asner |first14=D M |last15=Baer |first15=H |date=2022-08-08 |title=Review of Particle Physics |url=https://academic.oup.com/ptep/article/doi/10.1093/ptep/ptac097/6651666 |journal=Progress of Theoretical and Experimental Physics |language=en |volume=2022 |issue=8 |article-number=083C01 |doi=10.1093/ptep/ptac097 |issn=2050-3911|hdl=1854/LU-01HQG4F6CV7P2F3WWNH4RRN8HD |hdl-access=free }}</ref>{{rp|loc=24.2}} Deuterium is not radioactive, and is not a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called [[Chemistry:Heavy water|heavy water]]. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for {{chem|1|H}}-NMR spectroscopy.<ref>{{cite journal
+'''''', the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in the nucleus. Nearly all deuterium nuclei in the universe are thought to have been produced in Big Bang nucleosynthesis, and have endured since then.<ref>{{Cite journal |last1=Particle Data Group |last2=Workman |first2=R L |last3=Burkert |first3=V D |last4=Crede |first4=V |last5=Klempt |first5=E |last6=Thoma |first6=U |last7=Tiator |first7=L |last8=Agashe |first8=K |last9=Aielli |first9=G |last10=Allanach |first10=B C |last11=Amsler |first11=C |last12=Antonelli |first12=M |last13=Aschenauer |first13=E C |last14=Asner |first14=D M |last15=Baer |first15=H |date=2022-08-08 |title=Review of Particle Physics |url=https://academic.oup.com/ptep/article/doi/10.1093/ptep/ptac097/6651666 |journal=Progress of Theoretical and Experimental Physics |language=en |volume=2022 |issue=8 |article-number=083C01 |doi=10.1093/ptep/ptac097 |issn=2050-3911|hdl=1854/LU-01HQG4F6CV7P2F3WWNH4RRN8HD |hdl-access=free }}</ref>{{rp|loc=24.2}} Deuterium is not radioactive, and is not a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for -NMR spectroscopy.<ref>{{cite journal
 |author=Oda, Y.
 |author2=Nakamura, H.
@@ Line 104: / Line 108: @@
 |author7=Ikehara, M.
 |title=1H NMR studies of deuterated ribonuclease HI selectively labeled with protonated amino acids
-|journal=[[Chemistry:Journal of Biomolecular NMR|Journal of Biomolecular NMR]]
+|journal=Journal of Biomolecular NMR
 |date=1992|volume=2|issue=2|pages=137–47
 |doi=10.1007/BF01875525
 |pmid=1330130|s2cid=28027551
-}}</ref> Heavy water is used as a [[Physics:Neutron moderator|neutron moderator]] and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial [[Physics:Nuclear fusion|nuclear fusion]].<ref>{{cite news
+}}</ref> Heavy water is used as a neutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial nuclear fusion.<ref>{{cite news
 |last=Broad
 |first=W. J.
@@ Line 121: / Line 125: @@
 }}</ref>
-'''{{chem|3|H}}''' is known as [[Physics:Tritium|tritium]] and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into [[Physics:Helium-3|helium-3]] through [[Physics:Beta decay|beta decay]] with a [[Physics:Half-life|half-life]] of 12.32{{nbsp}}years.<ref name="Miessler" /> It is radioactive enough to be used in luminous paint to enhance the visibility of data displays, such as for painting the hands and dial-markers of watches. The watch glass prevents the small amount of radiation from escaping the case.<ref name="Traub95">{{cite web|last1=Traub|first1=R. J.|last2=Jensen|first2=J. A.|title=Tritium radioluminescent devices, Health and Safety Manual|url=http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/27/001/27001618.pdf|publisher=International Atomic Energy Agency|access-date=20 May 2015|page=2.4|date=June 1995|archive-url=https://web.archive.org/web/20150906043743/http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/27/001/27001618.pdf|archive-date=6 September 2015|url-status=live}}</ref> Small amounts of tritium are produced naturally by cosmic rays striking atmospheric gases; tritium has also been released in nuclear weapons tests.<ref>{{cite web| author=Staff| date=15 November 2007| url=http://www.epa.gov/rpdweb00/radionuclides/tritium.html| publisher=U.S. Environmental Protection Agency| title=Tritium| access-date=12 February 2008| archive-url=https://web.archive.org/web/20080102171148/http://www.epa.gov/rpdweb00/radionuclides/tritium.html| archive-date=2 January 2008}}</ref> It is used in nuclear fusion,<ref>{{cite web| last=Nave| first=C. R.| title=Deuterium-Tritium Fusion| work=HyperPhysics| publisher=Georgia State University| date=2006| url=http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fusion.html| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080316055852/http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fusion.html| archive-date=16 March 2008| url-status=live}}</ref> as a tracer in [[Chemistry:Isotope geochemistry|isotope geochemistry]],<ref>{{cite journal| first1=C.| last1=Kendall| first2=E.| last2=Caldwell| journal=Isotope Tracers in Catchment Hydrology| title=Chapter 2: Fundamentals of Isotope Geochemistry| editor1=C. Kendall| editor2=J. J. McDonnell| publisher=US Geological Survey| date=1998| doi=10.1016/B978-0-444-81546-0.50009-4| url=http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html#2.5.1| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080314192517/http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html#2.5.1| archive-date=14 March 2008| pages=51–86}}</ref> and in specialized self-powered lighting devices.<ref>{{cite web| title=The Tritium Laboratory| publisher=University of Miami| date=2008| url=http://www.rsmas.miami.edu/groups/tritium/| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080228061358/http://www.rsmas.miami.edu/groups/tritium/| archive-date=28 February 2008}}</ref> Tritium has also been used in chemical and biological labeling experiments as a radiolabel.<ref name="holte">{{cite journal| last1=Holte| first1=A. E.| last2=Houck| first2=M. A.| last3=Collie| first3=N. L.| title=Potential Role of Parasitism in the Evolution of Mutualism in Astigmatid Mites| journal=Experimental and Applied Acarology| volume=25| issue=2| pages=97–107| date=2004|doi=10.1023/A:1010655610575| pmid=11513367| s2cid=13159020}}</ref>
+'''''' is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium-3 through beta decay with a half-life of 12.32{{nbsp}}years.<ref name="Miessler" /> It is radioactive enough to be used in luminous paint to enhance the visibility of data displays, such as for painting the hands and dial-markers of watches. The watch glass prevents the small amount of radiation from escaping the case.<ref name="Traub95">{{cite web|last1=Traub|first1=R. J.|last2=Jensen|first2=J. A.|title=Tritium radioluminescent devices, Health and Safety Manual|url=http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/27/001/27001618.pdf|publisher=International Atomic Energy Agency|access-date=20 May 2015|page=2.4|date=June 1995|archive-url=https://web.archive.org/web/20150906043743/http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/27/001/27001618.pdf|archive-date=6 September 2015|url-status=live}}</ref> Small amounts of tritium are produced naturally by cosmic rays striking atmospheric gases; tritium has also been released in nuclear weapons tests.<ref>{{cite web| author=Staff| date=15 November 2007| url=http://www.epa.gov/rpdweb00/radionuclides/tritium.html| publisher=U.S. Environmental Protection Agency| title=Tritium| access-date=12 February 2008| archive-url=https://web.archive.org/web/20080102171148/http://www.epa.gov/rpdweb00/radionuclides/tritium.html| archive-date=2 January 2008}}</ref> It is used in nuclear fusion,<ref>{{cite web| last=Nave| first=C. R.| title=Deuterium-Tritium Fusion| work=HyperPhysics| publisher=Georgia State University| date=2006| url=http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fusion.html| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080316055852/http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fusion.html| archive-date=16 March 2008| url-status=live}}</ref> as a tracer in isotope geochemistry,<ref>{{cite journal| first1=C.| last1=Kendall| first2=E.| last2=Caldwell| journal=Isotope Tracers in Catchment Hydrology| title=Chapter 2: Fundamentals of Isotope Geochemistry| editor1=C. Kendall| editor2=J. J. McDonnell| publisher=US Geological Survey| date=1998| doi=10.1016/B978-0-444-81546-0.50009-4| url=http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html#2.5.1| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080314192517/http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html#2.5.1| archive-date=14 March 2008| pages=51–86}}</ref> and in specialized self-powered lighting devices.<ref>{{cite web| title=The Tritium Laboratory| publisher=University of Miami| date=2008| url=http://www.rsmas.miami.edu/groups/tritium/| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080228061358/http://www.rsmas.miami.edu/groups/tritium/| archive-date=28 February 2008}}</ref> Tritium has also been used in chemical and biological labeling experiments as a radiolabel.<ref name="holte">{{cite journal| last1=Holte| first1=A. E.| last2=Houck| first2=M. A.| last3=Collie| first3=N. L.| title=Potential Role of Parasitism in the Evolution of Mutualism in Astigmatid Mites| journal=Experimental and Applied Acarology| volume=25| issue=2| pages=97–107| date=2004|doi=10.1023/A:1010655610575| pmid=11513367| s2cid=13159020}}</ref>
-Unique among the elements, distinct names are assigned to hydrogen's isotopes in common use. During the early study of radioactivity, heavy radioisotopes were given their own names, but these are mostly no longer used. The symbols D and{{nbsp}}T (instead of {{chem|2|H}} and {{chem|3|H}}) are sometimes used for deuterium and tritium, but the symbol{{nbsp}}P was already used for [[Chemistry:Phosphorus|phosphorus]] and thus was not available for protium.<ref>{{cite web|last=van der Krogt|first=P.|date=5 May 2005|url=http://elements.vanderkrogt.net/element.php?sym=H|publisher=Elementymology & Elements Multidict|title=Hydrogen|access-date=20 December 2010|archive-url=https://web.archive.org/web/20100123001440/http://elements.vanderkrogt.net/element.php?sym=H|archive-date=23 January 2010}}</ref> In its nomenclatural guidelines, the [[Organization:International Union of Pure and Applied Chemistry|International Union of Pure and Applied Chemistry]]{{nbsp}}(IUPAC) allows any of D, T, {{chem|2|H}}, and {{chem|3|H}} to be used, though {{chem|2|H}} and {{chem|3|H}} are preferred.<ref>§ IR-3.3.2, [http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap3-3.04.pdf Provisional Recommendations] {{Webarchive|url=https://web.archive.org/web/20160209043933/http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap3-3.04.pdf |date=9 February 2016 }}, Nomenclature of Inorganic Chemistry, Chemical Nomenclature and Structure Representation Division, IUPAC. Accessed on line 3 October 2007.</ref>
+Unique among the elements, distinct names are assigned to hydrogen's isotopes in common use. During the early study of radioactivity, heavy radioisotopes were given their own names, but these are mostly no longer used. The symbols D and{{nbsp}}T (instead of  and ) are sometimes used for deuterium and tritium, but the symbol{{nbsp}}P was already used for phosphorus and thus was not available for protium.<ref>{{cite web|last=van der Krogt|first=P.|date=5 May 2005|url=http://elements.vanderkrogt.net/element.php?sym=H|publisher=Elementymology & Elements Multidict|title=Hydrogen|access-date=20 December 2010|archive-url=https://web.archive.org/web/20100123001440/http://elements.vanderkrogt.net/element.php?sym=H|archive-date=23 January 2010}}</ref> In its nomenclatural guidelines, the International Union of Pure and Applied Chemistry{{nbsp}}(IUPAC) allows any of D, T, , and  to be used, though  and  are preferred.<ref>§ IR-3.3.2, [http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap3-3.04.pdf Provisional Recommendations] {{Webarchive|url=https://web.archive.org/web/20160209043933/http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap3-3.04.pdf |date=9 February 2016 }}, Nomenclature of Inorganic Chemistry, Chemical Nomenclature and Structure Representation Division, IUPAC. Accessed on line 3 October 2007.</ref>
-[[Physics:Antihydrogen|Antihydrogen]] ({{physics particle|anti=yes|H}}) is the [[Physics:Antimatter|antimatter]] counterpart to hydrogen. It consists of an [[antiproton|{{shy|anti|proton}}]] with a [[Physics:Positron|positron]].<ref name="char15">{{cite journal|last1=Charlton|first1=Mike|last2=Van Der Werf|first2=Dirk Peter|title=Advances in antihydrogen physics|journal=Science Progress|date=1 March 2015|volume=98|issue=1|pages=34–62|doi=10.3184/003685015X14234978376369|pmid=25942774|pmc=10365473 |s2cid=23581065}}</ref><ref name="Keller15">{{cite journal|last1=Kellerbauer|first1=Alban|title=Why Antimatter Matters|journal=European Review|date=29 January 2015|volume=23|issue=1|pages=45–56|doi=10.1017/S1062798714000532|s2cid=58906869}}</ref> The [[Physics:Exotic atom|exotic atom]] [[Physics:Muonium|muonium]] (symbol Mu), composed of an antimuon and an [[Physics:Electron|electron]], is the {{shy|anti|matter}} analogue of hydrogen; {{abbr|IUPAC|international Union of Pure and Applied Chemistry}}{{nbsp}}nomenclature incorporates such hypothetical compounds as muonium chloride{{nbsp}}(MuCl) and sodium muonide{{nbsp}}(NaMu), analogous to [[Chemistry:Hydrogen chloride|hydrogen chloride]] and [[Chemistry:Sodium hydride|sodium hydride]] respectively.<ref name="iupac">{{cite journal
+Antihydrogen () is the antimatter counterpart to hydrogen. It consists of an  with a positron.<ref name="char15">{{cite journal|last1=Charlton|first1=Mike|last2=Van Der Werf|first2=Dirk Peter|title=Advances in antihydrogen physics|journal=Science Progress|date=1 March 2015|volume=98|issue=1|pages=34–62|doi=10.3184/003685015X14234978376369|pmid=25942774|pmc=10365473 |s2cid=23581065}}</ref><ref name="Keller15">{{cite journal|last1=Kellerbauer|first1=Alban|title=Why Antimatter Matters|journal=European Review|date=29 January 2015|volume=23|issue=1|pages=45–56|doi=10.1017/S1062798714000532|s2cid=58906869}}</ref> The exotic atom muonium (symbol Mu), composed of an antimuon and an electron, is the  analogue of hydrogen; {{nbsp}}nomenclature incorporates such hypothetical compounds as muonium chloride{{nbsp}}(MuCl) and sodium muonide{{nbsp}}(NaMu), analogous to hydrogen chloride and sodium hydride respectively.<ref name="iupac">{{cite journal
   |doi=10.1351/pac200173020377
   |author=W. H. Koppenol
@@ Line 132: / Line 136: @@
   |title=Names for muonium and hydrogen atoms and their ions
   |url=http://www.iupac.org/publications/pac/2001/pdf/7302x0377.pdf
-  |journal=[[Chemistry:Pure and Applied Chemistry|Pure and Applied Chemistry]]
+  |journal=Pure and Applied Chemistry
   |volume=73
   |issue=2
@@ Line 145: / Line 149: @@
 ===Dihydrogen===
-Under standard conditions, hydrogen is a [[Physics:Gas|gas]] of [[Chemistry:Diatomic molecule|diatomic molecule]]s with the [[Chemistry:Chemical formula|formula]]{{nbsp}}{{chem2|H2}}, officially called "dihydrogen",<ref>[http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005] - Full text (PDF)<br />2004 version with separate chapters as pdf: [http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (2004)] {{webarchive|url=https://web.archive.org/web/20080219122415/http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html |date=2008-02-19 }}</ref>{{rp|308}} but also called "molecular hydrogen",<ref name="britannica">{{Cite encyclopedia|title=Hydrogen|url=https://www.britannica.com/science/hydrogen|url-status=live|access-date=25 December 2021|encyclopedia=[[Encyclopædia Britannica]]|archive-date=24 December 2021|archive-url=https://web.archive.org/web/20211224165150/https://www.britannica.com/science/hydrogen}}</ref> or simply hydrogen. Dihydrogen is a colorless, odorless, flammable gas.<ref name="britannica"/>
+Under standard conditions, hydrogen is a gas of diatomic molecules with the formula{{nbsp}}{{chem2|H2}}, officially called "dihydrogen",<ref>[http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005] - Full text (PDF)<br />2004 version with separate chapters as pdf: [http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (2004)] {{webarchive|url=https://web.archive.org/web/20080219122415/http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html |date=2008-02-19 }}</ref>{{rp|308}} but also called "molecular hydrogen",<ref name="britannica">{{Cite encyclopedia|title=Hydrogen|url=https://www.britannica.com/science/hydrogen|url-status=live|access-date=25 December 2021|encyclopedia=Encyclopædia Britannica|archive-date=24 December 2021|archive-url=https://web.archive.org/web/20211224165150/https://www.britannica.com/science/hydrogen}}</ref> or simply hydrogen. Dihydrogen is a colorless, odorless, flammable gas.<ref name="britannica"/>
 ==== Combustion ====
 [[File:19. Експлозија на смеса од водород и воздух.webm|thumb|Combustion of hydrogen with the oxygen in the air. When the bottom cap is removed, allowing air to enter, hydrogen in the container rises and burns as it mixes with the air.]]
-Hydrogen gas is highly flammable, reacting with [[Chemistry:Oxygen|oxygen]] in air to produce liquid water:
+Hydrogen gas is highly flammable, reacting with oxygen in air to produce liquid water:
 {{bi|{{chem2|2 H2(g) + O2(g) → 2 H2O(l)}}}}
-The amount of heat released per [[Mole (unit)|mole]] of hydrogen is {{val|−286|u=kilojoules}}{{nbsp}}(kJ), or {{val|141.9|u=megajoules}}{{nbsp}}(MJ) for a {{convert|1|kilogram|adj=on|spell=in|abbr=out}} mass (based on the higher heating value measurement).<ref>{{cite book
+The amount of heat released per mole of hydrogen is {{val|−286|u=kilojoules}}{{nbsp}}(kJ), or {{val|141.9|u=megajoules}}{{nbsp}}(MJ) for a {{convert|1|kilogram|adj=on|spell=in|abbr=out}} mass (based on the higher heating value measurement).<ref>{{cite book
 |author=Committee on Alternatives and Strategies for Future Hydrogen Production and Use
 |date=2004
@@ Line 172: / Line 176: @@
 |doi=10.1016/j.energy.2004.02.012
 |bibcode=2005Ene....30.1439C
-}}</ref> and with chlorine at {{val|5|–|95|u=%}}. The hydrogen [[Chemistry:Autoignition temperature|autoignition temperature]], the temperature of spontaneous ignition in air, is {{convert|500|C|F}}.<ref>{{cite book
+}}</ref> and with chlorine at {{val|5|–|95|u=%}}. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is {{convert|500|C|F}}.<ref>{{cite book
 |url=https://books.google.com/books?id=-CRRJBVv5d0C&pg=PA402
 |page=402
@@ Line 187: / Line 191: @@
 }}</ref> In a high-pressure hydrogen leak, the shock wave from the leak itself can heat air to the autoignition temperature, leading to flaming and possibly explosion.<ref>{{Cite journal |last1=Yamada |first1=Eisuke |last2=Kitabayashi |first2=Naoki |last3=Hayashi |first3=A. Koichi |last4=Tsuboi |first4=Nobuyuki |date=2011-02-01 |title=Mechanism of high-pressure hydrogen auto-ignition when spouting into air |url=https://linkinghub.elsevier.com/retrieve/pii/S0360319910009468 |journal=International Journal of Hydrogen Energy |series=The Third Annual International Conference on Hydrogen Safety |volume=36 |issue=3 |pages=2560–2566 |doi=10.1016/j.ijhydene.2010.05.011 |bibcode=2011IJHE...36.2560Y |issn=0360-3199|url-access=subscription }}</ref>
-Hydrogen flames emit faint blue and [[Physics:Ultraviolet|ultraviolet]] light.<ref>{{cite journal |last1=Schefer |first1=E. W. |last2=Kulatilaka |first2=W. D. |last3=Patterson |first3=B. D. |last4=Settersten |first4=T. B. |date=June 2009 |title=Visible emission of hydrogen flames |url=https://zenodo.org/record/1258847 |url-status=live |journal=Combustion and Flame |volume=156 |issue=6 |pages=1234–1241 |bibcode=2009CoFl..156.1234S |doi=10.1016/j.combustflame.2009.01.011 |archive-url=https://web.archive.org/web/20210129015717/https://zenodo.org/record/1258847 |archive-date=29 January 2021 |access-date=30 June 2019}}</ref> [[Engineering:Flame detector|Flame detector]]s are used to detect hydrogen fires as they are nearly invisible to the naked eye in daylight.<ref>{{Cite web |title=Making Visible the Invisible {{!}} NASA Spinoff |url=https://spinoff.nasa.gov/spinoff1999/er5.htm |access-date=2025-02-09 |website=spinoff.nasa.gov}}</ref><ref name="spinoff-2016" />
+Hydrogen flames emit faint blue and ultraviolet light.<ref>{{cite journal |last1=Schefer |first1=E. W. |last2=Kulatilaka |first2=W. D. |last3=Patterson |first3=B. D. |last4=Settersten |first4=T. B. |date=June 2009 |title=Visible emission of hydrogen flames |url=https://zenodo.org/record/1258847 |url-status=live |journal=Combustion and Flame |volume=156 |issue=6 |pages=1234–1241 |bibcode=2009CoFl..156.1234S |doi=10.1016/j.combustflame.2009.01.011 |archive-url=https://web.archive.org/web/20210129015717/https://zenodo.org/record/1258847 |archive-date=29 January 2021 |access-date=30 June 2019}}</ref> Flame detectors are used to detect hydrogen fires as they are nearly invisible to the naked eye in daylight.<ref>{{Cite web |title=Making Visible the Invisible {{!}} NASA Spinoff |url=https://spinoff.nasa.gov/spinoff1999/er5.htm |access-date=2025-02-09 |website=spinoff.nasa.gov}}</ref><ref name="spinoff-2016" />
 ==== Spin isomers ====
 {{Main|Physics:Spin isomers of hydrogen}}
-Molecular {{chem2|H2}} exists as two [[Physics:Nuclear isomer|nuclear isomer]]s that differ in the [[Spin|spin states]] of their nuclei.<ref name="uigi">{{cite web|author=Staff|date=2003|url=http://www.uigi.com/hydrogen.html|title=Hydrogen (H<sub>2</sub>) Properties, Uses, Applications: Hydrogen Gas and Liquid Hydrogen|publisher=Universal Industrial Gases, Inc.|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080219073329/http://www.uigi.com/hydrogen.html|archive-date=19 February 2008|url-status=live}}</ref> In the '''{{shy|ortho|hydrogen}}''' form, the spins of the two nuclei are parallel, forming a spin [[Physics:Triplet state|triplet state]] having a total molecular spin <math>S = 1</math>; in the '''{{shy|para|hydrogen}}''' form the spins are {{shy|anti|parallel}} and form a spin [[Physics:Singlet state|singlet state]] having spin <math>S = 0</math>. The equilibrium ratio of ortho- to para-hydrogen depends on temperature. At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form.<ref name="Green2012">{{cite journal |last1=Green |first1=Richard A. |display-authors=0 |title=The theory and practice of hyperpolarization in magnetic resonance using ''para''hydrogen |journal=Prog. Nucl. Magn. Reson. Spectrosc. |date=2012 |volume=67 |pages=1–48 |doi=10.1016/j.pnmrs.2012.03.001 |pmid=23101588 |bibcode=2012PNMRS..67....1G |url=https://www.sciencedirect.com/science/article/abs/pii/S0079656512000477 |access-date=28 August 2021 |archive-date=28 August 2021 |archive-url=https://web.archive.org/web/20210828222611/https://www.sciencedirect.com/science/article/abs/pii/S0079656512000477 |url-status=live |url-access=subscription }}</ref> The ortho form is an [[Physics:Excited state|excited state]], having higher energy than the para form by {{val|1.455|ul=kJ/mol}},<ref name="PlanckInstitut">{{cite web |url=https://www.mpibpc.mpg.de/146336/para-Wasserstoff |language=de |website=Max-Planck-Institut für Biophysikalische Chemie |title=Die Entdeckung des para-Wasserstoffs (The discovery of para-hydrogen) |access-date=9 November 2020 |archive-date=16 November 2020 |archive-url=https://web.archive.org/web/20201116064055/https://www.mpibpc.mpg.de/146336/para-Wasserstoff |url-status=live }}</ref> and it converts to the para form over the course of several minutes when cooled to low temperature.<ref>{{cite journal|last1=Milenko|first1=Yu. Ya.|last2=Sibileva|first2=R. M.|last3=Strzhemechny|first3=M. A.|title=Natural ortho-para conversion rate in liquid and gaseous hydrogen|journal=Journal of Low Temperature Physics|date=1997|volume=107|issue=1–2|pages=77–92
+Molecular {{chem2|H2}} exists as two nuclear isomers that differ in the spin states of their nuclei.<ref name="uigi">{{cite web|author=Staff|date=2003|url=http://www.uigi.com/hydrogen.html|title=Hydrogen (H<sub>2</sub>) Properties, Uses, Applications: Hydrogen Gas and Liquid Hydrogen|publisher=Universal Industrial Gases, Inc.|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080219073329/http://www.uigi.com/hydrogen.html|archive-date=19 February 2008|url-status=live}}</ref> In the '''''' form, the spins of the two nuclei are parallel, forming a spin triplet state having a total molecular spin <math>S = 1</math>; in the '''''' form the spins are  and form a spin singlet state having spin <math>S = 0</math>. The equilibrium ratio of ortho- to para-hydrogen depends on temperature. At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form.<ref name="Green2012">{{cite journal |last1=Green |first1=Richard A. |display-authors=0 |title=The theory and practice of hyperpolarization in magnetic resonance using ''para''hydrogen |journal=Prog. Nucl. Magn. Reson. Spectrosc. |date=2012 |volume=67 |pages=1–48 |doi=10.1016/j.pnmrs.2012.03.001 |pmid=23101588 |bibcode=2012PNMRS..67....1G |url=https://www.sciencedirect.com/science/article/abs/pii/S0079656512000477 |access-date=28 August 2021 |archive-date=28 August 2021 |archive-url=https://web.archive.org/web/20210828222611/https://www.sciencedirect.com/science/article/abs/pii/S0079656512000477 |url-status=live |url-access=subscription }}</ref> The ortho form is an excited state, having higher energy than the para form by {{val|1.455|ul=kJ/mol}},<ref name="PlanckInstitut">{{cite web |url=https://www.mpibpc.mpg.de/146336/para-Wasserstoff |language=de |website=Max-Planck-Institut für Biophysikalische Chemie |title=Die Entdeckung des para-Wasserstoffs (The discovery of para-hydrogen) |access-date=9 November 2020 |archive-date=16 November 2020 |archive-url=https://web.archive.org/web/20201116064055/https://www.mpibpc.mpg.de/146336/para-Wasserstoff |url-status=live }}</ref> and it converts to the para form over the course of several minutes when cooled to low temperature.<ref>{{cite journal|last1=Milenko|first1=Yu. Ya.|last2=Sibileva|first2=R. M.|last3=Strzhemechny|first3=M. A.|title=Natural ortho-para conversion rate in liquid and gaseous hydrogen|journal=Journal of Low Temperature Physics|date=1997|volume=107|issue=1–2|pages=77–92
-|doi=10.1007/BF02396837|bibcode = 1997JLTP..107...77M |s2cid=120832814}}</ref> The thermal properties of these isomers differ because each has distinct [[Physics:Rotational–vibrational spectroscopy|rotational quantum states]].<!-- This link is less direct than [[Physics:Rotational spectroscopy|Rotational spectroscopy]] but presently the subject better (June 2021).--><ref name="NASA">{{cite web|last=Hritz|first=J.|date=March 2006|url=http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|title=CH. 6&nbsp;– Hydrogen|work=NASA Glenn Research Center Glenn Safety Manual, Document GRC-MQSA.001|publisher=NASA|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080216050326/http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|archive-date=16 February 2008}}</ref>
+|doi=10.1007/BF02396837|bibcode = 1997JLTP..107...77M |s2cid=120832814}}</ref> The thermal properties of these isomers differ because each has distinct rotational quantum states.<!-- This link is less direct than Rotational spectroscopy but presently the subject better (June 2021).--><ref name="NASA">{{cite web|last=Hritz|first=J.|date=March 2006|url=http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|title=CH. 6&nbsp;– Hydrogen|work=NASA Glenn Research Center Glenn Safety Manual, Document GRC-MQSA.001|publisher=NASA|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080216050326/http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|archive-date=16 February 2008}}</ref>
-The ortho-to-para ratio in {{chem2|H2}} is an important consideration in the [[Physics:Liquefaction|liquefaction]] and storage of [[Chemistry:Liquid hydrogen|liquid hydrogen]]: the conversion from ortho to para is exothermic, and produces sufficient heat to evaporate most of the liquid if the conversion to {{shy|para|hydrogen}} does not occur during the cooling process.<ref name="Amos98">{{cite web|url=http://www.nrel.gov/docs/fy99osti/25106.pdf|title=Costs of Storing and Transporting Hydrogen|publisher=National Renewable Energy Laboratory|date=1 November 1998|first1=Wade A.|last1=Amos|pages=6–9|access-date=19 May 2015|archive-url=https://web.archive.org/web/20141226131234/http://www.nrel.gov/docs/fy99osti/25106.pdf|archive-date=26 December 2014|url-status=live}}</ref> [[Software:Catalyst|Catalyst]]s for the ortho-para {{shy|inter|conversion}}, such as ferric oxide and [[Physics:Activated carbon|activated carbon]] compounds, are therefore used during hydrogen cooling to avoid this loss of liquid.<ref name="Svadlenak">{{cite journal|last1=Svadlenak|first1=R. E.|last2=Scott|first2=A. B.|title=The Conversion of Ortho- to Parahydrogen on Iron Oxide-Zinc Oxide Catalysts|journal=Journal of the American Chemical Society|date=1957|volume=79|issue=20|pages=5385–5388|doi=10.1021/ja01577a013|bibcode=1957JAChS..79.5385S }}</ref>
+The ortho-to-para ratio in {{chem2|H2}} is an important consideration in the liquefaction and storage of liquid hydrogen: the conversion from ortho to para is exothermic, and produces sufficient heat to evaporate most of the liquid if the conversion to  does not occur during the cooling process.<ref name="Amos98">{{cite web|url=http://www.nrel.gov/docs/fy99osti/25106.pdf|title=Costs of Storing and Transporting Hydrogen|publisher=National Renewable Energy Laboratory|date=1 November 1998|first1=Wade A.|last1=Amos|pages=6–9|access-date=19 May 2015|archive-url=https://web.archive.org/web/20141226131234/http://www.nrel.gov/docs/fy99osti/25106.pdf|archive-date=26 December 2014|url-status=live}}</ref> Catalysts for the ortho-para , such as ferric oxide and activated carbon compounds, are therefore used during hydrogen cooling to avoid this loss of liquid.<ref name="Svadlenak">{{cite journal|last1=Svadlenak|first1=R. E.|last2=Scott|first2=A. B.|title=The Conversion of Ortho- to Parahydrogen on Iron Oxide-Zinc Oxide Catalysts|journal=Journal of the American Chemical Society|date=1957|volume=79|issue=20|pages=5385–5388|doi=10.1021/ja01577a013|bibcode=1957JAChS..79.5385S }}</ref>
 ==== Phases ====
-[[File:Phase diagram of hydrogen.png|thumb|[[Phase diagram]] of hydrogen with a [[Logarithmic scale|logarithmic scale]]. The left edge corresponds to about one [[Physics:Atmosphere (unit)|atmosphere]].<ref>{{Cite journal |last=Stevenson |first=D J |date=May 1982 |title=Interiors of the Giant Planets |url=https://www.annualreviews.org/doi/10.1146/annurev.ea.10.050182.001353 |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=10 |issue=1 |pages=257–295 |doi=10.1146/annurev.ea.10.050182.001353 |bibcode=1982AREPS..10..257S |issn=0084-6597|url-access=subscription }}</ref>|alt=Phase diagram of hydrogen on logarithmic scales. Lines show boundaries between phases, with the end of the liquid-gas line indicating the critical point. The triple point of hydrogen is just off-scale to the left.]]
+[[File:Phase diagram of hydrogen.png|thumb|Phase diagram of hydrogen with a logarithmic scale. The left edge corresponds to about one atmosphere.<ref>{{Cite journal |last=Stevenson |first=D J |date=May 1982 |title=Interiors of the Giant Planets |url=https://www.annualreviews.org/doi/10.1146/annurev.ea.10.050182.001353 |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=10 |issue=1 |pages=257–295 |doi=10.1146/annurev.ea.10.050182.001353 |bibcode=1982AREPS..10..257S |issn=0084-6597|url-access=subscription }}</ref>|alt=Phase diagram of hydrogen on logarithmic scales. Lines show boundaries between phases, with the end of the liquid-gas line indicating the critical point. The triple point of hydrogen is just off-scale to the left.]]
-[[Chemistry:Liquid hydrogen|Liquid hydrogen]] can exist at temperatures below hydrogen's [[Critical point (thermodynamics)|critical point]] of {{convert|33|K|C F|abbr=out|lk=in}}.<ref>{{cite web|url=https://webbook.nist.gov/cgi/cbook.cgi?ID=C1333740&Mask=4 |title=Hydrogen |website=NIST Chemistry WebBook, SRD 69 |publisher=[[National Institute of Standards and Technology]] |access-date=2025-01-14 |year=2023}}</ref> However, for it to be in a fully liquid state at [[Physics:Atmospheric pressure|atmospheric pressure]], H<sub>2</sub> needs to be cooled to {{cvt|20.28|K|C F}}. Hydrogen was liquefied by James Dewar in{{nbsp}}1898 by using [[Physics:Regenerative cooling|regenerative cooling]] and his invention, the [[Physics:Vacuum flask|vacuum flask]].<ref>{{cite journal |author1=James Dewar |title=Liquid Hydrogen |journal=Science |date=1900 |volume=11 |issue=278 |pages=641–651 |doi=10.1126/science.11.278.641 |pmid=17813562 |bibcode=1900Sci....11..641D |language=en}}</ref>
+Liquid hydrogen can exist at temperatures below hydrogen's critical point of {{convert|33|K|C F|abbr=out|lk=in}}.<ref>{{cite web|url=https://webbook.nist.gov/cgi/cbook.cgi?ID=C1333740&Mask=4 |title=Hydrogen |website=NIST Chemistry WebBook, SRD 69 |publisher=National Institute of Standards and Technology |access-date=2025-01-14 |year=2023}}</ref> However, for it to be in a fully liquid state at atmospheric pressure, H<sub>2</sub> needs to be cooled to {{cvt|20.28|K|C F}}. Hydrogen was liquefied by James Dewar in{{nbsp}}1898 by using regenerative cooling and his invention, the vacuum flask.<ref>{{cite journal |author1=James Dewar |title=Liquid Hydrogen |journal=Science |date=1900 |volume=11 |issue=278 |pages=641–651 |doi=10.1126/science.11.278.641 |pmid=17813562 |bibcode=1900Sci....11..641D |language=en}}</ref>
-Liquid hydrogen becomes [[Physics:Solid hydrogen|solid hydrogen]] at standard pressure below hydrogen's [[Physics:Melting point|melting point]] of {{cvt|14.01|K|C F}}. Distinct solid phases exist, known as Phase{{nbsp}}I through Phase{{nbsp}}V, each exhibiting a characteristic molecular arrangement.<ref name="Helled2020">{{cite journal|first1=Ravit |last1=Helled |first2=Guglielmo |last2=Mazzola |first3=Ronald |last3=Redmer |title=Understanding dense hydrogen at planetary conditions |date=2020-09-01 |journal=Nature Reviews Physics |volume=2 |issue=10 |pages=562–574 |doi=10.1038/s42254-020-0223-3 |arxiv=2006.12219|bibcode=2020NatRP...2..562H }}</ref> Liquid and solid phases can exist in combination at the [[Chemistry:Triple point|triple point]]; this mixture is known as [[Physics:Slush hydrogen|slush hydrogen]].<ref>{{cite book |last=Ohira |first=K. |chapter=Slush hydrogen production, storage, and transportation |date=2016 |title=Compendium of Hydrogen Energy |pages=53–90 |publisher=Elsevier |doi=10.1016/b978-1-78242-362-1.00003-1 |isbn=978-1-78242-362-1}}</ref>
+Liquid hydrogen becomes solid hydrogen at standard pressure below hydrogen's melting point of {{cvt|14.01|K|C F}}. Distinct solid phases exist, known as Phase{{nbsp}}I through Phase{{nbsp}}V, each exhibiting a characteristic molecular arrangement.<ref name="Helled2020">{{cite journal|first1=Ravit |last1=Helled |first2=Guglielmo |last2=Mazzola |first3=Ronald |last3=Redmer |title=Understanding dense hydrogen at planetary conditions |date=2020-09-01 |journal=Nature Reviews Physics |volume=2 |issue=10 |pages=562–574 |doi=10.1038/s42254-020-0223-3 |arxiv=2006.12219|bibcode=2020NatRP...2..562H }}</ref> Liquid and solid phases can exist in combination at the triple point; this mixture is known as slush hydrogen.<ref>{{cite book |last=Ohira |first=K. |chapter=Slush hydrogen production, storage, and transportation |date=2016 |title=Compendium of Hydrogen Energy |pages=53–90 |publisher=Elsevier |doi=10.1016/b978-1-78242-362-1.00003-1 |isbn=978-1-78242-362-1}}</ref>
-[[Physics:Metallic hydrogen|Metallic hydrogen]], a phase obtained at extremely high pressures (in excess of 400 billion), is an electrical conductor. It is believed to exist deep within [[Astronomy:Giant planet|giant planet]]s like [[Astronomy:Jupiter|Jupiter]].<ref name="Helled2020"/><ref>{{cite book|last1=Frankoi |first1=A. |display-authors=0 |title=Astronomy 2e |year=2022 |publisher=OpenStax |chapter-url=https://openstax.org/books/astronomy-2e/pages/11-2-the-giant-planets |chapter=11.2 The Giant Planets |page=370 |isbn=978-1-951693-50-3}}</ref>
+Metallic hydrogen, a phase obtained at extremely high pressures (in excess of 400 billion), is an electrical conductor. It is believed to exist deep within giant planets like Jupiter.<ref name="Helled2020"/><ref>{{cite book|last1=Frankoi |first1=A. |display-authors=0 |title=Astronomy 2e |year=2022 |publisher=OpenStax |chapter-url=https://openstax.org/books/astronomy-2e/pages/11-2-the-giant-planets |chapter=11.2 The Giant Planets |page=370 |isbn=978-1-951693-50-3}}</ref>
-When ionized, hydrogen becomes a [[Software:Plasma|plasma]]. This is the form in which hydrogen exists within [[Astronomy:Star|star]]s.<ref>{{Cite book|last=Phillips |first=K. J. H. |date=1995 |title=Guide to the Sun |page=|publisher=Cambridge University Press |url=https://books.google.com/books?id=idwBChjVP0gC&q=Guide+to+the+Sun+phillips |isbn=978-0-521-39788-9 |url-status=live |archive-url= https://web.archive.org/web/20180115215631/https://books.google.com/books?id=idwBChjVP0gC&printsec=frontcover&dq=Guide+to+the+Sun+phillips&hl=en&sa=X&ved=0ahUKEwiBj4Gbj5bXAhXrrVQKHfnAAKUQ6AEIKDAA |archive-date=15 January 2018 }}</ref>
+When ionized, hydrogen becomes a plasma. This is the form in which hydrogen exists within stars.<ref>{{Cite book|last=Phillips |first=K. J. H. |date=1995 |title=Guide to the Sun |page=|publisher=Cambridge University Press |url=https://books.google.com/books?id=idwBChjVP0gC&q=Guide+to+the+Sun+phillips |isbn=978-0-521-39788-9 |url-status=live |archive-url= https://web.archive.org/web/20180115215631/https://books.google.com/books?id=idwBChjVP0gC&printsec=frontcover&dq=Guide+to+the+Sun+phillips&hl=en&sa=X&ved=0ahUKEwiBj4Gbj5bXAhXrrVQKHfnAAKUQ6AEIKDAA |archive-date=15 January 2018 }}</ref>
 {{clear}}
@@ Line 214: / Line 218: @@
 !Temperature (K)
 !Density (kg/m<sup>3</sup>)
-![[Physics:Specific heat|Specific heat]] (kJ/kg K)
+!Specific heat (kJ/kg K)
-![[Physics:Dynamic viscosity|Dynamic viscosity]] (kg/m s)
+!Dynamic viscosity (kg/m s)
-![[Physics:Kinematic viscosity|Kinematic viscosity]] (m<sup>2</sup>/s)
+!Kinematic viscosity (m<sup>2</sup>/s)
-![[Physics:Thermal conductivity|Thermal conductivity]] (W/m K)
+!Thermal conductivity (W/m K)
-![[Physics:Thermal diffusivity|Thermal diffusivity]] (m<sup>2</sup>/s)
+!Thermal diffusivity (m<sup>2</sup>/s)
 !Prandtl Number
 |-
@@ Line 451: / Line 455: @@
 === 18th century ===
-[[File:Portret van Robert Boyle, RP-P-OB-4578 (cropped).jpg|thumb|[[Biography:Robert Boyle|Robert Boyle]], who discovered the reaction between [[Chemistry:Iron filings|iron filings]] and dilute acids]]
+[[File:Portret van Robert Boyle, RP-P-OB-4578 (cropped).jpg|thumb|Robert Boyle, who discovered the reaction between iron filings and dilute acids]]
-In 1671, Irish scientist [[Biography:Robert Boyle|Robert Boyle]] discovered and described the reaction between [[Chemistry:Iron|iron]] filings and dilute [[Chemistry:Acid|acid]]s, which results in the production of hydrogen gas.<ref>{{Cite book |last=Boyle |first=R. |url=https://quod.lib.umich.edu/e/eebo2/A29057.0001.001?rgn=main;view=fulltext |title=Tracts written by the Honourable Robert Boyle containing new experiments, touching the relation betwixt flame and air, and about explosions, an hydrostatical discourse occasion'd by some objections of Dr. Henry More against some explications of new experiments made by the author of these tracts: To which is annex't, an hydrostatical letter, dilucidating an experiment about a way of weighing water in water, new experiments, of the positive or relative levity of bodies under water, of the air's spring on bodies under water, about the differing pressure of heavy solids and fluids |publisher=Printed for Richard Davis |year=1672 |pages=64–65}}</ref><ref>{{cite web
+In 1671, Irish scientist Robert Boyle discovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas.<ref>{{Cite book |last=Boyle |first=R. |url=https://quod.lib.umich.edu/e/eebo2/A29057.0001.001?rgn=main;view=fulltext |title=Tracts written by the Honourable Robert Boyle containing new experiments, touching the relation betwixt flame and air, and about explosions, an hydrostatical discourse occasion'd by some objections of Dr. Henry More against some explications of new experiments made by the author of these tracts: To which is annex't, an hydrostatical letter, dilucidating an experiment about a way of weighing water in water, new experiments, of the positive or relative levity of bodies under water, of the air's spring on bodies under water, about the differing pressure of heavy solids and fluids |publisher=Printed for Richard Davis |year=1672 |pages=64–65}}</ref><ref>{{cite web
 |first=M.
 |last=Winter
@@ Line 463: / Line 467: @@
 |archive-date=10 April 2008
 }}</ref>
-Boyle did not note that the gas was flammable, but hydrogen would play a key role in overturning the [[Physics:Phlogiston theory|phlogiston theory]] of combustion.<ref name=Ramsay-1896>{{Cite book |last=Ramsay |first=W. |url=https://www.gutenberg.org/files/52778/52778-h/52778-h.htm |title=The gases of the atmosphere: The history of their discovery |publisher=Macmillan |year=1896 |page=19}}</ref>
+Boyle did not note that the gas was flammable, but hydrogen would play a key role in overturning the phlogiston theory of combustion.<ref name=Ramsay-1896>{{Cite book |last=Ramsay |first=W. |url=https://www.gutenberg.org/files/52778/52778-h/52778-h.htm |title=The gases of the atmosphere: The history of their discovery |publisher=Macmillan |year=1896 |page=19}}</ref>
-In 1766, [[Biography:Henry Cavendish|Henry Cavendish]] was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "inflammable air". He speculated that "inflammable air" was in fact identical to the hypothetical substance "[[Physics:Phlogiston theory|phlogiston]]"<ref>{{cite book |last = Musgrave
+In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "inflammable air". He speculated that "inflammable air" was in fact identical to the hypothetical substance "phlogiston"<ref>{{cite book |last = Musgrave
   |first = A.
   |chapter = Why did oxygen supplant phlogiston? Research programmes in the Chemical Revolution
@@ Line 481: / Line 485: @@
 }}</ref><ref name="cav766">{{cite journal|last1=Cavendish|first1=Henry|title=Three Papers, Containing Experiments on Factitious Air, by the Hon. Henry Cavendish, F. R. S.|journal=Philosophical Transactions|date=12 May 1766|volume=56|pages=141–184|jstor=105491|bibcode=1766RSPT...56..141C|doi=10.1098/rstl.1766.0019|doi-access=free}}</ref> and further finding in{{nbsp}}1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.<ref name="Nostrand">{{cite encyclopedia| title=Hydrogen| encyclopedia=Van Nostrand's Encyclopedia of Chemistry| pages=797–799| publisher=Wylie-Interscience| year=2005| isbn=978-0-471-61525-5}}</ref><ref name="nbb">{{cite book| last=Emsley| first=John| title=Nature's Building Blocks| publisher=Oxford University Press| year=2001| location=Oxford| pages=183–191| isbn=978-0-19-850341-5}}</ref>
-[[File:Antoine-Laurent Lavoisier by Louis Jean Desire Delaistre (cropped).jpg|thumb|{{langr|fr|[[Biography:Antoine Lavoisier|Antoine Lavoisier]]}}, who identified the element that came to be known as hydrogen]]
+[[File:Antoine-Laurent Lavoisier by Louis Jean Desire Delaistre (cropped).jpg|thumb|, who identified the element that came to be known as hydrogen]]
-In 1783, {{langr|fr|[[Biography:Antoine Lavoisier|Antoine Lavoisier]]}} identified the element that came to be known as hydrogen<ref>{{cite book| last=Stwertka| first=Albert| title=A Guide to the Elements| url=https://archive.org/details/guidetoelements00stwe| url-access=registration| publisher=Oxford University Press| year=1996| pages=[https://archive.org/details/guidetoelements00stwe/page/16 16–21]| isbn=978-0-19-508083-4}}</ref> when he and [[Pierre-Simon Laplace|{{langr|fr|Laplace|cat=no}}]] reproduced Cavendish's finding that water is produced when hydrogen is burned.<ref name="nbb" /> {{langr|fr|Lavoisier}} produced hydrogen for his experiments on mass conservation by treating metallic [[Chemistry:Iron|iron]] with a stream of water through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:
+In 1783,  identified the element that came to be known as hydrogen<ref>{{cite book| last=Stwertka| first=Albert| title=A Guide to the Elements| url=https://archive.org/details/guidetoelements00stwe| url-access=registration| publisher=Oxford University Press| year=1996| pages=[https://archive.org/details/guidetoelements00stwe/page/16 16–21]| isbn=978-0-19-508083-4}}</ref> when he and Pierre-Simon Laplace reproduced Cavendish's finding that water is produced when hydrogen is burned.<ref name="nbb" />  produced hydrogen for his experiments on mass conservation by treating metallic iron with a stream of water through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:
 *{{chem2|Fe + H2O -> FeO + H2}}
 *{{chem2|2 Fe + 3 H2O -> Fe2O3 + 3 H2}}
@@ Line 491: / Line 495: @@
 ===19th century===
 By 1806 hydrogen was used to fill balloons.<ref>{{Cite journal |last=Szydło |first=Z. A. |date=2020 |title=Hydrogen - Some Historical Highlights |journal=Chemistry-Didactics-Ecology-Metrology |volume=25 |issue=1–2 |pages=5–34|doi=10.2478/cdem-2020-0001 |s2cid=231776282 |doi-access=free }}</ref>
-{{langr|fr|François Isaac de Rivaz}} built the first [[de Rivaz engine|{{langr|fr|de{{nbsp}}Rivaz|cat=no}} engine]], an internal combustion engine powered by a mixture of hydrogen and oxygen, in{{nbsp}}1806. Edward Daniel Clarke invented the hydrogen gas blowpipe in{{nbsp}}1819. The [[Döbereiner's lamp|{{langr|de|Döbereiner|cat=no}}'s lamp]] and [[Physics:Limelight|limelight]] were invented in{{nbsp}}1823. Hydrogen was [[Chemistry:Liquid hydrogen|liquefied]] for the first time by James Dewar in{{nbsp}}1898 by using [[Physics:Regenerative cooling|regenerative cooling]] and his invention, the [[Physics:Vacuum flask|vacuum flask]]. He produced [[Physics:Solid hydrogen|solid hydrogen]] the next year.<ref name="nbb" />
+ built the first {{langr|fr|de{{nbsp}}Rivaz|cat=no}} engine, an internal combustion engine powered by a mixture of hydrogen and oxygen, in{{nbsp}}1806. Edward Daniel Clarke invented the hydrogen gas blowpipe in{{nbsp}}1819. The 's lamp and limelight were invented in{{nbsp}}1823. Hydrogen was liquefied for the first time by James Dewar in{{nbsp}}1898 by using regenerative cooling and his invention, the vacuum flask. He produced solid hydrogen the next year.<ref name="nbb" />
-One of the first quantum effects to be explicitly noticed, although not understood at the time, was [[Biography:James Clerk Maxwell|James Clerk Maxwell]]'s observation that the [[Physics:Specific heat capacity|specific heat capacity]] of {{chem2|H2}} unaccountably departs from that of a diatomic gas below room temperature, and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the ([[Physics:Quantization|quantized]]) rotational energy levels, which are particularly wide-spaced in {{chem2|H2}} because of its low mass. These widely-spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.<ref name="Berman">{{cite journal
+One of the first quantum effects to be explicitly noticed, although not understood at the time, was James Clerk Maxwell's observation that the specific heat capacity of {{chem2|H2}} unaccountably departs from that of a diatomic gas below room temperature, and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in {{chem2|H2}} because of its low mass. These widely-spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.<ref name="Berman">{{cite journal
 |last1=Berman|first1=R.|last2=Cooke|first2=A. H.|last3=Hill|first3=R. W.
 |title=Cryogenics|journal=Annual Review of Physical Chemistry
@@ Line 500: / Line 504: @@
 ===20th century===
-The existence of the hydride anion was suggested by [[Biography:Gilbert N. Lewis|Gilbert{{nbsp}}N. Lewis]] in{{nbsp}}1916 for group{{nbsp}}1 and group{{nbsp}}2 salt-like compounds. In{{nbsp}}1920, {{langr|de|Moers}} electrolyzed molten [[Chemistry:Lithium hydride|lithium hydride]]{{nbsp}}(LiH), producing a stoichiometric quantity of hydrogen at the [[Physics:Anode|anode]].<ref name="Moers">{{cite journal|last=Moers|first=K.|title=Investigations on the Salt Character of Lithium Hydride|journal=Zeitschrift für Anorganische und Allgemeine Chemie|date=1920|volume=113|issue=191|pages=179–228|doi=10.1002/zaac.19201130116|url=https://zenodo.org/record/1428170|access-date=24 August 2019|archive-url=https://web.archive.org/web/20190824162148/https://zenodo.org/record/1428170/files/article.pdf|archive-date=24 August 2019|url-status=live}}</ref>
+The existence of the hydride anion was suggested by Gilbert{{nbsp}}N. Lewis in{{nbsp}}1916 for group{{nbsp}}1 and group{{nbsp}}2 salt-like compounds. In{{nbsp}}1920,  electrolyzed molten lithium hydride{{nbsp}}(LiH), producing a stoichiometric quantity of hydrogen at the anode.<ref name="Moers">{{cite journal|last=Moers|first=K.|title=Investigations on the Salt Character of Lithium Hydride|journal=Zeitschrift für Anorganische und Allgemeine Chemie|date=1920|volume=113|issue=191|pages=179–228|doi=10.1002/zaac.19201130116|url=https://zenodo.org/record/1428170|access-date=24 August 2019|archive-url=https://web.archive.org/web/20190824162148/https://zenodo.org/record/1428170/files/article.pdf|archive-date=24 August 2019|url-status=live}}</ref>
-[[File:Emission_spectrum-H_labeled.svg|thumb|Hydrogen emission spectrum lines in the four visible lines of the [[Balmer series]]|alt=A line spectrum showing black background with narrow lines superimposed on it: one violet, one blue, one cyan, and one red.]]
+[[File:Emission_spectrum-H_labeled.svg|thumb|Hydrogen emission spectrum lines in the four visible lines of the Balmer series|alt=A line spectrum showing black background with narrow lines superimposed on it: one violet, one blue, one cyan, and one red.]]
-Because of its simple atomic structure, consisting only of a proton and an electron, the [[Physics:Hydrogen atom|hydrogen atom]], together with the [[Physics:Hydrogen spectral series|spectrum of light]] produced from it or absorbed by it, has been central to the [[Physics:History of atomic theory|development of the theory of atomic structure]].<ref>{{cite book |last=Crepeau |first=R.
+Because of its simple atomic structure, consisting only of a proton and an electron, the [[Physics:Quantum atoms/hydrogen|hydrogen atom]], together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of atomic structure.<ref>{{cite book |last=Crepeau |first=R.
 |title=Niels Bohr: The Atomic Model |series=Great Scientific Minds
 |date=1 January 2006 |isbn=978-1-4298-0723-4
-}}</ref> The energy levels of hydrogen can be calculated fairly accurately using the [[Bohr model|{{langr|da|Bohr|cat=no}} model]] of the atom, in which the electron "orbits" the proton, just as Earth orbits the Sun. However, the electron and proton are held together by electrostatic attraction, while planets and celestial objects are held by [[Company:Gravity|gravity]]. Due to the discretization of [[Physics:Angular momentum|angular momentum]] postulated in early [[Physics:Quantum mechanics|quantum mechanics]] by {{langr|da|Bohr}}, the electron in the {{langr|da|Bohr}} model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.<ref>{{cite web
+}}</ref> The energy levels of hydrogen can be calculated fairly accurately using the model of the atom, in which the electron "orbits" the proton, just as Earth orbits the Sun. However, the electron and proton are held together by electrostatic attraction, while planets and celestial objects are held by gravity. Due to the discretization of angular momentum postulated in early [[Physics:Quantum mechanics|quantum mechanics]] by , the electron in the  model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.<ref>{{cite web
 |last=Stern
 |first=D. P.
@@ Line 518: / Line 522: @@
 }}</ref>
-Hydrogen's unique position as the only neutral atom for which the [[Schrödinger equation|{{langr|de|Schrödinger|cat=no}} equation]] can be directly solved has significantly contributed to the understanding of quantum mechanics through the exploration of its energetics.<ref name="Laursen04">{{cite web|last1=Laursen|first1=S.|last2=Chang|first2=J.|last3=Medlin|first3=W.|last4=Gürmen|first4=N.|last5=Fogler|first5=H. S.|title=An extremely brief introduction to computational quantum chemistry|url=http://www.umich.edu/~elements/5e/web_mod/quantum/introduction_3.htm|website=Molecular Modeling in Chemical Engineering|publisher=University of Michigan|access-date=4 May 2015|date=27 July 2004|archive-url=https://web.archive.org/web/20150520061846/http://www.umich.edu/~elements/5e/web_mod/quantum/introduction_3.htm|archive-date=20 May 2015|url-status=live}}</ref> Furthermore, study of the corresponding simplicity of the hydrogen molecule and the corresponding cation, [[H2+|{{chem2|H2+}}]], brought understanding of the nature of the [[Chemistry:Chemical bond|chemical bond]], which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.<ref>{{Cite journal |last=Wilson |first=E. Bright |date=1977 |title=Impact of the Heitler-London hydrogen molecule paper on chemistry |url=https://onlinelibrary.wiley.com/doi/10.1002/qua.560120807 |journal=International Journal of Quantum Chemistry |language=en |volume=12 |issue=S11 |pages=17–28 |doi=10.1002/qua.560120807 |issn=1097-461X|url-access=subscription }}</ref>
+Hydrogen's unique position as the only neutral atom for which the equation can be directly solved has significantly contributed to the understanding of quantum mechanics through the exploration of its energetics.<ref name="Laursen04">{{cite web|last1=Laursen|first1=S.|last2=Chang|first2=J.|last3=Medlin|first3=W.|last4=Gürmen|first4=N.|last5=Fogler|first5=H. S.|title=An extremely brief introduction to computational quantum chemistry|url=http://www.umich.edu/~elements/5e/web_mod/quantum/introduction_3.htm|website=Molecular Modeling in Chemical Engineering|publisher=University of Michigan|access-date=4 May 2015|date=27 July 2004|archive-url=https://web.archive.org/web/20150520061846/http://www.umich.edu/~elements/5e/web_mod/quantum/introduction_3.htm|archive-date=20 May 2015|url-status=live}}</ref> Furthermore, study of the corresponding simplicity of the hydrogen molecule and the corresponding cation, {{chem2|H2+}}, brought understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.<ref>{{Cite journal |last=Wilson |first=E. Bright |date=1977 |title=Impact of the Heitler-London hydrogen molecule paper on chemistry |url=https://onlinelibrary.wiley.com/doi/10.1002/qua.560120807 |journal=International Journal of Quantum Chemistry |language=en |volume=12 |issue=S11 |pages=17–28 |doi=10.1002/qua.560120807 |issn=1097-461X|url-access=subscription }}</ref>
 ==== Hydrogen-lifted airship ====
-[[File:Hindenburg over New York 1937 (cropped).jpg|alt=Airship Hindenburg over New York|thumb|The [[Hindenburg-class airship|{{lang|de|Hindenburg|cat=no}}]] over New York City in{{nbsp}}1937]]
+[[File:Hindenburg over New York 1937 (cropped).jpg|alt=Airship Hindenburg over New York|thumb|The {{lang|de|Hindenburg|cat=no}} over New York City in{{nbsp}}1937]]
-Because {{chem2|H2}} has only 7% the density of air, it was once widely used as a [[Physics:Lifting gas|lifting gas]] in balloons and [[Company:Airship|airship]]s.<ref name="Almqvist03">{{cite book |last1=Almqvist |first1=Ebbe |url={{Google books|OI0fTJhydh4C|page=|keywords=|text=|plainurl=yes}} |title=History of industrial gases |date=2003 |publisher=Kluwer Academic/Plenum Publishers |isbn=978-0-306-47277-0 |location=New York, N.Y. |pages=47–56 |access-date=20 May 2015}}</ref> The first hydrogen-filled balloon was invented by {{langr|fr|[[Biography:Jacques Charles|Jacques Charles]]}} in{{nbsp}}1783. Hydrogen provided the lift for the first reliable form of air-travel following the{{nbsp}}1852 invention of the first hydrogen-lifted airship by {{langr|fr|[[Biography:Henri Giffard|Henri Giffard]]}}. German count [[Ferdinand von Zeppelin|{{langr|de|Ferdinand von{{nbsp}}Zeppelin|cat=no}}]] promoted the idea of rigid airships lifted by hydrogen that later were called {{langr|de|Zeppelins}}, the first of which had its maiden flight in{{nbsp}}1900.<ref name="nbb" /> Regularly-scheduled flights started in{{nbsp}}1910 and by the outbreak of World War{{nbsp}}I in August{{nbsp}}1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships in the form of [[Chemistry:Blimp|blimp]]s were used as observation platforms and bombers during World War{{nbsp}}II, especially on the US{{nbsp}}Eastern seaboard.<ref>{{Cite web |last=Kratz |first=Jessie |date=2017-10-27 |title=Beyond the Hindenburg: Airships Throughout History |url=https://prologue.blogs.archives.gov/2017/10/27/beyond-the-hindenburg-airships-throughout-history/ |access-date=2025-04-09 |website=Pieces of History |language=en-US}}</ref>
+Because {{chem2|H2}} has only 7% the density of air, it was once widely used as a lifting gas in balloons and airships.<ref name="Almqvist03">{{cite book |last1=Almqvist |first1=Ebbe |url= |title=History of industrial gases |date=2003 |publisher=Kluwer Academic/Plenum Publishers |isbn=978-0-306-47277-0 |location=New York, N.Y. |pages=47–56 |access-date=20 May 2015}}</ref> The first hydrogen-filled balloon was invented by  in{{nbsp}}1783. Hydrogen provided the lift for the first reliable form of air-travel following the{{nbsp}}1852 invention of the first hydrogen-lifted airship by . German count {{langr|de|Ferdinand von{{nbsp}}Zeppelin|cat=no}} promoted the idea of rigid airships lifted by hydrogen that later were called , the first of which had its maiden flight in{{nbsp}}1900.<ref name="nbb" /> Regularly-scheduled flights started in{{nbsp}}1910 and by the outbreak of World War{{nbsp}}I in August{{nbsp}}1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships in the form of blimps were used as observation platforms and bombers during World War{{nbsp}}II, especially on the US{{nbsp}}Eastern seaboard.<ref>{{Cite web |last=Kratz |first=Jessie |date=2017-10-27 |title=Beyond the Hindenburg: Airships Throughout History |url=https://prologue.blogs.archives.gov/2017/10/27/beyond-the-hindenburg-airships-throughout-history/ |access-date=2025-04-09 |website=Pieces of History |language=en-US}}</ref>
-The first non-stop transatlantic crossing was made by the British airship{{nbsp}}''R34'' in{{nbsp}}1919 and  regular passenger service resumed in the{{nbsp}}1920s. Hydrogen was used in the [[LZ 129 Hindenburg|{{lang|de|Hindenburg|cat=no}}]], which caught fire over New Jersey on 6{{nbsp}}May 1937.<ref name="nbb" /> The hydrogen that filled the airship was ignited, possibly by static electricity, and burst into flames.<ref>{{Cite web |last=Follows |first=Mike |date=July 2, 2015 |title=What ignited the Hindenburg? |url=https://edu.rsc.org/feature/what-ignited-the-hindenburg/2000137.article |access-date=2025-02-19 |website=RSC Education |language=en}}</ref> Following this [[Engineering:Hindenburg disaster|disaster]], commercial hydrogen airship travel [[Engineering:Rigid airship#Demise|ceased]]. Hydrogen is still used, in preference to non-flammable but more expensive [[Chemistry:Helium|helium]], as a lifting gas for [[Earth:Weather balloon#Materials and equipment|weather balloons]].<ref>{{Cite web |last=Rappe |first=Mollie |date=May 9, 2023 |title=Researchers switch from helium to hydrogen weather balloons |url=https://phys.org/news/2023-05-helium-hydrogen-weather-balloons.html |access-date=2025-02-19 |website=phys.org |language=en}}</ref>
+The first non-stop transatlantic crossing was made by the British airship{{nbsp}}''R34'' in{{nbsp}}1919 and  regular passenger service resumed in the{{nbsp}}1920s. Hydrogen was used in the {{lang|de|Hindenburg|cat=no}}, which caught fire over New Jersey on 6{{nbsp}}May 1937.<ref name="nbb" /> The hydrogen that filled the airship was ignited, possibly by static electricity, and burst into flames.<ref>{{Cite web |last=Follows |first=Mike |date=July 2, 2015 |title=What ignited the Hindenburg? |url=https://edu.rsc.org/feature/what-ignited-the-hindenburg/2000137.article |access-date=2025-02-19 |website=RSC Education |language=en}}</ref> Following this disaster, commercial hydrogen airship travel ceased. Hydrogen is still used, in preference to non-flammable but more expensive helium, as a lifting gas for weather balloons.<ref>{{Cite web |last=Rappe |first=Mollie |date=May 9, 2023 |title=Researchers switch from helium to hydrogen weather balloons |url=https://phys.org/news/2023-05-helium-hydrogen-weather-balloons.html |access-date=2025-02-19 |website=phys.org |language=en}}</ref>
 ==== Deuterium and tritium ====
-[[Physics:Deuterium|Deuterium]] was discovered in December{{nbsp}}1931 by [[Biography:Harold Urey|Harold Urey]], and [[Physics:Tritium|tritium]] was prepared in{{nbsp}}1934 by [[Biography:Ernest Rutherford|Ernest Rutherford]], [[Biography:Mark Oliphant|Mark Oliphant]], and Paul Harteck.<ref name="Nostrand" /> [[Chemistry:Heavy water|Heavy water]], which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in{{nbsp}}1932.<ref name="nbb" />
+Deuterium was discovered in December{{nbsp}}1931 by Harold Urey, and tritium was prepared in{{nbsp}}1934 by Ernest Rutherford, Mark Oliphant, and Paul Harteck.<ref name="Nostrand" /> Heavy water, which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in{{nbsp}}1932.<ref name="nbb" />
 ==Chemistry==
 ===Reactions of H<sub>2</sub>===
-[[File:HFe H2 dppe 2.svg|thumb|right|A [[Chemistry:Dihydrogen complex|dihydrogen complex]] of iron, [HFe(H<sub>2</sub>)(dppe)<sub>2</sub>]<sup>+</sup>]]
+[[File:HFe H2 dppe 2.svg|thumb|right|A dihydrogen complex of iron, [HFe(H<sub>2</sub>)(dppe)<sub>2</sub>]<sup>+</sup>]]
-{{chem2|H2}} is relatively unreactive. The thermodynamic basis of this low reactivity is the very strong {{nowr|H–H bond}}, with a [[Physics:Bond dissociation energy|bond dissociation energy]] of {{val|435.7|ul=kJ/mol}}.<ref>{{RubberBible87th}}</ref>  It does form coordination complexes called [[Chemistry:Dihydrogen complex|dihydrogen complex]]es. These species provide insights into the early steps in the interactions of hydrogen with metal catalysts.  According to [[Physics:Neutron diffraction|neutron diffraction]], the metal and two H{{nbsp}}atoms form a triangle in these complexes. The {{nowr|H-H bond}} remains intact but is elongated. They are acidic.<ref>{{Cite book
+{{chem2|H2}} is relatively unreactive. The thermodynamic basis of this low reactivity is the very strong , with a bond dissociation energy of {{val|435.7|ul=kJ/mol}}.<ref></ref>  It does form coordination complexes called dihydrogen complexes. These species provide insights into the early steps in the interactions of hydrogen with metal catalysts.  According to neutron diffraction, the metal and two H{{nbsp}}atoms form a triangle in these complexes. The  remains intact but is elongated. They are acidic.<ref>{{Cite book
 | edition = 1
 | publisher = Springer
@@ Line 543: / Line 547: @@
 }}</ref>
-Although exotic on Earth, the {{chem2|H3+}}{{nbsp}}ion is common in the universe. It is a triangular species, like the aforementioned dihydrogen complexes. It is known as [[Chemistry:Trihydrogen cation|protonated molecular hydrogen]] or the trihydrogen cation.<ref name="Carrington">{{cite journal
+Although exotic on Earth, the {{chem2|H3+}}{{nbsp}}ion is common in the universe. It is a triangular species, like the aforementioned dihydrogen complexes. It is known as protonated molecular hydrogen or the trihydrogen cation.<ref name="Carrington">{{cite journal
 |last1=Carrington|first1=A.|last2=McNab|first2=I. R.
 |title=The infrared predissociation spectrum of triatomic hydrogen cation (H<sub>3</sub><sup>+</sup>)
@@ Line 550: / Line 554: @@
 |doi=10.1021/ar00162a004}}</ref>
-Hydrogen reacts with [[Chemistry:Chlorine|chlorine]] to produce{{nbsp}}HCl, and with [[Chemistry:Bromine|bromine]] to produce{{nbsp}}[[Chemistry:Hydrogen bromide|HBr]], via a [[Philosophy:Chain reaction|chain reaction]]. The reaction requires initiation. For example, in the case of Br<sub>2</sub>, the dibromine molecule is split apart: {{chem2|Br2 + (UV light)-> 2Br•}}. Propagating reactions consume hydrogen molecules and produce{{nbsp}}HBr, as well as Br{{nbsp}}and H{{nbsp}}atoms:
+Hydrogen reacts with chlorine to produce{{nbsp}}HCl, and with bromine to produce{{nbsp}}HBr, via a chain reaction. The reaction requires initiation. For example, in the case of Br<sub>2</sub>, the dibromine molecule is split apart: {{chem2|Br2 + (UV light)-> 2Br•}}. Propagating reactions consume hydrogen molecules and produce{{nbsp}}HBr, as well as Br{{nbsp}}and H{{nbsp}}atoms:
 {{bi|{{chem2|Br• + H2 -> HBr + H}}}}
 {{bi|{{chem2|H + Br2 -> HBr +Br}}}}
@@ Line 558: / Line 562: @@
 consumes the remaining atoms.<ref>{{Cite book |last=Laidler |first=Keith J. |url=https://archive.org/details/chemicalkinetics0000laid_s2p0/page/290/mode/2up |title=Chemical kinetics |date=1998 |publisher=HarperCollins |isbn=978-0-06-043862-3 |edition=3. ed., [Nachdr.] |location=New York, NY}}</ref>{{rp|289}}
-The addition of H<sub>2</sub> to [[Chemistry:Saturated and unsaturated compounds|unsaturated]] organic compounds, such as [[Chemistry:Alkene|alkene]]s and [[Chemistry:Alkyne|alkyne]]s, is called [[Chemistry:Hydrogenation|hydrogenation]]. Even if the reaction is energetically favorable, it does not occur spontaneously even at higher temperatures. In the presence of a [[Software:Catalyst|catalyst]] like finely divided [[Chemistry:Platinum|platinum]] or [[Chemistry:Nickel|nickel]], the reaction proceeds at room temperature.<ref>{{Cite book |last1=Vollhardt |first1=Kurt Peter C. |title=Organic chemistry: structure and function |last2=Schore |first2=Neil Eric |date=2003 |publisher=W.H. Freeman and Co |isbn=978-0-7167-4374-3 |edition=4. |location=New York}}</ref>{{rp|477}}
+The addition of H<sub>2</sub> to unsaturated organic compounds, such as alkenes and alkynes, is called hydrogenation. Even if the reaction is energetically favorable, it does not occur spontaneously even at higher temperatures. In the presence of a catalyst like finely divided platinum or nickel, the reaction proceeds at room temperature.<ref>{{Cite book |last1=Vollhardt |first1=Kurt Peter C. |title=Organic chemistry: structure and function |last2=Schore |first2=Neil Eric |date=2003 |publisher=W.H. Freeman and Co |isbn=978-0-7167-4374-3 |edition=4. |location=New York}}</ref>{{rp|477}}
 ===Hydrogen-containing compounds===
 {{Main|Chemistry:Hydrogen compounds}}
-Hydrogen can exist in both +1 and −1{{nbsp}}[[Chemistry:Oxidation state|oxidation state]]s, forming compounds through ionic and covalent bonding. The element is part of a wide range of substances, including water, [[Chemistry:Hydrocarbon|hydrocarbon]]s, and numerous other [[Chemistry:Organic compound|organic compound]]s.<ref name="hydrocarbon">{{cite web| title=Structure and Nomenclature of Hydrocarbons| publisher=Purdue University| url=https://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html| access-date=23 March 2008| archive-url=https://web.archive.org/web/20120611084045/http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html| archive-date=11 June 2012}}</ref> The H<sup>+</sup>{{nbsp}}ion—commonly referred to as a proton due to its single proton and absence of electrons—is central to [[Chemistry:Acid–base reaction|acid–base chemistry]], although the proton does not move freely. In the [[Brønsted–Lowry acids|{{langr|no|Brønsted|cat=no}}–Lowry]] framework, acids are defined by their ability to donate H<sup>+</sup>{{nbsp}}ions to bases.<ref>{{Cite book |last=Laurence |first=Christian |title=Lewis basicity and affinity scales: data and measurement |date=2010 |publisher=Wiley |isbn=978-0-470-68189-3 |location=Chichester}}</ref>
+Hydrogen can exist in both +1 and −1{{nbsp}}oxidation states, forming compounds through ionic and covalent bonding. The element is part of a wide range of substances, including water, hydrocarbons, and numerous other organic compounds.<ref name="hydrocarbon">{{cite web| title=Structure and Nomenclature of Hydrocarbons| publisher=Purdue University| url=https://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html| access-date=23 March 2008| archive-url=https://web.archive.org/web/20120611084045/http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html| archive-date=11 June 2012}}</ref> The H<sup>+</sup>{{nbsp}}ion—commonly referred to as a proton due to its single proton and absence of electrons—is central to acid–base chemistry, although the proton does not move freely. In the –Lowry framework, acids are defined by their ability to donate H<sup>+</sup>{{nbsp}}ions to bases.<ref>{{Cite book |last=Laurence |first=Christian |title=Lewis basicity and affinity scales: data and measurement |date=2010 |publisher=Wiley |isbn=978-0-470-68189-3 |location=Chichester}}</ref>
-Hydrogen forms a vast variety of compounds with [[Chemistry:Carbon|carbon]], known as hydrocarbons, and an even greater diversity with other elements ([[Chemistry:Heteroatom|heteroatom]]s), giving rise to the broad class of organic compounds often associated with living organisms.<ref name="hydrocarbon"/>
+Hydrogen forms a vast variety of compounds with carbon, known as hydrocarbons, and an even greater diversity with other elements (heteroatoms), giving rise to the broad class of organic compounds often associated with living organisms.<ref name="hydrocarbon"/>
-[[File:NaH.jpg|thumb|A sample of [[Chemistry:Sodium hydride|sodium hydride]] ]]
+[[File:NaH.jpg|thumb|A sample of sodium hydride ]]
-Hydrogen compounds with hydrogen in the oxidation state{{nbsp}}−1 are known as [[Physics:Hydride|hydride]]s, which are usually formed between hydrogen and metals. The hydrides can be ionic (aka saline), covalent, or metallic. With heating, H<sub>2</sub> reacts efficiently with the alkali and [[Chemistry:Alkaline earth metal|alkaline earth metal]]s to give the ionic hydrides of the formulas{{nbsp}}MH and MH<sub>2</sub>, respectively. These salt-like crystalline compounds have high melting points and all react with water to liberate hydrogen. Covalent hydrides include [[Chemistry:Borane|borane]]s and polymeric [[Chemistry:Aluminium hydride|aluminium hydride]]. [[Chemistry:Transition metal|Transition metal]]s form metal hydrides via continuous dissolution of hydrogen into the metal.<ref name=UllmannH2/> A well-known hydride is [[Chemistry:Lithium aluminium hydride|lithium aluminium hydride]]: the {{chem2|[AlH4]-}}{{nbsp}}anion carries hydridic centers firmly attached to the Al(III).<ref>{{Greenwood&Earnshaw2nd|page=228}}</ref> Perhaps the most extensive series of hydrides are the [[Chemistry:Borane|borane]]s, compounds consisting only of boron and hydrogen.<ref name="Downs">{{cite journal
+Hydrogen compounds with hydrogen in the oxidation state{{nbsp}}−1 are known as hydrides, which are usually formed between hydrogen and metals. The hydrides can be ionic (aka saline), covalent, or metallic. With heating, H<sub>2</sub> reacts efficiently with the alkali and alkaline earth metals to give the ionic hydrides of the formulas{{nbsp}}MH and MH<sub>2</sub>, respectively. These salt-like crystalline compounds have high melting points and all react with water to liberate hydrogen. Covalent hydrides include boranes and polymeric aluminium hydride. Transition metals form metal hydrides via continuous dissolution of hydrogen into the metal.<ref name=UllmannH2/> A well-known hydride is lithium aluminium hydride: the {{chem2|[AlH4]-}}{{nbsp}}anion carries hydridic centers firmly attached to the Al(III).<ref></ref> Perhaps the most extensive series of hydrides are the boranes, compounds consisting only of boron and hydrogen.<ref name="Downs">{{cite journal
 |last1=Downs|first1=A. J.
 |last2=Pulham|first2=C. R.
@@ Line 577: / Line 581: @@
 }}</ref>
-Hydrides can bond to these electropositive elements not only as a terminal [[Chemistry:Ligand|ligand]] but also as [[Chemistry:Bridging ligand|bridging ligand]]s. In diborane{{nbsp}}({{chem2|B2H6}}), four hydrogen atoms are terminal, while two bridge between the two boron atoms.<ref name="Miessler" />
+Hydrides can bond to these electropositive elements not only as a terminal ligand but also as bridging ligands. In diborane{{nbsp}}({{chem2|B2H6}}), four hydrogen atoms are terminal, while two bridge between the two boron atoms.<ref name="Miessler" />
 === Hydrogen bonding ===
 {{Main|Chemistry:Hydrogen bond}}
-When bonded to a more electronegative element, particularly [[Chemistry:Fluorine|fluorine]], [[Chemistry:Oxygen|oxygen]], or [[Chemistry:Nitrogen|nitrogen]], hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with a [[Chemistry:Lone pair|lone pair]] like oxygen or nitrogen. This phenomenon, called hydrogen bonding, is critical to the stability of many biological molecules.<ref>{{Cite journal |last1=Pimentel |first1=G C |last2=McClellan |first2=A L |date=October 1971 |title=Hydrogen Bonding |url=https://www.annualreviews.org/doi/10.1146/annurev.pc.22.100171.002023 |journal=Annual Review of Physical Chemistry |language=en |volume=22 |issue=1 |pages=347–385 |doi=10.1146/annurev.pc.22.100171.002023 |bibcode=1971ARPC...22..347P |issn=0066-426X|url-access=subscription }}</ref>{{rp|375}}<ref>IUPAC Compendium of Chemical Terminology, Electronic version, [http://goldbook.iupac.org/H02899.html Hydrogen Bond] {{Webarchive|url=https://web.archive.org/web/20080319045705/http://goldbook.iupac.org/H02899.html |date=19 March 2008 }}</ref> Hydrogen bonding alters molecule structures, [[Viscosity|viscosity]], [[Chemistry:Solubility|solubility]], melting and boiling points, and even [[Biology:Protein folding|protein folding]] dynamics.<ref>{{Cite journal |last1=Xie |first1=Zhenkai |last2=Luo |first2=Rui |last3=Ying |first3=Tianping |last4=Gao |first4=Yurui |last5=Song |first5=Boqin |last6=Yu |first6=Tongxu |last7=Chen |first7=Xu |last8=Hao |first8=Munan |last9=Chai |first9=Congcong |last10=Yan |first10=Jiashu |last11=Huang |first11=Zhiheng |last12=Chen |first12=Zhiguo |last13=Du |first13=Luojun |last14=Zhu |first14=Chongqin |last15=Guo |first15=Jiangang |date=November 2024 |title=Dynamic-to-static switch of hydrogen bonds induces a metal–insulator transition in an organic–inorganic superlattice |url=https://www.nature.com/articles/s41557-024-01566-1 |journal=Nature Chemistry |language=en |volume=16 |issue=11 |pages=1803–1810 |doi=10.1038/s41557-024-01566-1 |pmid=39143300 |bibcode=2024NatCh..16.1803X |issn=1755-4330|url-access=subscription }}</ref>
+When bonded to a more electronegative element, particularly fluorine, oxygen, or nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with a lone pair like oxygen or nitrogen. This phenomenon, called hydrogen bonding, is critical to the stability of many biological molecules.<ref>{{Cite journal |last1=Pimentel |first1=G C |last2=McClellan |first2=A L |date=October 1971 |title=Hydrogen Bonding |url=https://www.annualreviews.org/doi/10.1146/annurev.pc.22.100171.002023 |journal=Annual Review of Physical Chemistry |language=en |volume=22 |issue=1 |pages=347–385 |doi=10.1146/annurev.pc.22.100171.002023 |bibcode=1971ARPC...22..347P |issn=0066-426X|url-access=subscription }}</ref>{{rp|375}}<ref>IUPAC Compendium of Chemical Terminology, Electronic version, [http://goldbook.iupac.org/H02899.html Hydrogen Bond] {{Webarchive|url=https://web.archive.org/web/20080319045705/http://goldbook.iupac.org/H02899.html |date=19 March 2008 }}</ref> Hydrogen bonding alters molecule structures, viscosity, solubility, melting and boiling points, and even protein folding dynamics.<ref>{{Cite journal |last1=Xie |first1=Zhenkai |last2=Luo |first2=Rui |last3=Ying |first3=Tianping |last4=Gao |first4=Yurui |last5=Song |first5=Boqin |last6=Yu |first6=Tongxu |last7=Chen |first7=Xu |last8=Hao |first8=Munan |last9=Chai |first9=Congcong |last10=Yan |first10=Jiashu |last11=Huang |first11=Zhiheng |last12=Chen |first12=Zhiguo |last13=Du |first13=Luojun |last14=Zhu |first14=Chongqin |last15=Guo |first15=Jiangang |date=November 2024 |title=Dynamic-to-static switch of hydrogen bonds induces a metal–insulator transition in an organic–inorganic superlattice |url=https://www.nature.com/articles/s41557-024-01566-1 |journal=Nature Chemistry |language=en |volume=16 |issue=11 |pages=1803–1810 |doi=10.1038/s41557-024-01566-1 |pmid=39143300 |bibcode=2024NatCh..16.1803X |issn=1755-4330|url-access=subscription }}</ref>
 ===Protons and acids ===
-  [[File:Base pair AT.svg|thumb|right|An "A-T [[base pair]]" in DNA illustrating how hydrogen bonds are critical to the [[Biology:Genetic code|genetic code]]. The drawing illustrates that in many chemical depictions, {{nowr|C-H bonds}} are not always shown explicitly, an indication of their pervasiveness.]]
+  [[File:Base pair AT.svg|thumb|right|An "A-T [[base pair]]" in DNA illustrating how hydrogen bonds are critical to the genetic code. The drawing illustrates that in many chemical depictions,  are not always shown explicitly, an indication of their pervasiveness.]]
 In water, hydrogen bonding plays an important role in reaction thermodynamics. A hydrogen bond can shift over to proton transfer.
-Under the [[Brønsted–Lowry acid–base theory|{{langr|da|Brønsted|cat=no}}–Lowry acid–base theory]], acids are proton donors, while bases are proton acceptors.<ref>{{Cite book |last=Punekar |first=Narayan S. |url=https://link.springer.com/10.1007/978-981-97-8179-9_28 |title=ENZYMES: Catalysis, Kinetics and Mechanisms |date=2025 |publisher=Springer Nature Singapore |isbn=978-981-97-8178-2 |location=Singapore |pages=333–345 |language=en |doi=10.1007/978-981-97-8179-9_28}}</ref>{{rp|loc=28}}
+Under the –Lowry acid–base theory, acids are proton donors, while bases are proton acceptors.<ref>{{Cite book |last=Punekar |first=Narayan S. |url=https://link.springer.com/10.1007/978-981-97-8179-9_28 |title=ENZYMES: Catalysis, Kinetics and Mechanisms |date=2025 |publisher=Springer Nature Singapore |isbn=978-981-97-8178-2 |location=Singapore |pages=333–345 |language=en |doi=10.1007/978-981-97-8179-9_28}}</ref>{{rp|loc=28}}
-A bare proton{{nbsp}}({{chem2|H+}}) essentially cannot exist in anything other than a vacuum. Otherwise it attaches to other atoms, ions, or molecules. Even chemical species as inert as [[Chemistry:Methane|methane]] can be protonated. The term "proton" is used loosely and metaphorically to refer to solvated hydrogen cations attached to other solvated chemical species; it is denoted{{nbsp}}"{{chem2|H+}}" without any implication that any single protons exist freely in solution as a species. To avoid the implication of the naked proton in solution, acidic aqueous solutions are sometimes considered to contain the "[[Chemistry:Hydronium|hydronium]] ion"{{nbsp}}({{chem2|[H3O]+}}), or still more accurately, {{chem2|[H9O4]+}}.<ref name="Okumura">{{cite journal
+A bare proton{{nbsp}}({{chem2|H+}}) essentially cannot exist in anything other than a vacuum. Otherwise it attaches to other atoms, ions, or molecules. Even chemical species as inert as methane can be protonated. The term "proton" is used loosely and metaphorically to refer to solvated hydrogen cations attached to other solvated chemical species; it is denoted{{nbsp}}"{{chem2|H+}}" without any implication that any single protons exist freely in solution as a species. To avoid the implication of the naked proton in solution, acidic aqueous solutions are sometimes considered to contain the "hydronium ion"{{nbsp}}({{chem2|[H3O]+}}), or still more accurately, {{chem2|[H9O4]+}}.<ref name="Okumura">{{cite journal
 |last1=Okumura|first1=A. M.
 |last2=Yeh|first2=L. I.|last3=Myers|first3=J. D.|last4=Lee|first4=Y. T.
@@ Line 593: / Line 597: @@
 |journal=Journal of Physical Chemistry
 |date=1990|volume=94|issue=9|pages=3416–3427|doi=10.1021/j100372a014
-}}</ref> Other [[Chemistry:Oxonium ion|oxonium ion]]s are found when water is in acidic solution with other solvents.<ref name="Perdoncin">{{cite journal
+}}</ref> Other oxonium ions are found when water is in acidic solution with other solvents.<ref name="Perdoncin">{{cite journal
 |last1=Perdoncin|first1=G.|last2=Scorrano|first2=G.
 |title=Protonation Equilibria in Water at Several Temperatures of Alcohols, Ethers, Acetone, Dimethyl Sulfide, and Dimethyl Sulfoxide
@@ Line 601: / Line 605: @@
 |bibcode=1977JAChS..99.6983P }}</ref>
-The concentration of these solvated protons determines the [[Chemistry:PH|pH]] of a solution, a [[Logarithmic scale|logarithmic scale]] that reflects its acidity or basicity. Lower pH{{nbsp}}values indicate higher concentrations of hydronium ions, corresponding to more acidic conditions.<ref name="housecroft" />
+The concentration of these solvated protons determines the pH of a solution, a logarithmic scale that reflects its acidity or basicity. Lower pH{{nbsp}}values indicate higher concentrations of hydronium ions, corresponding to more acidic conditions.<ref name="housecroft" />
 ==Occurrence==
 ===Cosmic===
-[[File:Nursery of New Stars - GPN-2000-000972.jpg|right|thumb|[[NGC 604]], a giant [[Astronomy:H II region|region of ionized hydrogen]] in the [[Astronomy:Triangulum Galaxy|Triangulum Galaxy]]|alt=A white-green cotton-like clog on black background.]]
+[[File:Nursery of New Stars - GPN-2000-000972.jpg|right|thumb|NGC 604, a giant region of ionized hydrogen in the Triangulum Galaxy|alt=A white-green cotton-like clog on black background.]]
-Hydrogen, as atomic H, is the most [[Chemistry:Natural abundance|abundant]] [[Chemistry:Chemical element|chemical element]] in the universe, making up 75% of [[Physics:Baryon|normal matter]] by [[Mass|mass]].<ref>{{cite web
+Hydrogen, as atomic H, is the most abundant chemical element in the universe, making up 75% of normal matter by mass.<ref>{{cite web
 |last=Boyd
 |first=Padi
 |title=What is the chemical composition of stars?
 |url=https://imagine.gsfc.nasa.gov/ask_astro/stars.html#961112a
-|publisher=[[Organization:NASA|NASA]]
+|publisher=NASA
 |date=19 July 2014
 |access-date=5 February 2008
@@ Line 623: / Line 627: @@
 |publisher=Cambridge University Press
 |isbn=978-0-521-82381-4
-}}</ref> In the early universe, protons formed in the first second after the Big Bang; neutral hydrogen atoms formed about 370,000{{nbsp}}years later during the [[Astronomy:Recombination (cosmology)|recombination epoch]] as the universe expanded and plasma had cooled enough for electrons to remain bound to protons.<ref>{{cite journal |last=Tanabashi |first=M. |display-authors=0 |year=2018 |journal=[[Physics:Physical Review D|Physical Review D]] |volume=98 |issue=3 |via=[[Physics:Particle Data Group|Particle Data Group]] at [[Organization:Lawrence Berkeley National Laboratory|Lawrence Berkeley National Laboratory]] |url=http://pdg.lbl.gov/2018/reviews/rpp2018-rev-bbang-cosmology.pdf |page=358 |quote=Chapter 21.4.1 - This occurred when the age of the Universe was about 370,000 years. |title=Big-Bang Cosmology |url-status=live |archive-url=https://web.archive.org/web/20210629034426/https://pdg.lbl.gov/2018/reviews/rpp2018-rev-bbang-cosmology.pdf |archive-date=29 June 2021 |doi=10.1103/PhysRevD.98.030001 |doi-access=free |bibcode=2018PhRvD..98c0001T |hdl=11384/78286 }} (Revised September 2017) by [[Biography:Keith Olive|Keith A. Olive]] and [[Biography:John A. Peacock|John A. Peacock]].</ref>
+}}</ref> In the early universe, protons formed in the first second after the Big Bang; neutral hydrogen atoms formed about 370,000{{nbsp}}years later during the recombination epoch as the universe expanded and plasma had cooled enough for electrons to remain bound to protons.<ref>{{cite journal |last=Tanabashi |first=M. |display-authors=0 |year=2018 |journal=Physical Review D |volume=98 |issue=3 |via=Particle Data Group at Lawrence Berkeley National Laboratory |url=http://pdg.lbl.gov/2018/reviews/rpp2018-rev-bbang-cosmology.pdf |page=358 |quote=Chapter 21.4.1 - This occurred when the age of the Universe was about 370,000 years. |title=Big-Bang Cosmology |url-status=live |archive-url=https://web.archive.org/web/20210629034426/https://pdg.lbl.gov/2018/reviews/rpp2018-rev-bbang-cosmology.pdf |archive-date=29 June 2021 |doi=10.1103/PhysRevD.98.030001 |doi-access=free |bibcode=2018PhRvD..98c0001T |hdl=11384/78286 }} (Revised September 2017) by Keith A. Olive and John A. Peacock.</ref>
-In astrophysics, neutral hydrogen in the [[Astronomy:Interstellar medium|interstellar medium]] is called ''H{{nbsp}}I'' and ionized hydrogen is called ''H{{nbsp}}II''.<ref>{{Cite book |last1=Kaplan |first1=S. A. |url=https://www.degruyter.com/document/doi/10.4159/harvard.9780674493988/html |title=The Interstellar Medium |last2=Pikelner |first2=S. B. |date=1970-12-31 |publisher=Harvard University Press |isbn=978-0-674-49397-1 |pages=1–77 |chapter=1. Interstellar Hydrogen |doi=10.4159/harvard.9780674493988}}</ref> Radiation from stars ionizes H{{nbsp}}I to H{{nbsp}}II, creating [[Astronomy:Strömgren sphere|spheres of ionized H{{nbsp}}II]] around stars. In the [[Astronomy:Chronology of the universe|chronology of the universe]] neutral hydrogen dominated until the birth of stars during the era of [[Astronomy:Reionization|reionization]], which then produced bubbles of ionized hydrogen that grew and merged over hundreds of millions of years.<ref>{{Cite journal |last=Dijkstra |first=Mark |date=January 2014 |title=Lyα Emitting Galaxies as a Probe of Reionisation |url=https://www.cambridge.org/core/journals/publications-of-the-astronomical-society-of-australia/article/ly-emitting-galaxies-as-a-probe-of-reionisation/51F95FB047C1F0418D1DA56D39470C22 |journal=Publications of the Astronomical Society of Australia |language=en |volume=31 |article-number=e040 |doi=10.1017/pasa.2014.33 |arxiv=1406.7292 |bibcode=2014PASA...31...40D |issn=1323-3580}}</ref>
+In astrophysics, neutral hydrogen in the interstellar medium is called ''H{{nbsp}}I'' and ionized hydrogen is called ''H{{nbsp}}II''.<ref>{{Cite book |last1=Kaplan |first1=S. A. |url=https://www.degruyter.com/document/doi/10.4159/harvard.9780674493988/html |title=The Interstellar Medium |last2=Pikelner |first2=S. B. |date=1970-12-31 |publisher=Harvard University Press |isbn=978-0-674-49397-1 |pages=1–77 |chapter=1. Interstellar Hydrogen |doi=10.4159/harvard.9780674493988}}</ref> Radiation from stars ionizes H{{nbsp}}I to H{{nbsp}}II, creating spheres of ionized H{{nbsp}}II around stars. In the chronology of the universe neutral hydrogen dominated until the birth of stars during the era of reionization, which then produced bubbles of ionized hydrogen that grew and merged over hundreds of millions of years.<ref>{{Cite journal |last=Dijkstra |first=Mark |date=January 2014 |title=Lyα Emitting Galaxies as a Probe of Reionisation |url=https://www.cambridge.org/core/journals/publications-of-the-astronomical-society-of-australia/article/ly-emitting-galaxies-as-a-probe-of-reionisation/51F95FB047C1F0418D1DA56D39470C22 |journal=Publications of the Astronomical Society of Australia |language=en |volume=31 |article-number=e040 |doi=10.1017/pasa.2014.33 |arxiv=1406.7292 |bibcode=2014PASA...31...40D |issn=1323-3580}}</ref>
-These are the source of the 21-centimeter [[Physics:Hydrogen line|hydrogen line]], at {{val|1420|ul=MHz}}, that is detected in order to probe primordial hydrogen. The large amount of neutral hydrogen found in the [[Astronomy:Damped Lyman-alpha system|damped Lyman-alpha system]]s is thought to dominate the [[Astronomy:Physical cosmology|cosmological]] [[Physics:Baryon|baryon]]ic density of the universe up to a [[Astronomy:Redshift|redshift]] of {{nowr|''z'' {{=}} 4}}.<ref>{{cite journal
+These are the source of the 21-centimeter hydrogen line, at {{val|1420|ul=MHz}}, that is detected in order to probe primordial hydrogen. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the universe up to a redshift of {{nowr|''z'' {{=}} 4}}.<ref>{{cite journal
 |last1=Storrie-Lombardi|first1=L. J.
 |last2=Wolfe|first2=A. M.
@@ Line 638: / Line 642: @@
 }}</ref>
-Hydrogen is found in great abundance in stars and [[Astronomy:Gas giant|gas giant]] planets. [[Astronomy:Molecular cloud|Molecular cloud]]s of {{chem2|H2}} are associated with [[Astronomy:Star formation|star formation]]. Hydrogen plays a vital role in powering [[Astronomy:Star|star]]s through the proton-proton reaction in lower-mass stars, and through the [[Astronomy:CNO cycle|CNO{{nbsp}}cycle]] of [[Physics:Nuclear fusion|nuclear fusion]] in stars more massive than the [[Astronomy:Sun|Sun]].<ref>{{cite web
+Hydrogen is found in great abundance in stars and gas giant planets. Molecular clouds of {{chem2|H2}} are associated with star formation. Hydrogen plays a vital role in powering stars through the proton-proton reaction in lower-mass stars, and through the CNO{{nbsp}}cycle of nuclear fusion in stars more massive than the Sun.<ref>{{cite web
 |last1=Haubold|first1=H.|last2=Mathai|first2=A. M.
 |date=15 November 2007|url=http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter4.html |archive-url =https://web.archive.org/web/20111211073137/http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter4.html
   |archive-date=11 December 2011 |title=Solar Thermonuclear Energy Generation
-|publisher=[[Organization:Columbia University|Columbia University]]|access-date=12 February 2008
+|publisher=Columbia University|access-date=12 February 2008
 }}</ref>
-Protonated molecular hydrogen{{nbsp}}({{chem2|H3+}}) is found in the [[Astronomy:Interstellar medium|interstellar medium]], where it is generated by ionization of molecular hydrogen by [[Astronomy:Cosmic ray|cosmic ray]]s. This ion has also been observed in the [[Earth:Primary atmosphere|upper atmosphere of Jupiter]]. The ion is long-lived in outer space due to the low temperature and density. {{chem2|H3+}} is one of the most abundant ions in the universe, and it plays a notable role in the chemistry of the interstellar medium.<ref>{{cite web|author=McCall Group|author2=Oka Group|date=22 April 2005|url=http://h3plus.uiuc.edu/|title=H3+ Resource Center|publisher=Universities of Illinois and Chicago|access-date=5 February 2008|archive-url=https://web.archive.org/web/20071011211244/http://h3plus.uiuc.edu/|archive-date=11 October 2007}}</ref> Neutral [[Chemistry:Triatomic hydrogen|triatomic hydrogen]] {{chem2|H3}} can exist only in an excited form and is unstable.<ref name="couple">{{citation|year=2003|publisher=Department of Molecular and Optical Physics, University of Freiburg, Germany|author=Helm, H.|display-authors=0|title=Dissociative Recombination of Molecular Ions with Electrons|pages=275–288|doi=10.1007/978-1-4615-0083-4_27|chapter=Coupling of Bound States to Continuum States in Neutral Triatomic Hydrogen|isbn=978-1-4613-4915-0}}</ref>
+Protonated molecular hydrogen{{nbsp}}({{chem2|H3+}}) is found in the interstellar medium, where it is generated by ionization of molecular hydrogen by cosmic rays. This ion has also been observed in the upper atmosphere of Jupiter. The ion is long-lived in outer space due to the low temperature and density. {{chem2|H3+}} is one of the most abundant ions in the universe, and it plays a notable role in the chemistry of the interstellar medium.<ref>{{cite web|author=McCall Group|author2=Oka Group|date=22 April 2005|url=http://h3plus.uiuc.edu/|title=H3+ Resource Center|publisher=Universities of Illinois and Chicago|access-date=5 February 2008|archive-url=https://web.archive.org/web/20071011211244/http://h3plus.uiuc.edu/|archive-date=11 October 2007}}</ref> Neutral triatomic hydrogen {{chem2|H3}} can exist only in an excited form and is unstable.<ref name="couple">{{citation|year=2003|publisher=Department of Molecular and Optical Physics, University of Freiburg, Germany|author=Helm, H.|display-authors=0|title=Dissociative Recombination of Molecular Ions with Electrons|pages=275–288|doi=10.1007/978-1-4615-0083-4_27|chapter=Coupling of Bound States to Continuum States in Neutral Triatomic Hydrogen|isbn=978-1-4613-4915-0}}</ref>
 ===Terrestrial===
@@ Line 662: / Line 666: @@
 |archive-date=13 February 2008
 |bibcode=2004APS..MAR.m1001D
-}}</ref> mostly existing within [[Chemistry:Chemical compound|chemical compound]]s such as [[Chemistry:Hydrocarbon|hydrocarbon]]s and water.<ref name="Miessler">{{cite book|first1=G. L.|last1=Miessler|last2=Tarr|first2=D. A.|date=2003|title=Inorganic Chemistry|edition=3rd|publisher=Prentice Hall|isbn=978-0-13-035471-6|url-access=registration|url=https://archive.org/details/inorganicchemist03edmies}}</ref> Elemental hydrogen is normally in the form of a gas, {{chem2|H2}}, at standard conditions. It is present in a very low concentration in Earth's atmosphere (around {{val|0.53|u=[[Parts per million|parts per million]]}} on a [[Mole (unit)|molar basis]]<ref name="Grinter">{{cite journal |last1=Rhys Grinter |last2=Kropp |first2=A. |last3=Venugopal |display-authors=0 |date=2023 |title=Structural basis for bacterial energy extraction from atmospheric hydrogen |journal=Nature |volume=615 |issue=7952 |pages=541–547 |bibcode=2023Natur.615..541G |doi=10.1038/s41586-023-05781-7 |pmc=10017518 |pmid=36890228}}</ref>) because of its light weight, which enables it to [[Earth:Atmospheric escape|escape the atmosphere]] more rapidly than heavier gases. Despite its low concentration in the atmosphere, terrestrial hydrogen is sufficiently abundant to support the metabolism of several varieties of bacteria.<ref>{{cite journal |doi=10.1042/BST20230120 |title=Developing high-affinity, oxygen-insensitive &#91;NiFe&#93;-hydrogenases as biocatalysts for energy conversion |date=2023 |last1=Greening |first1=Chris |last2=Kropp |first2=Ashleigh |last3=Vincent |first3=Kylie |last4=Grinter |first4=Rhys |journal=Biochemical Society Transactions |volume=51 |issue=5 |pages=1921–1933 |pmid=37743798 |pmc=10657181 }}</ref>
+}}</ref> mostly existing within chemical compounds such as hydrocarbons and water.<ref name="Miessler">{{cite book|first1=G. L.|last1=Miessler|last2=Tarr|first2=D. A.|date=2003|title=Inorganic Chemistry|edition=3rd|publisher=Prentice Hall|isbn=978-0-13-035471-6|url-access=registration|url=https://archive.org/details/inorganicchemist03edmies}}</ref> Elemental hydrogen is normally in the form of a gas, {{chem2|H2}}, at standard conditions. It is present in a very low concentration in Earth's atmosphere (around {{val|0.53|u=parts per million}} on a molar basis<ref name="Grinter">{{cite journal |last1=Rhys Grinter |last2=Kropp |first2=A. |last3=Venugopal |display-authors=0 |date=2023 |title=Structural basis for bacterial energy extraction from atmospheric hydrogen |journal=Nature |volume=615 |issue=7952 |pages=541–547 |bibcode=2023Natur.615..541G |doi=10.1038/s41586-023-05781-7 |pmc=10017518 |pmid=36890228}}</ref>) because of its light weight, which enables it to escape the atmosphere more rapidly than heavier gases. Despite its low concentration in the atmosphere, terrestrial hydrogen is sufficiently abundant to support the metabolism of several varieties of bacteria.<ref>{{cite journal |doi=10.1042/BST20230120 |title=Developing high-affinity, oxygen-insensitive &#91;NiFe&#93;-hydrogenases as biocatalysts for energy conversion |date=2023 |last1=Greening |first1=Chris |last2=Kropp |first2=Ashleigh |last3=Vincent |first3=Kylie |last4=Grinter |first4=Rhys |journal=Biochemical Society Transactions |volume=51 |issue=5 |pages=1921–1933 |pmid=37743798 |pmc=10657181 }}</ref>
 Large underground deposits of hydrogen gas have been discovered in several countries including Mali, France and Australia.<ref name="Pearce-2024">{{Cite web |last=Pearce |first=Fred |date=January 25, 2024 |title=Natural Hydrogen: A Potential Clean Energy Source Beneath Our Feet |url=https://e360.yale.edu/features/natural-geologic-hydrogen-climate-change |access-date=2024-01-27 |website=Yale E360 |language=en-US}}</ref> As of 2024, it is uncertain how much underground hydrogen can be extracted economically.<ref name="Pearce-2024" />
@@ Line 678: / Line 682: @@
 | title=Principles of Modern Chemistry
 | edition=5th|publisher=Thomson Brooks/Cole
-| isbn=978-0-03-035373-4}}</ref> Thus, at high temperature ({{cvt|1000|–|1400|K|C F|disp=sqbr}}), [[Software:Steam|steam]] (water vapor) reacts with [[Chemistry:Methane|methane]] to yield [[Chemistry:Carbon monoxide|carbon monoxide]] and{{nbsp}}{{chem2|H2}}.
+| isbn=978-0-03-035373-4}}</ref> Thus, at high temperature ({{cvt|1000|–|1400|K|C F|disp=sqbr}}), steam (water vapor) reacts with methane to yield carbon monoxide and{{nbsp}}{{chem2|H2}}.
 {{bi|{{chem2|CH4 + H2O → CO + 3 H2}}}}
-Producing one [[Tonne|tonne]] of hydrogen through this process emits {{val|6.6|–|9.3}}{{nbsp}}tonnes of carbon dioxide.<ref name="Bonheure-2021">{{Cite web |last1=Bonheure |first1=Mike |last2=Vandewalle |first2=Laurien A. |last3=Marin |first3=Guy B. |last4=Van Geem |first4=Kevin M. |date=March 2021 |title=Dream or Reality? Electrification of the Chemical Process Industries |url=https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |url-status=live |archive-url=https://web.archive.org/web/20210717132733/https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |archive-date=17 July 2021 |access-date=6 July 2021 |website=CEP Magazine |publisher=[[Organization:American Institute of Chemical Engineers|American Institute of Chemical Engineers]]}}</ref> The production of natural gas feedstock also produces emissions such as [[Chemistry:Gas venting|vented]] and [[Chemistry:Fugitive gas emissions|fugitive methane]], which further contributes to the overall carbon footprint of hydrogen.<ref name="Griffiths-20212">{{Cite journal |last1=Griffiths |first1=Steve |last2=Sovacool |first2=Benjamin K. |last3=Kim |first3=Jinsoo |last4=Bazilian |first4=Morgan |last5=Uratani |first5=Joao M. |display-authors=4 |date=2021 |title=Industrial decarbonization via hydrogen: A critical and systematic review of developments, socio-technical systems and policy options |url=https://www.sciencedirect.com/science/article/pii/S2214629621003017?dgcid=coauthor |url-status=live |journal=[[Social:Energy Research & Social Science|Energy Research & Social Science]] |volume=80 |page=39 |article-number=102208 |bibcode=2021ERSS...8002208G |doi=10.1016/j.erss.2021.102208 |issn=2214-6296 |archive-url=https://web.archive.org/web/20211016205152/https://www.sciencedirect.com/science/article/abs/pii/S2214629621003017?dgcid=coauthor |archive-date=16 October 2021 |access-date=11 September 2021}}</ref>
+Producing one tonne of hydrogen through this process emits {{val|6.6|–|9.3}}{{nbsp}}tonnes of carbon dioxide.<ref name="Bonheure-2021">{{Cite web |last1=Bonheure |first1=Mike |last2=Vandewalle |first2=Laurien A. |last3=Marin |first3=Guy B. |last4=Van Geem |first4=Kevin M. |date=March 2021 |title=Dream or Reality? Electrification of the Chemical Process Industries |url=https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |url-status=live |archive-url=https://web.archive.org/web/20210717132733/https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |archive-date=17 July 2021 |access-date=6 July 2021 |website=CEP Magazine |publisher=American Institute of Chemical Engineers}}</ref> The production of natural gas feedstock also produces emissions such as vented and fugitive methane, which further contributes to the overall carbon footprint of hydrogen.<ref name="Griffiths-20212">{{Cite journal |last1=Griffiths |first1=Steve |last2=Sovacool |first2=Benjamin K. |last3=Kim |first3=Jinsoo |last4=Bazilian |first4=Morgan |last5=Uratani |first5=Joao M. |display-authors=4 |date=2021 |title=Industrial decarbonization via hydrogen: A critical and systematic review of developments, socio-technical systems and policy options |url=https://www.sciencedirect.com/science/article/pii/S2214629621003017?dgcid=coauthor |url-status=live |journal=Energy Research & Social Science |volume=80 |page=39 |article-number=102208 |bibcode=2021ERSS...8002208G |doi=10.1016/j.erss.2021.102208 |issn=2214-6296 |archive-url=https://web.archive.org/web/20211016205152/https://www.sciencedirect.com/science/article/abs/pii/S2214629621003017?dgcid=coauthor |archive-date=16 October 2021 |access-date=11 September 2021}}</ref>
-This reaction is favored at low pressures but is nonetheless conducted at high pressures{{nbsp}}({{cvt|2.0|MPa|atm inHg|lk=on|disp=sqbr}}) because high-pressure{{nbsp}}{{chem2|H2}} is the most marketable product, and [[Physics:Pressure swing adsorption|pressure swing adsorption]]{{nbsp}}(PSA) purification systems work better at higher pressures. The product mixture is known as "synthesis gas" because it is often used directly for the production of [[Chemistry:Methanol|methanol]] and many other compounds. [[Chemistry:Hydrocarbon|Hydrocarbon]]s other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly-optimized technology is the formation of [[Chemistry:Coke (fuel)|coke]] or carbon:
+This reaction is favored at low pressures but is nonetheless conducted at high pressures{{nbsp}}({{cvt|2.0|MPa|atm inHg|lk=on|disp=sqbr}}) because high-pressure{{nbsp}}{{chem2|H2}} is the most marketable product, and pressure swing adsorption{{nbsp}}(PSA) purification systems work better at higher pressures. The product mixture is known as "synthesis gas" because it is often used directly for the production of methanol and many other compounds. Hydrocarbons other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly-optimized technology is the formation of coke or carbon:
 {{bi|{{chem2|CH4 → C + 2 H2}}}}
-Therefore, steam reforming typically employs an excess of{{nbsp}}{{chem2|H2O}}. Additional hydrogen can be recovered from the steam by using carbon monoxide through the water gas shift reaction{{nbsp}}(WGS). This process requires an [[Chemistry:Iron oxide|iron oxide]] catalyst:<ref name="Oxtoby" />
+Therefore, steam reforming typically employs an excess of{{nbsp}}{{chem2|H2O}}. Additional hydrogen can be recovered from the steam by using carbon monoxide through the water gas shift reaction{{nbsp}}(WGS). This process requires an iron oxide catalyst:<ref name="Oxtoby" />
 {{bi|{{chem2|CO + H2O → CO2 + H2}}}}
-Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the [[Haber process|{{langr|de|Haber|cat=no}} process]] for [[Chemistry:Ammonia production|ammonia production]], hydrogen is generated from natural gas.<ref>{{cite web| last=Funderburg| first=E.| title=Why Are Nitrogen Prices So High?| publisher=The Samuel Roberts Noble Foundation| date=2008| url=http://www.noble.org/Ag/Soils/NitrogenPrices/Index.htm| access-date=11 March 2008| archive-url=https://web.archive.org/web/20010509065844/http://www.noble.org/ag/Soils/NitrogenPrices/Index.htm| archive-date=9 May 2001}}</ref>
+Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the process for ammonia production, hydrogen is generated from natural gas.<ref>{{cite web| last=Funderburg| first=E.| title=Why Are Nitrogen Prices So High?| publisher=The Samuel Roberts Noble Foundation| date=2008| url=http://www.noble.org/Ag/Soils/NitrogenPrices/Index.htm| access-date=11 March 2008| archive-url=https://web.archive.org/web/20010509065844/http://www.noble.org/ag/Soils/NitrogenPrices/Index.htm| archive-date=9 May 2001}}</ref>
 ====Partial oxidation of hydrocarbons====
@@ Line 699: / Line 703: @@
 {{bi|{{chem2|C + H2O → CO + H2}}}}
-Olefin production units may produce substantial quantities of byproduct hydrogen, particularly from cracking light feedstocks like [[Chemistry:Ethane|ethane]] or [[Chemistry:Propane|propane]].<ref>{{Cite journal |last=Hannula |first=Ilkka |date=2015 |title=Co-production of synthetic fuels and district heat from biomass residues, carbon dioxide and electricity: Performance and cost analysis |journal=Biomass and Bioenergy |volume=74 |pages=26–46 |doi=10.1016/j.biombioe.2015.01.006 |bibcode=2015BmBe...74...26H |issn=0961-9534}}</ref>
+Olefin production units may produce substantial quantities of byproduct hydrogen, particularly from cracking light feedstocks like ethane or propane.<ref>{{Cite journal |last=Hannula |first=Ilkka |date=2015 |title=Co-production of synthetic fuels and district heat from biomass residues, carbon dioxide and electricity: Performance and cost analysis |journal=Biomass and Bioenergy |volume=74 |pages=26–46 |doi=10.1016/j.biombioe.2015.01.006 |bibcode=2015BmBe...74...26H |issn=0961-9534}}</ref>
 ====Water electrolysis ====
 [[File:Hydrogen production via Electrolysis.png|thumb|Inputs and outputs of the electrolysis of water production of hydrogen]]
-[[Physics:Electrolysis of water|Electrolysis of water]] is a conceptually simple method of producing hydrogen.
+Electrolysis of water is a conceptually simple method of producing hydrogen.
 {{bi|{{chem2|2 H2O(l) → 2 H2(g) + O2(g)}}}}
-Commercial electrolyzers use [[Chemistry:Nickel|nickel]]-based catalysts in strongly alkaline solution. [[Chemistry:Platinum|Platinum]] is a better catalyst but is expensive.<ref>{{cite journal |doi=10.1038/ncomms5695 |title=Nanoscale nickel oxide/Nickel heterostructures for active hydrogen evolution electrocatalysis |date=2014 |last1=Gong |first1=Ming |last2=Zhou |first2=Wu |last3=Tsai |first3=Mon-Che |last4=Zhou |first4=Jigang |last5=Guan |first5=Mingyun |last6=Lin |first6=Meng-Chang |last7=Zhang |first7=Bo |last8=Hu |first8=Yongfeng |last9=Wang |first9=Di-Yan |last10=Yang |first10=Jiang |last11=Pennycook |first11=Stephen J. |last12=Hwang |first12=Bing-Joe |last13=Dai |first13=Hongjie |journal=Nature Communications |volume=5 |article-number=4695 |pmid=25146255 |bibcode=2014NatCo...5.4695G |s2cid=205329127 |doi-access=free }}</ref> The hydrogen created through electrolysis using renewable energy is commonly referred to as "[[Engineering:Green hydrogen|green hydrogen]]".<ref name="RoyalSociety-2021">{{Cite web |date=June 2021 |title=The role of hydrogen and ammonia in meeting the net zero challenge |url=https://royalsociety.org/-/media/policy/projects/climate-change-science-solutions/climate-science-solutions-hydrogen-ammonia.pdf |website=The Royal Society}}</ref>
+Commercial electrolyzers use nickel-based catalysts in strongly alkaline solution. Platinum is a better catalyst but is expensive.<ref>{{cite journal |doi=10.1038/ncomms5695 |title=Nanoscale nickel oxide/Nickel heterostructures for active hydrogen evolution electrocatalysis |date=2014 |last1=Gong |first1=Ming |last2=Zhou |first2=Wu |last3=Tsai |first3=Mon-Che |last4=Zhou |first4=Jigang |last5=Guan |first5=Mingyun |last6=Lin |first6=Meng-Chang |last7=Zhang |first7=Bo |last8=Hu |first8=Yongfeng |last9=Wang |first9=Di-Yan |last10=Yang |first10=Jiang |last11=Pennycook |first11=Stephen J. |last12=Hwang |first12=Bing-Joe |last13=Dai |first13=Hongjie |journal=Nature Communications |volume=5 |article-number=4695 |pmid=25146255 |bibcode=2014NatCo...5.4695G |s2cid=205329127 |doi-access=free }}</ref> The hydrogen created through electrolysis using renewable energy is commonly referred to as "green hydrogen".<ref name="RoyalSociety-2021">{{Cite web |date=June 2021 |title=The role of hydrogen and ammonia in meeting the net zero challenge |url=https://royalsociety.org/-/media/policy/projects/climate-change-science-solutions/climate-science-solutions-hydrogen-ammonia.pdf |website=The Royal Society}}</ref>
-[[Chemistry:Electrolysis|Electrolysis]] of [[Earth:Brine|brine]] to yield [[Chemistry:Chlorine|chlorine]]<ref>{{cite web| last=Lees| first=A.| title=Chemicals from salt| publisher=BBC|date=2007|url=http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml|access-date=11 March 2008|archive-url = https://web.archive.org/web/20071026052022/http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml |archive-date = 26 October 2007}}</ref> also produces high-purity hydrogen as a co-product, which is used for a variety of transformations such as [[Chemistry:Hydrogenation|hydrogenation]]s.<ref>{{Cite book |last1=Schmittinger |first1=Peter |chapter=Chlorine |date=2006-01-15 |title=Ullmann's Encyclopedia of Industrial Chemistry |place=Weinheim, Germany |publisher=Wiley-VCH Verlag GmbH & Co. KGaA |language=en |doi=10.1002/14356007.a06_399.pub2 |isbn=978-3-527-30673-2 |last2=Florkiewicz |first2=Thomas |last3=Curlin |first3=L. Calvert |last4=Lüke |first4=Benno |last5=Scannell |first5=Robert |last6=Navin |first6=Thomas |last7=Zelfel |first7=Erich |last8=Bartsch |first8=Rüdiger}}</ref>
+Electrolysis of brine to yield chlorine<ref>{{cite web| last=Lees| first=A.| title=Chemicals from salt| publisher=BBC|date=2007|url=http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml|access-date=11 March 2008|archive-url = https://web.archive.org/web/20071026052022/http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml |archive-date = 26 October 2007}}</ref> also produces high-purity hydrogen as a co-product, which is used for a variety of transformations such as hydrogenations.<ref>{{Cite book |last1=Schmittinger |first1=Peter |chapter=Chlorine |date=2006-01-15 |title=Ullmann's Encyclopedia of Industrial Chemistry |place=Weinheim, Germany |publisher=Wiley-VCH Verlag GmbH & Co. KGaA |language=en |doi=10.1002/14356007.a06_399.pub2 |isbn=978-3-527-30673-2 |last2=Florkiewicz |first2=Thomas |last3=Curlin |first3=L. Calvert |last4=Lüke |first4=Benno |last5=Scannell |first5=Robert |last6=Navin |first6=Thomas |last7=Zelfel |first7=Erich |last8=Bartsch |first8=Rüdiger}}</ref>
-The electrolysis process is more expensive than producing hydrogen from methane without [[Physics:Carbon capture and storage|carbon capture and storage]].<ref name="Evans-2020">{{Cite web |last1=Evans |first1=Simon |last2=Gabbatiss |first2=Josh |date=30 November 2020 |title=In-depth Q&A: Does the world need hydrogen to solve climate change? |url=https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |url-status=live |archive-url=https://web.archive.org/web/20201201155033/https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |archive-date=1 December 2020 |access-date=1 December 2020 |website=Carbon Brief}}</ref>
+The electrolysis process is more expensive than producing hydrogen from methane without carbon capture and storage.<ref name="Evans-2020">{{Cite web |last1=Evans |first1=Simon |last2=Gabbatiss |first2=Josh |date=30 November 2020 |title=In-depth Q&A: Does the world need hydrogen to solve climate change? |url=https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |url-status=live |archive-url=https://web.archive.org/web/20201201155033/https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |archive-date=1 December 2020 |access-date=1 December 2020 |website=Carbon Brief}}</ref>
 Innovation in hydrogen electrolyzers could make large-scale production of hydrogen from electricity more cost-competitive.<ref>{{Cite book|author1-link=International Energy Agency|last1=IEA|title=Net Zero by 2050: A Roadmap for the Global Energy Sector|year=2021|url=https://iea.blob.core.windows.net/assets/ad0d4830-bd7e-47b6-838c-40d115733c13/NetZeroby2050-ARoadmapfortheGlobalEnergySector.pdf|archive-date=23 May 2021|archive-url=https://web.archive.org/web/20210523155010/https://iea.blob.core.windows.net/assets/ad0d4830-bd7e-47b6-838c-40d115733c13/NetZeroby2050-ARoadmapfortheGlobalEnergySector.pdf|url-status=live
@@ Line 715: / Line 719: @@
 ====Methane pyrolysis====
-Hydrogen can be produced by [[Chemistry:Pyrolysis|pyrolysis]] of [[Chemistry:Natural gas|natural gas]] (methane), producing hydrogen gas and solid carbon with the aid of a catalyst and {{val|74|ul=kJ/mol}} input heat:
+Hydrogen can be produced by pyrolysis of natural gas (methane), producing hydrogen gas and solid carbon with the aid of a catalyst and {{val|74|ul=kJ/mol}} input heat:
 {{bi|{{chem2|CH4(g) → C(s) + 2 H2(g)}} (Δ''H''° {{=}} 74 kJ/mol)}}
 The carbon may be sold as a manufacturing feedstock or fuel, or landfilled. This route could have a lower carbon footprint than existing hydrogen production processes, but mechanisms for removing the carbon and preventing it from reacting with the catalyst remain obstacles for industrial-scale use.<ref>{{Cite journal |last1=Rasul |first1=M. G. |last2=Hazrat |first2=M. A |last3=Sattar |first3=M. A. |last4=Jahirul |first4=M. I. |last5=Shearer |first5=M. J. |date=2022-11-15 |title=The future of hydrogen: Challenges on production, storage and applications |url=https://linkinghub.elsevier.com/retrieve/pii/S0196890422011049 |journal=Energy Conversion and Management |volume=272 |article-number=116326 |doi=10.1016/j.enconman.2022.116326 |bibcode=2022ECM...27216326R |issn=0196-8904|url-access=subscription }}</ref>{{rp|17}}<ref>{{cite journal |last1=Schneider |first1=Stefan |title=State of the Art of Hydrogen Production via Pyrolysis of Natural Gas |journal=ChemBioEng Reviews |year=2020 |volume=7 |issue=5 |pages=150–158 |publisher=Wiley Online Library |doi=10.1002/cben.202000014 |doi-access=free }}</ref>
 ====Thermochemical====
-[[Chemistry:Water splitting|Water splitting]] is the process by which water is decomposed into its components. Relevant to the biological scenario is this equation:
+Water splitting is the process by which water is decomposed into its components. Relevant to the biological scenario is this equation:
 {{bi|{{chem2|2 H2O  ->  4 H+ + O2 + 4 e-}}}}
-The reaction occurs in the [[Biology:Light-dependent reactions|light-dependent reactions]] in all photosynthetic organisms. A few organisms, including the alga {{lang|la|[[Biology:Chlamydomonas reinhardtii|Chlamydomonas reinhardtii]]}} and [[Biology:Cyanobacteria|cyanobacteria]], have evolved a second step in the dark reactions in which protons and electrons are reduced to form {{chem2|H2}}{{nbsp}}gas by specialized [[Biology:Hydrogenase|hydrogenase]]s in the [[Biology:Chloroplast|chloroplast]].<ref>{{cite journal|last1=Kruse|first1=O.|last2=Rupprecht|first2=J.|last3=Bader|first3=K.|last4=Thomas-Hall|first4=S.|last5=Schenk|first5=P. M.|last6=Finazzi|first6=G.|last7=Hankamer|first7=B.|title=Improved photobiological H<sub>2</sub> production in engineered green algal cells|journal=The Journal of Biological Chemistry|date=2005|volume=280|issue=40|pages=34170–7|doi=10.1074/jbc.M503840200|pmid=16100118|s2cid=5373909|url=http://espace.library.uq.edu.au/view/UQ:75490/UQ75490_OA.pdf|access-date=24 August 2019|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015735/https://espace.library.uq.edu.au/data/UQ_75490/UQ75490_OA.pdf?Expires=1611885542&Key-Pair-Id=APKAJKNBJ4MJBJNC6NLQ&Signature=Qmpjq4YH0rwOJNqiSZ5M7-E5cYH~Dm2B-4kasb1eH66pVWPlvPNRj7TfcTKR1lDhF0--bkJdtE~yrSWwcZAA8FzxAA3MXY99mHTIOxyD3s73Dai1bwrLNuOkibXTVo6WbY5RKv7JAhXJ2sUV~TDIphC4Qikr0AWk5z-dwdY997n0NzcdTlqr0sn5n9WsOari3pJ0wRuL0w6Ged~HhrQ6ClrheilhtRo43U6HuaATFKEAuUM682rv4gvRCEVR1ljVOW0jwruB0SAJszTOZAbqNtb3V0SJh0x7wI8~ZZrp-XYqqzLDsWOB9w3ttyGSpLjcE2LvI7ty5vUljlfBGbnnLg__|url-status=live|doi-access=free}}</ref>
+The reaction occurs in the light-dependent reactions in all photosynthetic organisms. A few organisms, including the alga {{lang|la|Chlamydomonas reinhardtii}} and cyanobacteria, have evolved a second step in the dark reactions in which protons and electrons are reduced to form {{chem2|H2}}{{nbsp}}gas by specialized hydrogenases in the chloroplast.<ref>{{cite journal|last1=Kruse|first1=O.|last2=Rupprecht|first2=J.|last3=Bader|first3=K.|last4=Thomas-Hall|first4=S.|last5=Schenk|first5=P. M.|last6=Finazzi|first6=G.|last7=Hankamer|first7=B.|title=Improved photobiological H<sub>2</sub> production in engineered green algal cells|journal=The Journal of Biological Chemistry|date=2005|volume=280|issue=40|pages=34170–7|doi=10.1074/jbc.M503840200|pmid=16100118|s2cid=5373909|url=http://espace.library.uq.edu.au/view/UQ:75490/UQ75490_OA.pdf|access-date=24 August 2019|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015735/https://espace.library.uq.edu.au/data/UQ_75490/UQ75490_OA.pdf?Expires=1611885542&Key-Pair-Id=APKAJKNBJ4MJBJNC6NLQ&Signature=Qmpjq4YH0rwOJNqiSZ5M7-E5cYH~Dm2B-4kasb1eH66pVWPlvPNRj7TfcTKR1lDhF0--bkJdtE~yrSWwcZAA8FzxAA3MXY99mHTIOxyD3s73Dai1bwrLNuOkibXTVo6WbY5RKv7JAhXJ2sUV~TDIphC4Qikr0AWk5z-dwdY997n0NzcdTlqr0sn5n9WsOari3pJ0wRuL0w6Ged~HhrQ6ClrheilhtRo43U6HuaATFKEAuUM682rv4gvRCEVR1ljVOW0jwruB0SAJszTOZAbqNtb3V0SJh0x7wI8~ZZrp-XYqqzLDsWOB9w3ttyGSpLjcE2LvI7ty5vUljlfBGbnnLg__|url-status=live|doi-access=free}}</ref>
 Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to more efficiently generate {{chem2|H2}}{{nbsp}}gas even in the presence of oxygen.<ref>{{cite web
@@ Line 738: / Line 742: @@
 |archive-date= 29 December 2016
 |url-status= live
-}}</ref> Efforts have also been undertaken with genetically‐modified alga in a [[Biology:Bioreactor|bioreactor]].<ref>{{cite news| last=Williams| first=C.| title=Pond life: the future of energy| work=Science| publisher=The Register| date=24 February 2006| url=https://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| access-date=24 March 2008| archive-url=https://web.archive.org/web/20110509143055/http://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| archive-date=9 May 2011| url-status=live}}</ref>
+}}</ref> Efforts have also been undertaken with genetically‐modified alga in a bioreactor.<ref>{{cite news| last=Williams| first=C.| title=Pond life: the future of energy| work=Science| publisher=The Register| date=24 February 2006| url=https://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| access-date=24 March 2008| archive-url=https://web.archive.org/web/20110509143055/http://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| archive-date=9 May 2011| url-status=live}}</ref>
 Relevant to the thermal water-splitting scenario is this simple equation:
 {{bi|{{chem2|2 H2O  ->  2 H2 + O2}}}}
-Over 200 thermochemical cycles can be used for [[Chemistry:Water splitting|water splitting]]. Many of these cycles such as the [[Chemistry:Iron oxide cycle|iron oxide cycle]], [[Chemistry:Ceria based thermochemical cycles|cerium(IV) oxide–cerium(III) oxide cycle]], [[Chemistry:Zinc–zinc oxide cycle|zinc–zinc oxide cycle]], [[Chemistry:Sulfur–iodine cycle|sulfur–iodine cycle]], [[Chemistry:Copper–chlorine cycle|copper–chlorine cycle]] and [[Chemistry:Hybrid sulfur cycle|hybrid sulfur cycle]] have been evaluated for their commercial potential to produce hydrogen and oxygen from water and heat without using electricity.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|title=Development of solar-powered thermochemical production of hydrogen from water|first1=Al|last1=Weimer|date=25 May 2005|publisher=Solar Thermochemical Hydrogen Generation Project|access-date=21 December 2008|archive-url=https://web.archive.org/web/20070417134156/http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|archive-date=17 April 2007|url-status=live}}</ref> A number of labs (including in France, Germany, Greece, [[Earth:Japan|Japan]], and the [[United States]]) are developing thermochemical methods to produce hydrogen from solar energy and water.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|title=Development of Solar-Powered Thermochemical Production of Hydrogen from Water, DOE Hydrogen Program, 2007|author=Perret, R.|access-date=17 May 2008|archive-url=https://web.archive.org/web/20100527212241/http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|archive-date=27 May 2010}}</ref>
+Over 200 thermochemical cycles can be used for water splitting. Many of these cycles such as the iron oxide cycle, cerium(IV) oxide–cerium(III) oxide cycle, zinc–zinc oxide cycle, sulfur–iodine cycle, copper–chlorine cycle and hybrid sulfur cycle have been evaluated for their commercial potential to produce hydrogen and oxygen from water and heat without using electricity.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|title=Development of solar-powered thermochemical production of hydrogen from water|first1=Al|last1=Weimer|date=25 May 2005|publisher=Solar Thermochemical Hydrogen Generation Project|access-date=21 December 2008|archive-url=https://web.archive.org/web/20070417134156/http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|archive-date=17 April 2007|url-status=live}}</ref> A number of labs (including in France, Germany, Greece, Japan, and the United States) are developing thermochemical methods to produce hydrogen from solar energy and water.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|title=Development of Solar-Powered Thermochemical Production of Hydrogen from Water, DOE Hydrogen Program, 2007|author=Perret, R.|access-date=17 May 2008|archive-url=https://web.archive.org/web/20100527212241/http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|archive-date=27 May 2010}}</ref>
 ===Natural routes===
 ====Biohydrogen====
-  {{chem2|H2}} is produced in organisms by enzymes called [[Biology:Hydrogenase|hydrogenase]]s. This process allows the host organism to use [[Biology:Fermentation|fermentation]] as a source of energy.<ref>{{cite journal |doi=10.1021/cr050186q |title=&#91;NiFe&#93; and &#91;FeFe&#93; Hydrogenases Studied by Advanced Magnetic Resonance Techniques |date=2007 |last1=Lubitz |first1=Wolfgang |last2=Reijerse |first2=Eduard |last3=Van Gastel |first3=Maurice |journal=Chemical Reviews |volume=107 |issue=10 |pages=4331–4365 |pmid=17845059 }}</ref> These same enzymes also can oxidize{{nbsp}}H<sub>2</sub>, such that the host organisms can subsist by reducing oxidized substrates using electrons extracted from{{nbsp}}H<sub>2</sub>.<ref>{{cite journal |author1=Chris Greening |author2=Ambarish Biswas |author3=Carlo R Carere |author4=Colin J Jackson |author5=Matthew C Taylor |author6=Matthew B Stott |author7=Gregory M Cook |author8=Sergio E Morales |title=Genomic and metagenomic surveys of hydrogenase distribution indicate H2 is a widely utilised energy source for microbial growth and survival |journal=The ISME Journal |date=2016 |volume=10 |issue=3 |pages=761–777 |doi=10.1038/ismej.2015.153 |pmid=26405831 |pmc=4817680 |bibcode=2016ISMEJ..10..761G |language=en}}</ref>
+  {{chem2|H2}} is produced in organisms by enzymes called hydrogenases. This process allows the host organism to use fermentation as a source of energy.<ref>{{cite journal |doi=10.1021/cr050186q |title=&#91;NiFe&#93; and &#91;FeFe&#93; Hydrogenases Studied by Advanced Magnetic Resonance Techniques |date=2007 |last1=Lubitz |first1=Wolfgang |last2=Reijerse |first2=Eduard |last3=Van Gastel |first3=Maurice |journal=Chemical Reviews |volume=107 |issue=10 |pages=4331–4365 |pmid=17845059 }}</ref> These same enzymes also can oxidize{{nbsp}}H<sub>2</sub>, such that the host organisms can subsist by reducing oxidized substrates using electrons extracted from{{nbsp}}H<sub>2</sub>.<ref>{{cite journal |author1=Chris Greening |author2=Ambarish Biswas |author3=Carlo R Carere |author4=Colin J Jackson |author5=Matthew C Taylor |author6=Matthew B Stott |author7=Gregory M Cook |author8=Sergio E Morales |title=Genomic and metagenomic surveys of hydrogenase distribution indicate H2 is a widely utilised energy source for microbial growth and survival |journal=The ISME Journal |date=2016 |volume=10 |issue=3 |pages=761–777 |doi=10.1038/ismej.2015.153 |pmid=26405831 |pmc=4817680 |bibcode=2016ISMEJ..10..761G |language=en}}</ref>
-Hydrogenase enzymes feature [[Chemistry:Iron|iron]] or [[Astronomy:Iron–nickel alloy|iron–nickel]] centers at their [[Biology:Active site|active site]]s.<ref>{{cite book|first1=R.|last1=Cammack|url=https://books.google.com/books?id=GTzajKoBoNwC&pg=PA202|last2=Robson|first2=R. L.|date=2001|pages=202–203|title=Hydrogen as a Fuel: Learning from Nature|publisher=Taylor & Francis Ltd|isbn=978-0-415-24242-4|access-date=3 September 2020|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015731/https://books.google.com/books?id=GTzajKoBoNwC&pg=PA202|url-status=live}}</ref> The natural cycle of hydrogen production and consumption by organisms is called the [[Biology:Hydrogen cycle|hydrogen cycle]].<ref name="Rhee6">{{cite journal|last1=Rhee|first1=T. S.|last2=Brenninkmeijer|first2=C. A. M.|last3=Röckmann|first3=T.|title=The overwhelming role of soils in the global atmospheric hydrogen cycle|journal=Atmospheric Chemistry and Physics|date=19 May 2006|volume=6|issue=6|pages=1611–1625|doi=10.5194/acp-6-1611-2006|bibcode=2006ACP.....6.1611R|url=https://hal.archives-ouvertes.fr/hal-00301903/file/acpd-5-11215-2005.pdf|access-date=24 August 2019|archive-url=https://web.archive.org/web/20190824162153/https://hal.archives-ouvertes.fr/hal-00301903/file/acpd-5-11215-2005.pdf|archive-date=24 August 2019|url-status=live|doi-access=free}}</ref>
+Hydrogenase enzymes feature iron or iron–nickel centers at their active sites.<ref>{{cite book|first1=R.|last1=Cammack|url=https://books.google.com/books?id=GTzajKoBoNwC&pg=PA202|last2=Robson|first2=R. L.|date=2001|pages=202–203|title=Hydrogen as a Fuel: Learning from Nature|publisher=Taylor & Francis Ltd|isbn=978-0-415-24242-4|access-date=3 September 2020|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015731/https://books.google.com/books?id=GTzajKoBoNwC&pg=PA202|url-status=live}}</ref> The natural cycle of hydrogen production and consumption by organisms is called the hydrogen cycle.<ref name="Rhee6">{{cite journal|last1=Rhee|first1=T. S.|last2=Brenninkmeijer|first2=C. A. M.|last3=Röckmann|first3=T.|title=The overwhelming role of soils in the global atmospheric hydrogen cycle|journal=Atmospheric Chemistry and Physics|date=19 May 2006|volume=6|issue=6|pages=1611–1625|doi=10.5194/acp-6-1611-2006|bibcode=2006ACP.....6.1611R|url=https://hal.archives-ouvertes.fr/hal-00301903/file/acpd-5-11215-2005.pdf|access-date=24 August 2019|archive-url=https://web.archive.org/web/20190824162153/https://hal.archives-ouvertes.fr/hal-00301903/file/acpd-5-11215-2005.pdf|archive-date=24 August 2019|url-status=live|doi-access=free}}</ref>
 Some bacteria such as {{lang|la|Mycobacterium smegmatis}} can use the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking. Their hydrogenases feature small channels that exclude oxygen from the active site, permitting the reaction to occur even though the hydrogen concentration is very low and the oxygen concentration is as in normal air.<ref name=Grinter/><ref>{{cite journal |last1=Alex Wilkins |title=Soil bacteria enzyme generates electricity from hydrogen in the air |journal=New Scientist |date=Mar 8, 2023 |volume=257 |issue=3430 |page=13 |doi=10.1016/S0262-4079(23)00459-1 |bibcode=2023NewSc.257...13W |s2cid=257625443 |url=https://www.newscientist.com/article/2363552-soil-bacteria-enzyme-generates-electricity-from-hydrogen-in-the-air/}}</ref>
-Confirming the existence of hydrogenase‐employing microbes in the human gut, {{chem2|H2}} occurs in human breath. The concentration in the breath of fasting people at rest is typically under {{val|5|u=[[Parts per million|parts per million]]}}{{nbsp}}(ppm), but can reach {{val|50|u=ppm}} when people with intestinal disorders consume molecules they cannot absorb during diagnostic [[Physics:Hydrogen breath test|hydrogen breath test]]s.<ref>{{cite journal|doi=10.1088/1752-7155/2/4/046002|title=Implementation and interpretation of hydrogen breath tests|year=2008|last1=Eisenmann|first1=Alexander|last2=Amann|first2=Anton|last3=Said|first3=Michael|last4=Datta|first4=Bettina|last5=Ledochowski|first5=Maximilian|journal=Journal of Breath Research|volume=2|issue=4|article-number=046002|pmid=21386189|bibcode=2008JBR.....2d6002E|s2cid=31706721|url=http://pdfs.semanticscholar.org/2f16/5a981d54c41da92c1ae81af44021a88f1b95.pdf|access-date=26 December 2020|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015732/http://pdfs.semanticscholar.org/2f16/5a981d54c41da92c1ae81af44021a88f1b95.pdf}}</ref>
+Confirming the existence of hydrogenase‐employing microbes in the human gut, {{chem2|H2}} occurs in human breath. The concentration in the breath of fasting people at rest is typically under {{val|5|u=parts per million}}{{nbsp}}(ppm), but can reach {{val|50|u=ppm}} when people with intestinal disorders consume molecules they cannot absorb during diagnostic hydrogen breath tests.<ref>{{cite journal|doi=10.1088/1752-7155/2/4/046002|title=Implementation and interpretation of hydrogen breath tests|year=2008|last1=Eisenmann|first1=Alexander|last2=Amann|first2=Anton|last3=Said|first3=Michael|last4=Datta|first4=Bettina|last5=Ledochowski|first5=Maximilian|journal=Journal of Breath Research|volume=2|issue=4|article-number=046002|pmid=21386189|bibcode=2008JBR.....2d6002E|s2cid=31706721|url=http://pdfs.semanticscholar.org/2f16/5a981d54c41da92c1ae81af44021a88f1b95.pdf|access-date=26 December 2020|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015732/http://pdfs.semanticscholar.org/2f16/5a981d54c41da92c1ae81af44021a88f1b95.pdf}}</ref>
 ====Serpentinization====
-[[Earth:Serpentinization|Serpentinization]] is a geological mechanism which produces highly-reducing conditions.<ref name=FrostBeard2007>{{cite journal |last1=Frost |first1=B. R. |last2=Beard |first2=J. S. |title=On Silica Activity and Serpentinization |journal=Journal of Petrology |date=3 April 2007 |volume=48 |issue=7 |pages=1351–1368 |doi=10.1093/petrology/egm021|url=http://petrology.oxfordjournals.org/content/48/7/1351.full.pdf }}</ref> Under these conditions, water is capable of oxidizing ferrous{{nbsp}}({{chem|Fe|2+}}) ions in [[Chemistry:Fayalite|fayalite]], generating hydrogen gas:<ref name="Dincer-2015">{{Cite journal |last1=Dincer |first1=Ibrahim |last2=Acar |first2=Canan |date=14 September 2015 |title=Review and evaluation of hydrogen production methods for better sustainability |url=https://www.sciencedirect.com/science/article/pii/S0360319914034119 |url-status=live |journal=International Journal of Hydrogen Energy |language=en |volume=40 |issue=34 |pages=11094–11111 |bibcode=2015IJHE...4011094D |doi=10.1016/j.ijhydene.2014.12.035 |issn=0360-3199 |archive-url=https://web.archive.org/web/20220215183915/https://www.sciencedirect.com/science/article/abs/pii/S0360319914034119 |archive-date=15 February 2022 |access-date=4 February 2022|url-access=subscription }}</ref><ref>{{Cite journal| author1 = Sleep, N. H.| author2 = Meibom, A. | author3 = Fridriksson, Th. | author4 = Coleman, R. G. | author5 = Bird, D. K.| year = 2004| title = H<sub>2</sub>-rich fluids from serpentinization: Geochemical and biotic implications| journal = Proceedings of the National Academy of Sciences of the United States of America| volume = 101| issue = 35| pages = 12818–12823| doi = 10.1073/pnas.0405289101|bibcode = 2004PNAS..10112818S| pmid=15326313| pmc=516479| doi-access = free}}</ref>
+Serpentinization is a geological mechanism which produces highly-reducing conditions.<ref name=FrostBeard2007>{{cite journal |last1=Frost |first1=B. R. |last2=Beard |first2=J. S. |title=On Silica Activity and Serpentinization |journal=Journal of Petrology |date=3 April 2007 |volume=48 |issue=7 |pages=1351–1368 |doi=10.1093/petrology/egm021|url=http://petrology.oxfordjournals.org/content/48/7/1351.full.pdf }}</ref> Under these conditions, water is capable of oxidizing ferrous{{nbsp}}() ions in fayalite, generating hydrogen gas:<ref name="Dincer-2015">{{Cite journal |last1=Dincer |first1=Ibrahim |last2=Acar |first2=Canan |date=14 September 2015 |title=Review and evaluation of hydrogen production methods for better sustainability |url=https://www.sciencedirect.com/science/article/pii/S0360319914034119 |url-status=live |journal=International Journal of Hydrogen Energy |language=en |volume=40 |issue=34 |pages=11094–11111 |bibcode=2015IJHE...4011094D |doi=10.1016/j.ijhydene.2014.12.035 |issn=0360-3199 |archive-url=https://web.archive.org/web/20220215183915/https://www.sciencedirect.com/science/article/abs/pii/S0360319914034119 |archive-date=15 February 2022 |access-date=4 February 2022|url-access=subscription }}</ref><ref>{{Cite journal| author1 = Sleep, N. H.| author2 = Meibom, A. | author3 = Fridriksson, Th. | author4 = Coleman, R. G. | author5 = Bird, D. K.| year = 2004| title = H<sub>2</sub>-rich fluids from serpentinization: Geochemical and biotic implications| journal = Proceedings of the National Academy of Sciences of the United States of America| volume = 101| issue = 35| pages = 12818–12823| doi = 10.1073/pnas.0405289101|bibcode = 2004PNAS..10112818S| pmid=15326313| pmc=516479| doi-access = free}}</ref>
 {{bi|{{chem2|Fe2SiO4 + H2O → 2 Fe3O4 + SiO2 + H2}}}}
-Closely related to this geological process is the [[Schikorr reaction|{{langr|de|Schikorr|cat=no}} reaction]]:
+Closely related to this geological process is the reaction:
 {{bi|{{chem2|3 Fe(OH)2 → Fe3O4 + 2 H2O + H2}}}}
-This process also is relevant to the corrosion of [[Chemistry:Iron|iron]] and [[Chemistry:Steel|steel]] in [[Earth:Anoxic waters|oxygen-free]] [[Earth:Groundwater|groundwater]] and in reducing soils below the [[Earth:Water table|water table]].<ref>{{cite journal |author1=Stephan Kaufhold |author2=Stephen Klimke |author3=Stefan Schloemer |author4=Theodor Alpermann |author5=Franz Renz |author6=Reiner Dohrmann |title=About the Corrosion Mechanism of Metal Iron in Contact with Bentonite |journal=ACS Earth and Space Chemistry |date=2020 |volume=4 |issue=5 |pages=711–721 |doi=10.1021/acsearthspacechem.0c00005 |bibcode=2020ESC.....4..711K |language=en}}</ref>
+This process also is relevant to the corrosion of iron and steel in oxygen-free groundwater and in reducing soils below the water table.<ref>{{cite journal |author1=Stephan Kaufhold |author2=Stephen Klimke |author3=Stefan Schloemer |author4=Theodor Alpermann |author5=Franz Renz |author6=Reiner Dohrmann |title=About the Corrosion Mechanism of Metal Iron in Contact with Bentonite |journal=ACS Earth and Space Chemistry |date=2020 |volume=4 |issue=5 |pages=711–721 |doi=10.1021/acsearthspacechem.0c00005 |bibcode=2020ESC.....4..711K |language=en}}</ref>
 ===Laboratory syntheses===
-{{chem2|H2}} is produced in laboratory settings, such as in the small-scale [[Physics:Electrolysis of water|electrolysis of water]] using metal electrodes and water containing an [[Physics:Electrolyte|electrolyte]], which liberates hydrogen gas at the [[Physics:Cathode|cathode]]:<ref name="housecroft" />
+{{chem2|H2}} is produced in laboratory settings, such as in the small-scale electrolysis of water using metal electrodes and water containing an electrolyte, which liberates hydrogen gas at the cathode:<ref name="housecroft" />
 {{bi|{{chem2|2H+(aq) + 2 e− → H2(g)}}}}
-Hydrogen is also often a by-product of other reactions. Many metals react with water to produce{{nbsp}}{{chem2|H2}}, but the rate of hydrogen evolution depends on the metal, the pH, and the presence of alloying agents. Most often, hydrogen evolution is induced by acids. The alkali and alkaline earth metals as well as [[Chemistry:Aluminium|aluminium]], [[Chemistry:Zinc|zinc]], [[Chemistry:Manganese|manganese]], and [[Chemistry:Iron|iron]], react readily with aqueous acids.<ref name="housecroft">{{ cite book | title = Inorganic Chemistry | last1 = Housecroft | first1 = C. E. | last2 = Sharpe | first2 = A. G. | year = 2018 | publisher = Prentice Hall | edition = 5th | isbn = 978-1-292-13414-7 |pages = 219, 318–319}}</ref>
+Hydrogen is also often a by-product of other reactions. Many metals react with water to produce{{nbsp}}{{chem2|H2}}, but the rate of hydrogen evolution depends on the metal, the pH, and the presence of alloying agents. Most often, hydrogen evolution is induced by acids. The alkali and alkaline earth metals as well as aluminium, zinc, manganese, and iron, react readily with aqueous acids.<ref name="housecroft">{{ cite book | title = Inorganic Chemistry | last1 = Housecroft | first1 = C. E. | last2 = Sharpe | first2 = A. G. | year = 2018 | publisher = Prentice Hall | edition = 5th | isbn = 978-1-292-13414-7 |pages = 219, 318–319}}</ref>
 {{bi|{{chem2|Zn + 2 H+ → Zn(2+) + H2}}}}
-Many metals, such as aluminium, are slow to react with water because they form [[Engineering:Passivation|passivated]] oxide coatings. An alloy of aluminium and [[Chemistry:Gallium|gallium]], however, does react with water. In high-pH solutions, aluminium can react with {{nbsp}}{{chem2|H2}}:<ref name="housecroft" />
+Many metals, such as aluminium, are slow to react with water because they form passivated oxide coatings. An alloy of aluminium and gallium, however, does react with water. In high-pH solutions, aluminium can react with {{nbsp}}{{chem2|H2}}:<ref name="housecroft" />
 {{bi|{{chem2|2 Al + 6 H2O + 2 OH- → 2 [Al(OH)4]- + 3 H2}}}}
 ===Storage===
-If H<sub>2</sub> is to be used as an energy source, its storage is important. It dissolves only poorly in solvents. For example, at [[Physics:Room temperature|room temperature]] and {{convert|0.1|mPa|atm|abbr=out|lk=on}}, {{approx}}{{nbsp}}{{val|0.05|u=[[Mole (unit)|moles]]}} of hydrogen dissolve into {{convert|1|kg|abbr=out|spell=in}} of [[Chemistry:Diethyl ether|diethyl ether]].<ref name="UllmannH2">{{cite book |doi=10.1002/14356007.a13_297.pub3 |chapter=Hydrogen, 1. Properties and Occurrence |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2013 |last1=Lauermann |first1=Gerhard |last2=Häussinger |first2=Peter |last3=Lohmüller |first3=Reiner |last4=Watson |first4=Allan M. |pages=1–15 |isbn=978-3-527-30673-2 }}</ref> H<sub>2</sub> can be stored in compressed form, although compressing costs energy. Liquefaction is impractical given hydrogen's low [[Physics:Critical temperature|critical temperature]]. In contrast, ammonia and many hydrocarbons can be liquified at room temperature under pressure. For these reasons, hydrogen ''carriers''—materials that reversibly bind{{nbsp}}H<sub>2</sub>—have attracted much attention. The key question is then the weight percent of H<sub>2</sub>-equivalents within the carrier material. For example, hydrogen can be reversibly absorbed into many rare earths and [[Chemistry:Transition metal|transition metal]]s<ref name="Takeshita">{{cite journal
+If H<sub>2</sub> is to be used as an energy source, its storage is important. It dissolves only poorly in solvents. For example, at room temperature and {{convert|0.1|mPa|atm|abbr=out|lk=on}}, {{nbsp}}{{val|0.05|u=moles}} of hydrogen dissolve into {{convert|1|kg|abbr=out|spell=in}} of diethyl ether.<ref name="UllmannH2">{{cite book |doi=10.1002/14356007.a13_297.pub3 |chapter=Hydrogen, 1. Properties and Occurrence |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2013 |last1=Lauermann |first1=Gerhard |last2=Häussinger |first2=Peter |last3=Lohmüller |first3=Reiner |last4=Watson |first4=Allan M. |pages=1–15 |isbn=978-3-527-30673-2 }}</ref> H<sub>2</sub> can be stored in compressed form, although compressing costs energy. Liquefaction is impractical given hydrogen's low critical temperature. In contrast, ammonia and many hydrocarbons can be liquified at room temperature under pressure. For these reasons, hydrogen ''carriers''—materials that reversibly bind{{nbsp}}H<sub>2</sub>—have attracted much attention. The key question is then the weight percent of H<sub>2</sub>-equivalents within the carrier material. For example, hydrogen can be reversibly absorbed into many rare earths and transition metals<ref name="Takeshita">{{cite journal
 |last1=Takeshita|first1=T.
 |last2=Wallace|first2=W. E.
@@ Line 782: / Line 786: @@
 |date=1974
 |doi=10.1021/ic50139a050
-}}</ref> and is soluble in both nanocrystalline and [[Chemistry:Amorphous metal|amorphous metal]]s.<ref name="Kirchheim1">{{cite journal |last1=Kirchheim |first1=R. |last2=Mutschele |first2=T. |last3=Kieninger |first3=W. |title=Hydrogen in amorphous and nanocrystalline metals |journal=Materials Science and Engineering |date=1988 |volume=99 |issue=1–2 |pages=457–462 |doi=10.1016/0025-5416(88)90377-1 |last4=Gleiter |first4=H. |last5=Birringer |first5=R. |last6=Koble |first6=T. |bibcode=1988MSEng..99..457K }}</ref> Hydrogen [[Chemistry:Solubility|solubility]] in metals is influenced by local distortions or impurities in the [[Physics:Crystal lattice|crystal lattice]].<ref name="Kirchheim2">{{cite journal
+}}</ref> and is soluble in both nanocrystalline and amorphous metals.<ref name="Kirchheim1">{{cite journal |last1=Kirchheim |first1=R. |last2=Mutschele |first2=T. |last3=Kieninger |first3=W. |title=Hydrogen in amorphous and nanocrystalline metals |journal=Materials Science and Engineering |date=1988 |volume=99 |issue=1–2 |pages=457–462 |doi=10.1016/0025-5416(88)90377-1 |last4=Gleiter |first4=H. |last5=Birringer |first5=R. |last6=Koble |first6=T. |bibcode=1988MSEng..99..457K }}</ref> Hydrogen solubility in metals is influenced by local distortions or impurities in the crystal lattice.<ref name="Kirchheim2">{{cite journal
 |last=Kirchheim|first=R.
 |title=Hydrogen solubility and diffusivity in defective and amorphous metals
-|journal=[[Physics:Progress in Materials Science|Progress in Materials Science]]
+|journal=Progress in Materials Science
 |volume=32|issue=4|pages=262–325
 |date=1988
 |doi=10.1016/0079-6425(88)90010-2
 |bibcode=1988PrMS...32..261K
-}}</ref> These properties may be useful when hydrogen is purified by passage through hot [[Chemistry:Palladium|palladium]] disks, but the gas's high solubility is also a metallurgical problem, contributing to the [[Chemistry:Hydrogen embrittlement|embrittlement]] of many metals,<ref name="Rogers 1999 1057–1064">{{cite journal |last=Rogers|first=H. C. |title=Hydrogen Embrittlement of Metals |journal=Science |volume=159|issue=3819|pages=1057–1064 |date=1999 |doi=10.1126/science.159.3819.1057 |pmid=17775040 |bibcode=1968Sci...159.1057R |s2cid=19429952}}</ref> complicating the design of pipelines and storage tanks.<ref name="Christensen">{{cite news
+}}</ref> These properties may be useful when hydrogen is purified by passage through hot palladium disks, but the gas's high solubility is also a metallurgical problem, contributing to the embrittlement of many metals,<ref name="Rogers 1999 1057–1064">{{cite journal |last=Rogers|first=H. C. |title=Hydrogen Embrittlement of Metals |journal=Science |volume=159|issue=3819|pages=1057–1064 |date=1999 |doi=10.1126/science.159.3819.1057 |pmid=17775040 |bibcode=1968Sci...159.1057R |s2cid=19429952}}</ref> complicating the design of pipelines and storage tanks.<ref name="Christensen">{{cite news
 |last1=Christensen
 |first1=C. H.
@@ Line 807: / Line 811: @@
 }}</ref>
-The most problematic aspect of metal hydrides for storage is their modest H<sub>2</sub>{{nbsp}}content, often on the order of{{nbsp}}1%. For this reason, there is interest in storage of H<sub>2</sub> in compounds of low [[Physics:Molecular weight|molecular weight]]. For example, [[Chemistry:Ammonia borane|ammonia borane]] ({{chem2|H3N\sBH3}}) contains 19.8{{nbsp}}weight percent of{{nbsp}}H<sub>2</sub>. The problem with this material is that after release of H<sub>2</sub>, the resulting boron nitride does not re-add H<sub>2</sub>: i.e., ammonia borane is an irreversible hydrogen carrier.<ref>{{cite journal |doi=10.1016/j.gee.2022.03.011 |title=Ammonia borane-enabled hydrogen transfer processes: Insights into catalytic strategies and mechanisms |date=2023 |last1=Zhao |first1=Wenfeng |last2=Li |first2=Hu |last3=Zhang |first3=Heng |last4=Yang |first4=Song |last5=Riisager |first5=Anders |journal=Green Energy & Environment |volume=8 |issue=4 |pages=948–971 |bibcode=2023GrEE....8..948Z |doi-access=free }}</ref> More attractive are [[Chemistry:Hydrocarbon|hydrocarbon]]s such as [[Chemistry:Tetrahydroquinoline|tetrahydroquinoline]], which reversibly release some{{nbsp}}H<sub>2</sub> when heated in the presence of a catalyst:<ref>{{cite journal |doi=10.1021/acscatal.7b03547 |title=NHC-Based Iridium Catalysts for Hydrogenation and Dehydrogenation of N-Heteroarenes in Water under Mild Conditions |date=2018 |last1=Vivancos |first1=Ángela |last2=Beller |first2=Matthias |last3=Albrecht |first3=Martin |journal=ACS Catalysis |volume=8 |pages=17–21 }}</ref>
+The most problematic aspect of metal hydrides for storage is their modest H<sub>2</sub>{{nbsp}}content, often on the order of{{nbsp}}1%. For this reason, there is interest in storage of H<sub>2</sub> in compounds of low molecular weight. For example, ammonia borane ({{chem2|H3N\sBH3}}) contains 19.8{{nbsp}}weight percent of{{nbsp}}H<sub>2</sub>. The problem with this material is that after release of H<sub>2</sub>, the resulting boron nitride does not re-add H<sub>2</sub>: i.e., ammonia borane is an irreversible hydrogen carrier.<ref>{{cite journal |doi=10.1016/j.gee.2022.03.011 |title=Ammonia borane-enabled hydrogen transfer processes: Insights into catalytic strategies and mechanisms |date=2023 |last1=Zhao |first1=Wenfeng |last2=Li |first2=Hu |last3=Zhang |first3=Heng |last4=Yang |first4=Song |last5=Riisager |first5=Anders |journal=Green Energy & Environment |volume=8 |issue=4 |pages=948–971 |bibcode=2023GrEE....8..948Z |doi-access=free }}</ref> More attractive are hydrocarbons such as tetrahydroquinoline, which reversibly release some{{nbsp}}H<sub>2</sub> when heated in the presence of a catalyst:<ref>{{cite journal |doi=10.1021/acscatal.7b03547 |title=NHC-Based Iridium Catalysts for Hydrogenation and Dehydrogenation of N-Heteroarenes in Water under Mild Conditions |date=2018 |last1=Vivancos |first1=Ángela |last2=Beller |first2=Matthias |last3=Albrecht |first3=Martin |journal=ACS Catalysis |volume=8 |pages=17–21 }}</ref>
 {{bi|{{chem2|C9H10NH <-> C9H7N + 2 H2}}}}
@@ Line 815: / Line 819: @@
 === Petrochemical industry ===
-Large quantities of {{chem2|H2}} are used in the "upgrading" of fossil fuels. Key consumers of {{chem2|H2}} include [[Chemistry:Hydrodesulfurization|hydrodesulfurization]] and hydrocracking. Many of these reactions can be classified as [[Chemistry:Hydrogenolysis|hydrogenolysis]], i.e., the cleavage of bonds by hydrogen. Illustrative is the separation of sulfur from liquid fossil fuels:<ref name=KO>{{cite book |doi=10.1002/0471238961.0825041803262116.a01.pub2 |chapter=Hydrogen |title=Kirk-Othmer Encyclopedia of Chemical Technology |date=2001 |last1=Baade |first1=William F. |last2=Parekh |first2=Uday N. |last3=Raman |first3=Venkat S. |isbn=978-0-471-48494-3 }}</ref><ref name="UllmannHuse">{{cite book |author1=Peter Häussinger |author2=Reiner Lohmüller |author3=Allan M. Watson |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2011 |publisher=Wiley |isbn=978-3-527-30673-2 |language=en |chapter= Hydrogen, 6. Uses |doi=10.1002/14356007.o13_o07}}</ref>
+Large quantities of {{chem2|H2}} are used in the "upgrading" of fossil fuels. Key consumers of {{chem2|H2}} include hydrodesulfurization and hydrocracking. Many of these reactions can be classified as hydrogenolysis, i.e., the cleavage of bonds by hydrogen. Illustrative is the separation of sulfur from liquid fossil fuels:<ref name=KO>{{cite book |doi=10.1002/0471238961.0825041803262116.a01.pub2 |chapter=Hydrogen |title=Kirk-Othmer Encyclopedia of Chemical Technology |date=2001 |last1=Baade |first1=William F. |last2=Parekh |first2=Uday N. |last3=Raman |first3=Venkat S. |isbn=978-0-471-48494-3 }}</ref><ref name="UllmannHuse">{{cite book |author1=Peter Häussinger |author2=Reiner Lohmüller |author3=Allan M. Watson |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2011 |publisher=Wiley |isbn=978-3-527-30673-2 |language=en |chapter= Hydrogen, 6. Uses |doi=10.1002/14356007.o13_o07}}</ref>
 {{bi|{{chem2|R2S + 2 H2 → H2S + 2 RH}}}}
 === Hydrogenation ===
-[[Chemistry:Hydrogenation|Hydrogenation]], the addition of {{chem2|H2}} to various substrates, is done on a large scale. Hydrogenation of {{chem2|N2}} produces ammonia by the [[Haber process|{{langr|de|Haber|cat=no}} process]]:<ref name="UllmannHuse" />
+Hydrogenation, the addition of {{chem2|H2}} to various substrates, is done on a large scale. Hydrogenation of {{chem2|N2}} produces ammonia by the process:<ref name="UllmannHuse" />
 {{bi|{{chem2|N2 + 3 H2 → 2 NH3}}}}
-This process consumes a few percent of the energy budget in the entire industry and is the biggest consumer of hydrogen. The resulting ammonia is used extensively in [[Chemistry:Fertilizer|fertilizer]] production; these fertilizers have become essential feedstocks in modern agriculture.<ref name="Smil_2004_Enriching">{{cite book |last1=Smil |first1=Vaclav |title=Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production |date=2004 |publisher=MIT |location=Cambridge, MA |isbn=978-0-262-69313-4 |edition=1st}}</ref> Hydrogenation is also used to convert [[Chemistry:Unsaturated fat|unsaturated fat]]s and oils to saturated fats and oils. The major application is the production of margarine. [[Chemistry:Methanol|Methanol]] is produced by hydrogenation of carbon dioxide; the mixture of hydrogen and carbon dioxide used for this process is known as [[Chemistry:Syngas|syngas]]. It is similarly the source of hydrogen in the manufacture of [[Chemistry:Hydrochloric acid|hydrochloric acid]]. {{chem2|H2}} is also used as a [[Chemistry:Reducing agent|reducing agent]] for the conversion of some [[Earth:Ore|ore]]s to the metals.<ref>{{cite web|author=Chemistry Operations|date=15 December 2003|url=http://periodic.lanl.gov/1.shtml|title=Hydrogen|publisher=Los Alamos National Laboratory|access-date=5 February 2008|archive-url=https://web.archive.org/web/20110304203439/http://periodic.lanl.gov/1.shtml|archive-date=4 March 2011}}</ref><ref name="housecroft" />
+This process consumes a few percent of the energy budget in the entire industry and is the biggest consumer of hydrogen. The resulting ammonia is used extensively in fertilizer production; these fertilizers have become essential feedstocks in modern agriculture.<ref name="Smil_2004_Enriching">{{cite book |last1=Smil |first1=Vaclav |title=Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production |date=2004 |publisher=MIT |location=Cambridge, MA |isbn=978-0-262-69313-4 |edition=1st}}</ref> Hydrogenation is also used to convert unsaturated fats and oils to saturated fats and oils. The major application is the production of margarine. Methanol is produced by hydrogenation of carbon dioxide; the mixture of hydrogen and carbon dioxide used for this process is known as syngas. It is similarly the source of hydrogen in the manufacture of hydrochloric acid. {{chem2|H2}} is also used as a reducing agent for the conversion of some ores to the metals.<ref>{{cite web|author=Chemistry Operations|date=15 December 2003|url=http://periodic.lanl.gov/1.shtml|title=Hydrogen|publisher=Los Alamos National Laboratory|access-date=5 February 2008|archive-url=https://web.archive.org/web/20110304203439/http://periodic.lanl.gov/1.shtml|archive-date=4 March 2011}}</ref><ref name="housecroft" />
 === Fuel ===
-The potential for using hydrogen{{nbsp}}(H<sub>2</sub>) as a fuel has been widely discussed. Hydrogen can be used in fuel cells to produce electricity,<ref>{{cite journal |doi=10.1155/2024/7271748 |title=A Recent Comprehensive Review of Fuel Cells: History, Types, and Applications |date=2024 |last1=Qasem |first1=Naef A. A. |last2=Abdulrahman |first2=Gubran A. Q. |journal=International Journal of Energy Research |issue=1 |article-number=7271748 |doi-access=free |bibcode=2024IJER.202471748Q }}</ref> or burned to generate heat.<ref name="Lewis-2021">{{Cite journal |last=Lewis |first=Alastair C. |date=10 June 2021 |title=Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NO x emissions |journal=Environmental Science: Atmospheres |language=en |volume=1 |issue=5 |pages=201–207 |bibcode=2021ESAt....1..201L |doi=10.1039/D1EA00037C |doi-access=free}}{{Creative Commons text attribution notice|cc=by3|from this source=yes|url=|authors=|vrt=}}</ref> When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapor.<ref name="Lewis-2021" /> When burned, hydrogen produces relatively little pollution at the point of combustion, but can lead to thermal formation of harmful [[Earth:NOx|nitrogen oxides]].<ref name="Lewis-2021" />
+The potential for using hydrogen{{nbsp}}(H<sub>2</sub>) as a fuel has been widely discussed. Hydrogen can be used in fuel cells to produce electricity,<ref>{{cite journal |doi=10.1155/2024/7271748 |title=A Recent Comprehensive Review of Fuel Cells: History, Types, and Applications |date=2024 |last1=Qasem |first1=Naef A. A. |last2=Abdulrahman |first2=Gubran A. Q. |journal=International Journal of Energy Research |issue=1 |article-number=7271748 |doi-access=free |bibcode=2024IJER.202471748Q }}</ref> or burned to generate heat.<ref name="Lewis-2021">{{Cite journal |last=Lewis |first=Alastair C. |date=10 June 2021 |title=Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NO x emissions |journal=Environmental Science: Atmospheres |language=en |volume=1 |issue=5 |pages=201–207 |bibcode=2021ESAt....1..201L |doi=10.1039/D1EA00037C |doi-access=free}}</ref> When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapor.<ref name="Lewis-2021" /> When burned, hydrogen produces relatively little pollution at the point of combustion, but can lead to thermal formation of harmful nitrogen oxides.<ref name="Lewis-2021" />
-If hydrogen is produced with low or zero greenhouse gas emissions ([[Engineering:Green hydrogen|green hydrogen]]), it can play a significant role in decarbonizing energy systems where there are challenges and limitations to replacing fossil fuels with direct use of electricity.<ref name="IPCC-2022" /><ref name="Evans-2020" />
+If hydrogen is produced with low or zero greenhouse gas emissions (green hydrogen), it can play a significant role in decarbonizing energy systems where there are challenges and limitations to replacing fossil fuels with direct use of electricity.<ref name="IPCC-2022" /><ref name="Evans-2020" />
-Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals, thus contributing to the decarbonization of industry alongside other technologies, such as [[Physics:Electric arc furnace|electric arc furnace]]s for steelmaking.<ref>{{Cite web |last=Kjellberg-Motton |first=Brendan |date=2022-02-07 |title=Steel decarbonisation gathers speed {{!}} Argus Media |url=https://www.argusmedia.com/en//news/2299399-steel-decarbonisation-gathers-speed |access-date=2023-09-07 |website=www.argusmedia.com |language=en}}</ref> However, it is likely to play a larger role in providing industrial feedstock for cleaner production of ammonia and organic chemicals.<ref name="IPCC-2022">{{Cite book |author=IPCC |url=https://ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_FullReport.pdf |title=Climate Change 2022: Mitigation of Climate Change |publisher=Cambridge University Press (In Press) |year=2022 |isbn=978-1-009-15792-6 |editor1-last=Shukla |editor1-first=P.R. |series=Contribution of Working Group III to the [[Earth:IPCC Sixth Assessment Report|Sixth Assessment Report]] of the Intergovernmental Panel on Climate Change |place=Cambridge, UK and New York, NY, US |pages=91–92 |doi=10.1017/9781009157926 |ref={{harvid|IPCC AR6 WG3|2022}} |editor2-last=Skea |editor2-first=J. |editor3-last=Slade |editor3-first=R. |editor4-last=Al Khourdajie |editor4-first=A. |editor5-last=van Diemen |editor5-first=R. |editor6-last=McCollum |editor6-first=D. |editor7-last=Pathak |editor7-first=M. |editor8-last=Some |editor8-first=S. |editor9-last=Vyas |editor9-first=P. |display-editors=4 |editor10-first=R. |editor10-last=Fradera |editor11-first=M. |editor11-last=Belkacemi |editor12-first=A. |editor12-last=Hasija |editor13-first=G. |editor13-last=Lisboa |editor14-first=S. |editor14-last=Luz |editor15-first=J. |editor15-last=Malley}}</ref> For example, in [[Engineering:Steelmaking|steelmaking]], hydrogen could function as a clean fuel and also as a low-carbon catalyst, replacing coal-derived [[Chemistry:Coke (fuel)|coke]] (carbon):<ref>{{Cite web |last1=Blank |first1=Thomas |last2=Molly |first2=Patrick |date=January 2020 |title=Hydrogen's Decarbonization Impact for Industry |url=https://rmi.org/wp-content/uploads/2020/01/hydrogen_insight_brief.pdf |url-status=live |archive-url=https://web.archive.org/web/20200922115313/https://rmi.org/wp-content/uploads/2020/01/hydrogen_insight_brief.pdf |archive-date=22 September 2020 |publisher=Rocky Mountain Institute |pages=2, 7, 8}}</ref>
+Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals, thus contributing to the decarbonization of industry alongside other technologies, such as electric arc furnaces for steelmaking.<ref>{{Cite web |last=Kjellberg-Motton |first=Brendan |date=2022-02-07 |title=Steel decarbonisation gathers speed {{!}} Argus Media |url=https://www.argusmedia.com/en//news/2299399-steel-decarbonisation-gathers-speed |access-date=2023-09-07 |website=www.argusmedia.com |language=en}}</ref> However, it is likely to play a larger role in providing industrial feedstock for cleaner production of ammonia and organic chemicals.<ref name="IPCC-2022">{{Cite book |author=IPCC |url=https://ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_FullReport.pdf |title=Climate Change 2022: Mitigation of Climate Change |publisher=Cambridge University Press (In Press) |year=2022 |isbn=978-1-009-15792-6 |editor1-last=Shukla |editor1-first=P.R. |series=Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |place=Cambridge, UK and New York, NY, US |pages=91–92 |doi=10.1017/9781009157926 |ref={{harvid|IPCC AR6 WG3|2022}} |editor2-last=Skea |editor2-first=J. |editor3-last=Slade |editor3-first=R. |editor4-last=Al Khourdajie |editor4-first=A. |editor5-last=van Diemen |editor5-first=R. |editor6-last=McCollum |editor6-first=D. |editor7-last=Pathak |editor7-first=M. |editor8-last=Some |editor8-first=S. |editor9-last=Vyas |editor9-first=P. |display-editors=4 |editor10-first=R. |editor10-last=Fradera |editor11-first=M. |editor11-last=Belkacemi |editor12-first=A. |editor12-last=Hasija |editor13-first=G. |editor13-last=Lisboa |editor14-first=S. |editor14-last=Luz |editor15-first=J. |editor15-last=Malley}}</ref> For example, in steelmaking, hydrogen could function as a clean fuel and also as a low-carbon catalyst, replacing coal-derived coke (carbon):<ref>{{Cite web |last1=Blank |first1=Thomas |last2=Molly |first2=Patrick |date=January 2020 |title=Hydrogen's Decarbonization Impact for Industry |url=https://rmi.org/wp-content/uploads/2020/01/hydrogen_insight_brief.pdf |url-status=live |archive-url=https://web.archive.org/web/20200922115313/https://rmi.org/wp-content/uploads/2020/01/hydrogen_insight_brief.pdf |archive-date=22 September 2020 |publisher=Rocky Mountain Institute |pages=2, 7, 8}}</ref>
 {{bi|{{chem2|2FeO  +  C -> 2Fe  +  CO2}}}}
-{{bi|vs}}
 {{bi|{{chem2|FeO  +  H2 -> Fe  +  H2O}}}}
-Hydrogen used to decarbonize transportation is likely to find its largest applications in shipping, aviation and, to a lesser extent, heavy goods vehicles, through the use of hydrogen-derived synthetic fuels such as ammonia and methanol and fuel cell technology.<ref name="IPCC-2022" /> For light-duty vehicles including cars, hydrogen is far behind other [[Engineering:Alternative fuel vehicle|alternative fuel vehicle]]s, especially compared with the rate of adoption of battery electric vehicles, and may not play a significant role in future.<ref>{{Cite journal |last=Plötz |first=Patrick |date=2022-01-31 |title=Hydrogen technology is unlikely to play a major role in sustainable road transport |url=https://www.nature.com/articles/s41928-021-00706-6 |journal=Nature Electronics |language=en |volume=5 |issue=1 |pages=8–10 |doi=10.1038/s41928-021-00706-6 |s2cid=246465284 |issn=2520-1131|url-access=subscription }}</ref>
+Hydrogen used to decarbonize transportation is likely to find its largest applications in shipping, aviation and, to a lesser extent, heavy goods vehicles, through the use of hydrogen-derived synthetic fuels such as ammonia and methanol and fuel cell technology.<ref name="IPCC-2022" /> For light-duty vehicles including cars, hydrogen is far behind other alternative fuel vehicles, especially compared with the rate of adoption of battery electric vehicles, and may not play a significant role in future.<ref>{{Cite journal |last=Plötz |first=Patrick |date=2022-01-31 |title=Hydrogen technology is unlikely to play a major role in sustainable road transport |url=https://www.nature.com/articles/s41928-021-00706-6 |journal=Nature Electronics |language=en |volume=5 |issue=1 |pages=8–10 |doi=10.1038/s41928-021-00706-6 |s2cid=246465284 |issn=2520-1131|url-access=subscription }}</ref>
-[[File:Shuttle Main Engine Test Firing cropped edited and reduced.jpg|thumb|A [[Space Shuttle Main Engine]] burns hydrogen with oxygen, producing a nearly invisible flame at full thrust.|alt=A black inverted funnel with blue glow emerging from its opening.]]
+[[File:Shuttle Main Engine Test Firing cropped edited and reduced.jpg|thumb|A Space Shuttle Main Engine burns hydrogen with oxygen, producing a nearly invisible flame at full thrust.|alt=A black inverted funnel with blue glow emerging from its opening.]]
-[[Chemistry:Liquid hydrogen|Liquid hydrogen]] and [[Physics:Liquid oxygen|liquid oxygen]] together serve as cryogenic propellants in [[Engineering:Liquid-propellant rocket|liquid-propellant rocket]]s, as in the Space Shuttle main engines. [[Organization:NASA|NASA]] has investigated the use of [[Chemistry:Rocket propellant|rocket propellant]] made from atomic hydrogen, boron or carbon that is frozen into solid molecular hydrogen particles suspended in liquid helium. Upon warming, the mixture vaporizes to allow the atomic species to recombine, heating the mixture to high temperature.<ref>{{Cite web |url=https://ntrs.nasa.gov/api/citations/20030005922/downloads/20030005922.pdf |title=NASA/TM—2002-211915: Solid Hydrogen Experiments for Atomic Propellants |access-date=2 July 2021 |archive-date=9 July 2021 |archive-url=https://web.archive.org/web/20210709183557/https://ntrs.nasa.gov/api/citations/20030005922/downloads/20030005922.pdf |url-status=live }}</ref>
+Liquid hydrogen and liquid oxygen together serve as cryogenic propellants in liquid-propellant rockets, as in the Space Shuttle main engines. NASA has investigated the use of rocket propellant made from atomic hydrogen, boron or carbon that is frozen into solid molecular hydrogen particles suspended in liquid helium. Upon warming, the mixture vaporizes to allow the atomic species to recombine, heating the mixture to high temperature.<ref>{{Cite web |url=https://ntrs.nasa.gov/api/citations/20030005922/downloads/20030005922.pdf |title=NASA/TM—2002-211915: Solid Hydrogen Experiments for Atomic Propellants |access-date=2 July 2021 |archive-date=9 July 2021 |archive-url=https://web.archive.org/web/20210709183557/https://ntrs.nasa.gov/api/citations/20030005922/downloads/20030005922.pdf |url-status=live }}</ref>
-Hydrogen produced when there is a surplus of variable renewable electricity could in principle be stored and later used to generate heat or to re-generate electricity.<ref>{{Cite journal |last1=Palys |first1=Matthew J. |last2=Daoutidis |first2=Prodromos |date=2020 |title=Using hydrogen and ammonia for renewable energy storage: A geographically comprehensive techno-economic study |journal=[[Engineering:Computers & Chemical Engineering|Computers & Chemical Engineering]] |volume=136 |article-number=106785 |doi=10.1016/j.compchemeng.2020.106785 |issn=0098-1354 |doi-access=free}}</ref>  It can be further transformed into [[Chemistry:Synthetic fuel|synthetic fuel]]s such as [[Chemistry:Ammonia|ammonia]] and [[Chemistry:Methanol|methanol]].<ref>{{cite book |author=IRENA |url=https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2021/March/IRENA_World_Energy_Transitions_Outlook_2021.pdf |title=World Energy Transitions Outlook: 1.5°C Pathway |year=2021 |isbn=978-92-9260-334-2 |pages=12, 22 |archive-url=https://web.archive.org/web/20210611230855/https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2021/March/IRENA_World_Energy_Transitions_Outlook_2021.pdf |archive-date=11 June 2021 |url-status=live}}</ref> Disadvantages of hydrogen fuel include high costs of storage and distribution due to hydrogen's explosivity, its large volume compared to other fuels, and its tendency to [[Chemistry:Hydrogen embrittlement|embrittle materials]].<ref name="Griffiths-20212"/>
+Hydrogen produced when there is a surplus of variable renewable electricity could in principle be stored and later used to generate heat or to re-generate electricity.<ref>{{Cite journal |last1=Palys |first1=Matthew J. |last2=Daoutidis |first2=Prodromos |date=2020 |title=Using hydrogen and ammonia for renewable energy storage: A geographically comprehensive techno-economic study |journal=Computers & Chemical Engineering |volume=136 |article-number=106785 |doi=10.1016/j.compchemeng.2020.106785 |issn=0098-1354 |doi-access=free}}</ref>  It can be further transformed into synthetic fuels such as ammonia and methanol.<ref>{{cite book |author=IRENA |url=https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2021/March/IRENA_World_Energy_Transitions_Outlook_2021.pdf |title=World Energy Transitions Outlook: 1.5°C Pathway |year=2021 |isbn=978-92-9260-334-2 |pages=12, 22 |archive-url=https://web.archive.org/web/20210611230855/https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2021/March/IRENA_World_Energy_Transitions_Outlook_2021.pdf |archive-date=11 June 2021 |url-status=live}}</ref> Disadvantages of hydrogen fuel include high costs of storage and distribution due to hydrogen's explosivity, its large volume compared to other fuels, and its tendency to embrittle materials.<ref name="Griffiths-20212"/>
 === Nickel–hydrogen battery ===
-The very long-lived, rechargeable [[Physics:Nickel–hydrogen battery|nickel–hydrogen battery]] developed for satellite power systems uses pressurized gaseous{{nbsp}}H<sub>2</sub>.<ref>{{Cite book |last=Zimmerman |first=Albert H. |title=Nickel-hydrogen batteries: principles and practice |date=2009 |publisher=Aerospace press |isbn=978-1-884989-20-9 |location=El Segundo, Calif}}</ref> The [[Astronomy:International Space Station|International Space Station]],<ref>{{cite conference|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020070612_2002115777.pdf|work=IECEC '02. 2002 37th Intersociety Energy Conversion Engineering Conference, 2002|pages=45–50|date=July 2002|access-date=11 November 2011|doi=10.1109/IECEC.2002.1391972|title=Validation of international space station electrical performance model via on-orbit telemetry|last1=Jannette|first1=A. G.|last2=Hojnicki|first2=J. S.|last3=McKissock|first3=D. B.|last4=Fincannon|first4=J.|last5=Kerslake|first5=T. W.|last6=Rodriguez|first6=C. D.|isbn=0-7803-7296-4|archive-url=https://web.archive.org/web/20100514100504/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020070612_2002115777.pdf|archive-date=14 May 2010|url-status=live|hdl=2060/20020070612|hdl-access=free}}</ref> [[Astronomy:2001 Mars Odyssey|Mars Odyssey]]<ref>{{cite book|doi=10.1109/AERO.2002.1035418 |date=2002|last1=Anderson|first1=P. M.|last2=Coyne|first2=J. W.|title=Proceedings, IEEE Aerospace Conference |chapter=A lightweight, high reliability, single battery power system for interplanetary spacecraft |isbn=978-0-7803-7231-3|volume=5|pages=5–2433|s2cid=108678345}}</ref> and the [[Astronomy:Mars Global Surveyor|Mars Global Surveyor]]<ref>{{cite web|url=http://www.astronautix.com/craft/marveyor.htm|title=Mars Global Surveyor|publisher=Astronautix.com|access-date=6 April 2009|archive-url=https://web.archive.org/web/20090810180658/http://www.astronautix.com/craft/marveyor.htm|archive-date=10 August 2009}}</ref> are equipped with nickel-hydrogen batteries. In the dark part of its orbit, the [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]] is also powered by nickel-hydrogen batteries, which were finally replaced in May{{nbsp}}2009,<ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/servicing/SM4/main/SM4_Essentials.html|title=Hubble servicing mission 4 essentials|date=7 May 2009|editor=Lori Tyahla|access-date=19 May 2015|publisher=NASA|archive-url=https://web.archive.org/web/20150313073737/http://www.nasa.gov/mission_pages/hubble/servicing/SM4/main/SM4_Essentials.html|archive-date=13 March 2015|url-status=live}}</ref> more than 19{{nbsp}}years after launch and 13{{nbsp}}years beyond their design life.<ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/servicing/series/battery_story.html|title=Extending Hubble's mission life with new batteries|date=25 November 2008|first1=Susan|last1=Hendrix|editor=Lori Tyahla|access-date=19 May 2015|publisher=NASA|archive-url=https://web.archive.org/web/20160305002850/http://www.nasa.gov/mission_pages/hubble/servicing/series/battery_story.html|archive-date=5 March 2016|url-status=live}}</ref>
+The very long-lived, rechargeable nickel–hydrogen battery developed for satellite power systems uses pressurized gaseous{{nbsp}}H<sub>2</sub>.<ref>{{Cite book |last=Zimmerman |first=Albert H. |title=Nickel-hydrogen batteries: principles and practice |date=2009 |publisher=Aerospace press |isbn=978-1-884989-20-9 |location=El Segundo, Calif}}</ref> The International Space Station,<ref>{{cite conference|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020070612_2002115777.pdf|work=IECEC '02. 2002 37th Intersociety Energy Conversion Engineering Conference, 2002|pages=45–50|date=July 2002|access-date=11 November 2011|doi=10.1109/IECEC.2002.1391972|title=Validation of international space station electrical performance model via on-orbit telemetry|last1=Jannette|first1=A. G.|last2=Hojnicki|first2=J. S.|last3=McKissock|first3=D. B.|last4=Fincannon|first4=J.|last5=Kerslake|first5=T. W.|last6=Rodriguez|first6=C. D.|isbn=0-7803-7296-4|archive-url=https://web.archive.org/web/20100514100504/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020070612_2002115777.pdf|archive-date=14 May 2010|url-status=live|hdl=2060/20020070612|hdl-access=free}}</ref> Mars Odyssey<ref>{{cite book|doi=10.1109/AERO.2002.1035418 |date=2002|last1=Anderson|first1=P. M.|last2=Coyne|first2=J. W.|title=Proceedings, IEEE Aerospace Conference |chapter=A lightweight, high reliability, single battery power system for interplanetary spacecraft |isbn=978-0-7803-7231-3|volume=5|pages=5–2433|s2cid=108678345}}</ref> and the Mars Global Surveyor<ref>{{cite web|url=http://www.astronautix.com/craft/marveyor.htm|title=Mars Global Surveyor|publisher=Astronautix.com|access-date=6 April 2009|archive-url=https://web.archive.org/web/20090810180658/http://www.astronautix.com/craft/marveyor.htm|archive-date=10 August 2009}}</ref> are equipped with nickel-hydrogen batteries. In the dark part of its orbit, the Hubble Space Telescope is also powered by nickel-hydrogen batteries, which were finally replaced in May{{nbsp}}2009,<ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/servicing/SM4/main/SM4_Essentials.html|title=Hubble servicing mission 4 essentials|date=7 May 2009|editor=Lori Tyahla|access-date=19 May 2015|publisher=NASA|archive-url=https://web.archive.org/web/20150313073737/http://www.nasa.gov/mission_pages/hubble/servicing/SM4/main/SM4_Essentials.html|archive-date=13 March 2015|url-status=live}}</ref> more than 19{{nbsp}}years after launch and 13{{nbsp}}years beyond their design life.<ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/servicing/series/battery_story.html|title=Extending Hubble's mission life with new batteries|date=25 November 2008|first1=Susan|last1=Hendrix|editor=Lori Tyahla|access-date=19 May 2015|publisher=NASA|archive-url=https://web.archive.org/web/20160305002850/http://www.nasa.gov/mission_pages/hubble/servicing/series/battery_story.html|archive-date=5 March 2016|url-status=live}}</ref>
 === Semiconductor industry ===
-Hydrogen is employed in semiconductor manufacturing to saturate broken ("dangling") bonds of [[Chemistry:Amorphous silicon|amorphous silicon]] and [[Chemistry:Amorphous carbon|amorphous carbon]], which helps in stabilizing the materials' properties.<ref>{{cite journal
+Hydrogen is employed in semiconductor manufacturing to saturate broken ("dangling") bonds of amorphous silicon and amorphous carbon, which helps in stabilizing the materials' properties.<ref>{{cite journal
 |last1=Le Comber| first1=P. G.
 |title=Hall effect and impurity conduction in substitutionally doped amorphous silicon
@@ Line 854: / Line 858: @@
 |first2=D. I.
 |last3=Spear
-|first3=W. E.|bibcode = 1977PMag...35.1173C }}</ref> Hydrogen, introduced as an unintended side-effect of production, acts as a shallow [[Chemistry:Electron donor|electron donor]] leading to [[N-type semiconductor|{{nowr|n-type}}]] conductivity in ZnO, with important uses in transducers and phosphors.<ref>{{cite journal|last=Van de Walle|first=C. G.|title=Hydrogen as a cause of doping in zinc oxide|journal=Physical Review Letters|volume=85|issue=5|doi=10.1103/PhysRevLett.85.1012|pages=1012–1015|date=2000|pmid=10991462|bibcode=2000PhRvL..85.1012V|hdl=11858/00-001M-0000-0026-D0E6-E|url=http://pubman.mpdl.mpg.de/pubman/item/escidoc:741885/component/escidoc:932688/PRL-85-1012-2000.pdf|access-date=1 August 2018|archive-url=https://web.archive.org/web/20170815000602/http://pubman.mpdl.mpg.de/pubman/item/escidoc:741885/component/escidoc:932688/PRL-85-1012-2000.pdf|archive-date=15 August 2017|url-status=live|hdl-access=free}}</ref><ref>{{Cite journal |last1=Spencer |first1=Joseph A. |last2=Mock |first2=Alyssa L. |last3=Jacobs |first3=Alan G. |last4=Schubert |first4=Mathias |last5=Zhang |first5=Yuhao |last6=Tadjer |first6=Marko J. |date=2022-03-04 |title=A review of band structure and material properties of transparent conducting and semiconducting oxides: Ga2O3, Al2O3, In2O3, ZnO, SnO2, CdO, NiO, CuO, and Sc2O3 |url=https://pubs.aip.org/aip/apr/article-abstract/9/1/011315/2835450/A-review-of-band-structure-and-material-properties |journal=Applied Physics Reviews |volume=9 |issue=1 |page=011315 |doi=10.1063/5.0078037 |issn=1931-9401|url-access=subscription }}</ref> Detailed analysis of ZnO and of MgO shows evidence of four and six-fold hydrogen multicentre bonds.<ref>{{cite journal
+|first3=W. E.|bibcode = 1977PMag...35.1173C }}</ref> Hydrogen, introduced as an unintended side-effect of production, acts as a shallow electron donor leading to N-type semiconductor conductivity in ZnO, with important uses in transducers and phosphors.<ref>{{cite journal|last=Van de Walle|first=C. G.|title=Hydrogen as a cause of doping in zinc oxide|journal=Physical Review Letters|volume=85|issue=5|doi=10.1103/PhysRevLett.85.1012|pages=1012–1015|date=2000|pmid=10991462|bibcode=2000PhRvL..85.1012V|hdl=11858/00-001M-0000-0026-D0E6-E|url=http://pubman.mpdl.mpg.de/pubman/item/escidoc:741885/component/escidoc:932688/PRL-85-1012-2000.pdf|access-date=1 August 2018|archive-url=https://web.archive.org/web/20170815000602/http://pubman.mpdl.mpg.de/pubman/item/escidoc:741885/component/escidoc:932688/PRL-85-1012-2000.pdf|archive-date=15 August 2017|url-status=live|hdl-access=free}}</ref><ref>{{Cite journal |last1=Spencer |first1=Joseph A. |last2=Mock |first2=Alyssa L. |last3=Jacobs |first3=Alan G. |last4=Schubert |first4=Mathias |last5=Zhang |first5=Yuhao |last6=Tadjer |first6=Marko J. |date=2022-03-04 |title=A review of band structure and material properties of transparent conducting and semiconducting oxides: Ga2O3, Al2O3, In2O3, ZnO, SnO2, CdO, NiO, CuO, and Sc2O3 |url=https://pubs.aip.org/aip/apr/article-abstract/9/1/011315/2835450/A-review-of-band-structure-and-material-properties |journal=Applied Physics Reviews |volume=9 |issue=1 |page=011315 |doi=10.1063/5.0078037 |issn=1931-9401|url-access=subscription }}</ref> Detailed analysis of ZnO and of MgO shows evidence of four and six-fold hydrogen multicentre bonds.<ref>{{cite journal
 |last1=Janotti|first1= A.
 |title=Hydrogen multicentre bonds|doi=10.1038/nmat1795
@@ Line 878: / Line 882: @@
 === Niche and evolving uses ===
 Beyond than the uses mentioned above, hydrogen is used in smaller scales in the following applications:
-* Shielding gas: Hydrogen is used as a [[Chemistry:Shielding gas|shielding gas]] in [[Chemistry:Welding|welding]] methods such as [[Physics:Atomic hydrogen welding|atomic hydrogen welding]].<ref>{{cite journal
+* Shielding gas: Hydrogen is used as a shielding gas in welding methods such as atomic hydrogen welding.<ref>{{cite journal
 |last=Durgutlu| first=A.
 |title=Experimental investigation of the effect of hydrogen in argon as a shielding gas on TIG welding of austenitic stainless steel
@@ Line 888: / Line 892: @@
 |doi=10.1016/j.matdes.2003.07.004}}</ref><ref>{{Cite book |last1=Ujah |first1=Chika Oliver |url=https://onlinelibrary.wiley.com/doi/10.1002/9781394331925.ch6 |title=Advanced Welding Technologies |last2=N'Dedji Sodokin |first2=Rodolphe |last3=von Kallon |first3=Daramy Vandi |date=2025-05-05 |publisher=Wiley |isbn=978-1-394-33189-5 |editor-last=Kunar |editor-first=Sandip |edition=1 |pages=107–126 |language=en |chapter=Chapter 6 Atomic Hydrogen Welding |doi=10.1002/9781394331925.ch6 |editor-last2=Mandal |editor-first2=Gurudas}}</ref>
-* Coolant: Hydrogen is used as a [[Physics:Coolant|coolant]] in large electrical generators due to its high [[Physics:Thermal conductivity|thermal conductivity]] and low density.<ref>{{Cite conference |last1=Kumar |first1=Rajendar |last2=Kumar |first2=Ashwani |date=June 2015 |title=Assessment of impact of hydrogen cooled generator on power system loadability enhancement |conference=2015 International Conference on Energy, Power and Environment: Towards Sustainable Growth (ICEPE) |publisher=IEEE |pages=1–6 |doi=10.1109/EPETSG.2015.7510166 |isbn=978-1-4673-6503-1}}</ref> The first hydrogen-cooled turbogenerator went into service using gaseous hydrogen as a coolant in the rotor and the stator in{{nbsp}}1937 in Dayton, Ohio.<ref>{{cite book|url=https://archive.org/stream/chronologicalhis00natirich/chronologicalhis00natirich_djvu.txt|title=A chronological history of electrical development from 600 B.C|author=National Electrical Manufacturers Association|year=1946|page=102|publisher=New York, N.Y., National Electrical Manufacturers Association|access-date=9 February 2016|archive-url=https://web.archive.org/web/20160304141424/http://www.archive.org/stream/chronologicalhis00natirich/chronologicalhis00natirich_djvu.txt|archive-date=4 March 2016|url-status=live}}</ref>
+* Coolant: Hydrogen is used as a coolant in large electrical generators due to its high thermal conductivity and low density.<ref>{{Cite conference |last1=Kumar |first1=Rajendar |last2=Kumar |first2=Ashwani |date=June 2015 |title=Assessment of impact of hydrogen cooled generator on power system loadability enhancement |conference=2015 International Conference on Energy, Power and Environment: Towards Sustainable Growth (ICEPE) |publisher=IEEE |pages=1–6 |doi=10.1109/EPETSG.2015.7510166 |isbn=978-1-4673-6503-1}}</ref> The first hydrogen-cooled turbogenerator went into service using gaseous hydrogen as a coolant in the rotor and the stator in{{nbsp}}1937 in Dayton, Ohio.<ref>{{cite book|url=https://archive.org/stream/chronologicalhis00natirich/chronologicalhis00natirich_djvu.txt|title=A chronological history of electrical development from 600 B.C|author=National Electrical Manufacturers Association|year=1946|page=102|publisher=New York, N.Y., National Electrical Manufacturers Association|access-date=9 February 2016|archive-url=https://web.archive.org/web/20160304141424/http://www.archive.org/stream/chronologicalhis00natirich/chronologicalhis00natirich_djvu.txt|archive-date=4 March 2016|url-status=live}}</ref>
-* Cryogenic research: Liquid {{chem2|H2}} is used in cryogenic research, including [[Physics:Superconductivity|superconductivity]] studies.<ref>{{cite journal
+* Cryogenic research: Liquid {{chem2|H2}} is used in cryogenic research, including superconductivity studies.<ref>{{cite journal
 |last=Hardy
 |first=W. N.
@@ Line 898: / Line 902: @@
 |date=2003
 |doi=10.1016/S0921-4534(02)02591-1|bibcode = 2003PhyC..388....1H }}</ref>
-* Food industry: Hydrogen is an authorized [[Chemistry:Food additive|food additive]] ([[Chemistry:E number#E900–E999 (glazing agents, gases and sweeteners)|E949]])<ref>{{cite web |title=Report from the Commission on Dietary Food Additive Intake |url=http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf |url-status=live |archive-url=https://web.archive.org/web/20080216050325/http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf |archive-date=16 February 2008 |access-date=5 February 2008 |publisher=[[Social:European Union|European Union]]}}</ref> that is used as a packaging gas,<ref>{{Cite journal |last1=EFSA Panel on Food Additives and Flavourings (FAF) |last2=Castle |first2=Laurence |last3=Andreassen |first3=Monica |last4=Aquilina |first4=Gabriele |last5=Bastos |first5=Maria Lourdes |last6=Boon |first6=Polly |last7=Fallico |first7=Biagio |last8=FitzGerald |first8=Reginald |last9=Frutos Fernandez |first9=Maria Jose |last10=Grasl-Kraupp |first10=Bettina |last11=Gundert-Remy |first11=Ursula |last12=Gürtler |first12=Rainer |last13=Houdeau |first13=Eric |last14=Kurek |first14=Marcin |last15=Louro |first15=Henriqueta |date=2025 |title=Re-evaluation of oxygen (E 948) and hydrogen (E 949) as food additives |journal=EFSA Journal |language=en |volume=23 |issue=8 |article-number=e9595 |doi=10.2903/j.efsa.2025.9595 |issn=1831-4732 |pmc=12329423 |pmid=40777209}}</ref> and also has [[Biology:Antioxidant|antioxidant]] properties.<ref>{{Cite journal |last1=Yıldız |first1=Fatmanur |last2=LeBaron |first2=Tyler W. |last3=Alwazeer |first3=Duried |date=2025-03-01 |title=A comprehensive review of molecular hydrogen as a novel nutrition therapy in relieving oxidative stress and diseases: Mechanisms and perspectives |journal=Biochemistry and Biophysics Reports |volume=41 |article-number=101933 |doi=10.1016/j.bbrep.2025.101933 |issn=2405-5808 |pmc=11795818 |pmid=39911528}}</ref>
+* Food industry: Hydrogen is an authorized food additive (E949)<ref>{{cite web |title=Report from the Commission on Dietary Food Additive Intake |url=http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf |url-status=live |archive-url=https://web.archive.org/web/20080216050325/http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf |archive-date=16 February 2008 |access-date=5 February 2008 |publisher=European Union}}</ref> that is used as a packaging gas,<ref>{{Cite journal |last1=EFSA Panel on Food Additives and Flavourings (FAF) |last2=Castle |first2=Laurence |last3=Andreassen |first3=Monica |last4=Aquilina |first4=Gabriele |last5=Bastos |first5=Maria Lourdes |last6=Boon |first6=Polly |last7=Fallico |first7=Biagio |last8=FitzGerald |first8=Reginald |last9=Frutos Fernandez |first9=Maria Jose |last10=Grasl-Kraupp |first10=Bettina |last11=Gundert-Remy |first11=Ursula |last12=Gürtler |first12=Rainer |last13=Houdeau |first13=Eric |last14=Kurek |first14=Marcin |last15=Louro |first15=Henriqueta |date=2025 |title=Re-evaluation of oxygen (E 948) and hydrogen (E 949) as food additives |journal=EFSA Journal |language=en |volume=23 |issue=8 |article-number=e9595 |doi=10.2903/j.efsa.2025.9595 |issn=1831-4732 |pmc=12329423 |pmid=40777209}}</ref> and also has antioxidant properties.<ref>{{Cite journal |last1=Yıldız |first1=Fatmanur |last2=LeBaron |first2=Tyler W. |last3=Alwazeer |first3=Duried |date=2025-03-01 |title=A comprehensive review of molecular hydrogen as a novel nutrition therapy in relieving oxidative stress and diseases: Mechanisms and perspectives |journal=Biochemistry and Biophysics Reports |volume=41 |article-number=101933 |doi=10.1016/j.bbrep.2025.101933 |issn=2405-5808 |pmc=11795818 |pmid=39911528}}</ref>
-* Leak detection: Pure or mixed with nitrogen (sometimes called [[Physics:Forming gas|forming gas]]), hydrogen is a tracer gas for detection of minute leaks. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries;<ref>{{cite conference
+* Leak detection: Pure or mixed with nitrogen (sometimes called forming gas), hydrogen is a tracer gas for detection of minute leaks. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries;<ref>{{cite conference
 |first=M.
 |last=Block
@@ Line 913: / Line 917: @@
 }}</ref> it also allows for leak testing in food packaging.{{cn|date=February 2026}}
-* Neutron moderation: [[Physics:Deuterium|Deuterium]] (hydrogen-2) is used in [[Engineering:CANDU reactor|nuclear fission applications]] as a [[Physics:Neutron moderator|moderator to slow neutrons]].{{cn|date=February 2026}}
+* Neutron moderation: Deuterium (hydrogen-2) is used in nuclear fission applications as a moderator to slow neutrons.{{cn|date=February 2026}}
-* Nuclear fusion fuel: Deuterium is used in [[Physics:Nuclear fusion|nuclear fusion]] reactions.<ref name="nbb" />
+* Nuclear fusion fuel: Deuterium is used in nuclear fusion reactions.<ref name="nbb" />
-* Isotopic labeling: Deuterium compounds have applications in chemistry and biology in studies of [[Chemistry:Kinetic isotope effect|isotope effects]] on reaction rates.<ref>{{cite journal|last1=Reinsch| first1=J.|first2=A. |last2=Katz|first3=J.|last3=Wean|first4=G.|last4=Aprahamian|first5=J. T.|last5=MacFarland
+* Isotopic labeling: Deuterium compounds have applications in chemistry and biology in studies of isotope effects on reaction rates.<ref>{{cite journal|last1=Reinsch| first1=J.|first2=A. |last2=Katz|first3=J.|last3=Wean|first4=G.|last4=Aprahamian|first5=J. T.|last5=MacFarland
 |title=The deuterium isotope effect upon the reaction of fatty acyl-CoA dehydrogenase and butyryl-CoA| journal=J. Biol. Chem.|volume=255
 |issue=19|pages=9093–97|date=1980| doi=10.1016/S0021-9258(19)70531-6|pmid=7410413|doi-access=free}}</ref>
-* Tritium uses: [[Physics:Tritium|Tritium]] (hydrogen-3), produced in [[Physics:Nuclear reactor|nuclear reactor]]s, is used in the production of hydrogen bombs,<ref>{{cite journal| last=Bergeron| first=K. D.| title=The Death of no-dual-use| journal=Bulletin of the Atomic Scientists| volume=60| issue=1| pages=15–17| date=2004| url=http://find.galegroup.com/itx/start.do?prodId=SPJ.SP06| doi=10.2968/060001004| access-date=13 April 2008| archive-url=https://web.archive.org/web/20080419051641/http://find.galegroup.com/itx/start.do?prodId=SPJ.SP06| archive-date=19 April 2008| url-status=live| bibcode=2004BuAtS..60a..15B}}</ref> as an isotopic label in the biosciences,<ref name="holte" /> and as a source of beta radiation in [[Earth:Tritium radioluminescence|radioluminescent paint]] for instrument dials and emergency signage.<ref name="Traub95" />
+* Tritium uses: Tritium (hydrogen-3), produced in nuclear reactors, is used in the production of hydrogen bombs,<ref>{{cite journal| last=Bergeron| first=K. D.| title=The Death of no-dual-use| journal=Bulletin of the Atomic Scientists| volume=60| issue=1| pages=15–17| date=2004| url=http://find.galegroup.com/itx/start.do?prodId=SPJ.SP06| doi=10.2968/060001004| access-date=13 April 2008| archive-url=https://web.archive.org/web/20080419051641/http://find.galegroup.com/itx/start.do?prodId=SPJ.SP06| archive-date=19 April 2008| url-status=live| bibcode=2004BuAtS..60a..15B}}</ref> as an isotopic label in the biosciences,<ref name="holte" /> and as a source of beta radiation in radioluminescent paint for instrument dials and emergency signage.<ref name="Traub95" />
 == Safety and precautions ==
@@ Line 938: / Line 942: @@
   }}
 }}
-In hydrogen pipelines and steel storage vessels, hydrogen molecules are prone to reacting with metals, causing [[Chemistry:Hydrogen embrittlement|hydrogen embrittlement]] and leaks in the pipeline or storage vessel.<ref name="Li-2022">{{Cite journal |last1=Li |first1=Hao |last2=Cao |first2=Xuewen |last3=Liu |first3=Yang |last4=Shao |first4=Yanbo |last5=Nan |first5=Zilong |last6=Teng |first6=Lin |last7=Peng |first7=Wenshan |last8=Bian |first8=Jiang |date=2022-11-01 |title=Safety of hydrogen storage and transportation: An overview on mechanisms, techniques, and challenges |journal=Energy Reports |volume=8 |pages=6258–6269 |doi=10.1016/j.egyr.2022.04.067 |issn=2352-4847|doi-access=free |bibcode=2022EnRep...8.6258L }}Text was copied from this source, which is available under a [[Creativecommons:by/4.0/|Creative Commons Attribution 4.0 International License]]</ref> Since it is lighter than air, hydrogen does not easily accumulate to form a combustible gas mixture.<ref name="Li-2022" /> However, even without ignition sources, high-pressure hydrogen leakage may cause spontaneous combustion and [[Physics:Detonation|detonation]].<ref name="Li-2022" />
+In hydrogen pipelines and steel storage vessels, hydrogen molecules are prone to reacting with metals, causing hydrogen embrittlement and leaks in the pipeline or storage vessel.<ref name="Li-2022">{{Cite journal |last1=Li |first1=Hao |last2=Cao |first2=Xuewen |last3=Liu |first3=Yang |last4=Shao |first4=Yanbo |last5=Nan |first5=Zilong |last6=Teng |first6=Lin |last7=Peng |first7=Wenshan |last8=Bian |first8=Jiang |date=2022-11-01 |title=Safety of hydrogen storage and transportation: An overview on mechanisms, techniques, and challenges |journal=Energy Reports |volume=8 |pages=6258–6269 |doi=10.1016/j.egyr.2022.04.067 |issn=2352-4847|doi-access=free |bibcode=2022EnRep...8.6258L }}Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License</ref> Since it is lighter than air, hydrogen does not easily accumulate to form a combustible gas mixture.<ref name="Li-2022" /> However, even without ignition sources, high-pressure hydrogen leakage may cause spontaneous combustion and detonation.<ref name="Li-2022" />
 Hydrogen is flammable when mixed even in small amounts with air. Ignition can occur at a volumetric ratio of hydrogen to air as low as 4%.<ref>{{Cite journal |last1=Yang |first1=Fuyuan |last2=Wang |first2=Tianze |last3=Deng |first3=Xintao |last4=Dang |first4=Jian |last5=Huang |first5=Zhaoyuan |last6=Hu |first6=Song |last7=Li |first7=Yangyang |last8=Ouyang |first8=Minggao |date=2021-09-03 |title=Review on hydrogen safety issues: Incident statistics, hydrogen diffusion, and detonation process |url=https://linkinghub.elsevier.com/retrieve/pii/S0360319921025520 |journal=International Journal of Hydrogen Energy |volume=46 |issue=61 |pages=31467–31488 |doi=10.1016/j.ijhydene.2021.07.005 |bibcode=2021IJHE...4631467Y |issn=0360-3199|url-access=subscription }}</ref> In approximately 70% of hydrogen ignition accidents, the ignition source cannot be determined.<ref name="Li-2022" />
-Hydrogen fire, while being extremely hot, is almost invisible to the human eye, and thus can lead to accidental burns.<ref name="spinoff-2016">{{Cite web |date=2016 |title=Hydrogen Detection Tape Saves Time and Lives {{!}} NASA Spinoff |url=https://spinoff.nasa.gov/Spinoff2016/ps_5.html |access-date=2025-02-23 |website=spinoff.nasa.gov}}</ref> Hydrogen is non-toxic,<ref>{{Cite journal |last1=Abohamzeh |first1=Elham |last2=Salehi |first2=Fatemeh |last3=Sheikholeslami |first3=Mohsen |last4=Abbassi |first4=Rouzbeh |last5=Khan |first5=Faisal |date=2021-09-01 |title=Review of hydrogen safety during storage, transmission, and applications processes |journal=Journal of Loss Prevention in the Process Industries |volume=72 |article-number=104569 |doi=10.1016/j.jlp.2021.104569 |bibcode=2021JLPPI..7204569A |issn=0950-4230|doi-access=free }}</ref> but like most gases [[Chemistry:Inert gas asphyxiation|it can cause asphyxiation]] in the absence of adequate ventilation.<ref>{{Cite web |last=U.S. Department of Energy |title=Current Safe Operating Practices |url=https://www.energy.gov/eere/fuelcells/current-safe-operating-practices |access-date=2025-02-24 |website=Energy.gov |language=en}}</ref>
+Hydrogen fire, while being extremely hot, is almost invisible to the human eye, and thus can lead to accidental burns.<ref name="spinoff-2016">{{Cite web |date=2016 |title=Hydrogen Detection Tape Saves Time and Lives {{!}} NASA Spinoff |url=https://spinoff.nasa.gov/Spinoff2016/ps_5.html |access-date=2025-02-23 |website=spinoff.nasa.gov}}</ref> Hydrogen is non-toxic,<ref>{{Cite journal |last1=Abohamzeh |first1=Elham |last2=Salehi |first2=Fatemeh |last3=Sheikholeslami |first3=Mohsen |last4=Abbassi |first4=Rouzbeh |last5=Khan |first5=Faisal |date=2021-09-01 |title=Review of hydrogen safety during storage, transmission, and applications processes |journal=Journal of Loss Prevention in the Process Industries |volume=72 |article-number=104569 |doi=10.1016/j.jlp.2021.104569 |bibcode=2021JLPPI..7204569A |issn=0950-4230|doi-access=free }}</ref> but like most gases it can cause asphyxiation in the absence of adequate ventilation.<ref>{{Cite web |last=U.S. Department of Energy |title=Current Safe Operating Practices |url=https://www.energy.gov/eere/fuelcells/current-safe-operating-practices |access-date=2025-02-24 |website=Energy.gov |language=en}}</ref>
 =See also=
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 == Further reading ==
-{{Library resources box
- |onlinebooks=yes
- |by=no
- |lcheading= Hydrogen
- |label=Hydrogen
- }}
 * {{cite book| title=Chart of the Nuclides| edition=17th| publisher= Knolls Atomic Power Laboratory|date=2010| url=http://www.nuclidechart.com/|isbn=978-0-9843653-0-2}}
 * {{cite book|last=Newton|first=David E.|date=1994|title=The Chemical Elements|publisher=Franklin Watts|location=New York|isbn=978-0-531-12501-4|url=https://archive.org/details/chemicalelements00newt}}
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 * [http://hyperphysics.phy-astr.gsu.edu/Hbase/quantum/hydwf.html#c3 Wavefunction of hydrogen] at ''HyperPhysics'' (Georgia State University)
-{{Subject bar
-|portal1=Chemistry
-|portal2=Energy
-|book1=Hydrogen
-|book2=Period 1 elements
-|book3=Chemical elements (sorted&nbsp;alphabetically)
-|book4=Chemical elements (sorted by number)
-|commons=y
-|wikt=y
-|wikt-search=hydrogen
-|v=y
-|v-search=Hydrogen atom
-|b=y
-|b-search=Wikijunior:The Elements/Hydrogen
-}}
 {{Periodic table (navbox)}}
-{{Hydrogen compounds}}

Physics:Quantum atoms/hydrogen: Difference between revisions