An ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net electric charge.^[1]

A positively charged ion is called a cation.^[2]
A negatively charged ion is called an anion.^[3]

Ions play a central role in plasma physics, where matter exists as a collection of free electrons and ions.

Description

The charge of an electron is negative by convention, while the charge of a proton is positive. An ion has a nonzero net charge because the number of electrons is not equal to the number of protons.

A cation has fewer electrons than protons, giving it a positive charge. Examples include K⁺, Na⁺, and Li⁺. An anion has more electrons than protons, giving it a negative charge. Examples include Cl⁻, F⁻, and OH⁻.

Ions consisting of one atom are called monatomic ions or atomic ions. Ions consisting of two or more atoms are called polyatomic ions or molecular ions.

The net charge of an ion is written as a superscript. For example, Na⁺ indicates a sodium cation, F⁻ indicates a fluoride anion, and He²⁺ indicates a doubly charged helium ion.^[8]

Formation

Ions are formed through processes such as:

ionization, involving loss of electrons
electron attachment
electron transfer between atoms or molecules
collisions in high-energy environments
dissolution of salts in liquids
radiation interacting with gases or matter

In physical ionization in a gas or liquid, ion pairs may be created, consisting of a free electron and a positive ion.^[7]

In chemistry, monatomic ions are commonly formed by the gain or loss of electrons from the valence shell. Sodium, for example, tends to lose one electron:

N a ⟶ N a^{+} + e^{-}

Chlorine tends to gain one electron:

C l + e^{-} ⟶ C l^{-}

The resulting sodium and chloride ions can combine to form sodium chloride:

N a^{+} + C l^{-} ⟶ N a C l

Cations and anions

An anion is an ion with more electrons than protons, giving it a net negative charge.^[9]

A cation is an ion with fewer electrons than protons, giving it a net positive charge.^[10]

The words anion and cation come from Greek roots connected with motion toward electrodes. They were introduced by Michael Faraday following consultation with William Whewell.^[11]

Because electrons are light and spread out as matter waves, anions are usually larger than their neutral parent atoms, while cations are usually smaller. A hydrogen cation, H⁺, is essentially a bare proton.

Ionic bonding

Oppositely charged ions attract through electrostatic forces. This attraction can form ionic compounds and crystal lattices. Ionic bonding is especially common between metals, which tend to lose electrons, and nonmetals, which tend to gain electrons.

Ionic compounds contain characteristic distances between neighboring ions, from which ionic radii may be inferred.

Ionization potential

The energy required to remove an electron from an atom or molecule is called the ionization potential or ionization energy. Each successive ionization energy is usually larger than the previous one, especially after a stable shell or subshell has been emptied.

Sodium commonly forms Na⁺ because it has one valence electron outside a stable inner configuration. Chlorine commonly forms Cl⁻ because it needs one electron to complete a stable valence shell. Caesium has one of the lowest measured ionization energies among the elements, while helium has one of the highest.^[12]

Role in plasma physics

In plasmas:

ions and electrons move collectively
long-range electromagnetic forces dominate
charge neutrality is approximately maintained
collisions, ionization, and recombination control the plasma state

The behavior of ions is described by kinetic theory and fluid models such as magnetohydrodynamics.

Ions in plasmas can be accelerated, confined, heated, and transported by electric and magnetic fields. Their motion is central to fusion plasmas, astrophysical plasmas, ion beams, and space physics.

Detection of ionizing radiation

The ionizing effect of radiation in gases is widely used for radiation detection. Radiation can create ion pairs, and an applied electric field can collect the charges. The ionization chamber is one of the simplest detectors.^[7]

Devices such as Geiger–Müller tubes and proportional counters use ionization and electron multiplication to detect alpha particles, beta particles, gamma rays, and X-rays.

Natural and technological occurrence

Ions occur throughout nature. They are important in seawater, biological systems, atmospheric chemistry, the ionosphere, plasmas, and stellar environments.

Ions are also used in technology, including:

mass spectrometers
particle accelerators
ion sources
ion implantation
ion engines
optical emission spectrometers
air ionisers
radiation detectors

In biological systems, ion gradients across cell membranes are essential for signaling, metabolism, and cell function.

History

The word ion was introduced by Michael Faraday in 1834 for charged species that move through an aqueous medium between electrodes.^[13]^[14]

In 1884, Svante Arrhenius explained that crystalline salts dissociate into charged particles when dissolved. This work contributed to his 1903 Nobel Prize in Chemistry.^[15]^[16]^[17]^[18]

Physical interpretation

An ion represents a deviation from electrical neutrality at the atomic or molecular level. Its dynamics are governed by electromagnetic forces and play a fundamental role in plasma physics, chemistry, condensed matter physics, radiation detection, and biological systems.