Physics:Quantum atomic nucleus: Difference between revisions
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Protons repel each other through the electromagnetic force. A nucleus can exist because the residual strong force, also called the nuclear force, is attractive at typical nucleon separations and is stronger than the proton-proton electric repulsion at short distances. | Protons repel each other through the electromagnetic force. A nucleus can exist because the residual strong force, also called the nuclear force, is attractive at typical nucleon separations and is stronger than the proton-proton electric repulsion at short distances. | ||
Revision as of 07:25, 20 May 2026
The atomic nucleus is the compact, dense quantum system at the center of an atom. It consists mainly of protons and neutrons, collectively called nucleons. Nearly all of an atom's mass is concentrated in the nucleus, while the surrounding electrons occupy quantum states in the much larger atomic region.
The nucleus is not a small classical cluster of hard balls. In modern nuclear physics it is described as a many-body quantum system whose constituents occupy allowed nuclear states and interact through the residual strong force. The number of protons determines the chemical element, while different numbers of neutrons give rise to isotopes.
Discovery and basic idea
The atomic nucleus was discovered by Ernest Rutherford in 1911, following the Geiger–Marsden gold foil experiment. The large-angle scattering of alpha particles showed that most of the atom's mass and positive charge must be concentrated in a very small central region, rather than spread through the atom as in Thomson's earlier plum-pudding model.
After the discovery of the neutron in 1932, nuclear models based on protons and neutrons were developed rapidly by Dmitri Ivanenko and Werner Heisenberg.[1][2]
Composition
A nucleus contains protons and neutrons. Protons carry positive electric charge, while neutrons are electrically neutral. Both are composite particles made of quarks, but in low-energy nuclear physics they are usually treated as nucleons interacting through an effective nuclear force.
The number of protons is the atomic number Z and defines the element. The total number of protons and neutrons is the mass number A. The number of neutrons is therefore
Neutrons help reduce the relative importance of electrostatic repulsion between protons and make many nuclei stable. Nuclei with the same proton number but different neutron number are isotopes.
Size and density
Nuclear dimensions are measured in femtometres, where
The diameter of a nucleus ranges from about the size of a single proton in hydrogen to roughly ten femtometres for heavy nuclei such as uranium.[3] This is tens of thousands of times smaller than the full atom, whose size is set mainly by the electron cloud.
For many stable nuclei, the nuclear radius is approximately
where A is the mass number and is about 1.25 fm.[4] This relation reflects the nearly constant density of nuclear matter.
Nuclear force
Protons repel each other through the electromagnetic force. A nucleus can exist because the residual strong force, also called the nuclear force, is attractive at typical nucleon separations and is stronger than the proton-proton electric repulsion at short distances.
The nuclear force is short-ranged. It acts over distances of only a few femtometres and falls rapidly outside the nucleus. This limited range explains why very large nuclei become unstable: the attractive nuclear force does not grow indefinitely with nuclear size, while electric repulsion between many protons becomes increasingly important.
Quantum structure
The atomic nucleus is a quantum many-body system. Protons and neutrons are fermions, and their allowed states are constrained by the Pauli exclusion principle. Nuclear states can be organized into shells, somewhat analogous to electron shells in atoms, although the nuclear potential and forces are different.
The nuclear shell model explains special stability at certain proton or neutron numbers called magic numbers. Other models emphasize collective or clustered behavior, including liquid-drop models, cluster models, and deformed nuclear shapes.
Nuclear shapes
Many nuclei are approximately spherical, but nuclei can also be deformed. Possible shapes include prolate forms, oblate forms, triaxial shapes, and pear-shaped nuclei. Such deformations are important in nuclear spectroscopy, collective excitations, and searches for physics beyond the Standard Model.[5]
Halo nuclei
Near the limits of nuclear stability, some nuclei form halo structures. In a halo nucleus, one or more weakly bound nucleons occupy spatially extended quantum states far outside the compact nuclear core. Examples include neutron-rich nuclei such as lithium-11. These systems show clearly that a nucleus is not simply a dense classical object, but a quantum system with wave-like spatial structure.
Nuclear models
Several complementary models are used to describe nuclei:
- The liquid-drop model treats the nucleus as a dense drop of nuclear matter and helps explain bulk properties such as binding energy and fission.
- The shell model describes nucleons occupying quantum orbitals and explains magic numbers and many patterns of nuclear stability.
- The cluster model describes some nuclei as arrangements of substructures such as alpha-particle clusters.
- Modern ab initio and effective-field-theory methods attempt to derive nuclear properties from nucleon interactions connected to quantum chromodynamics.
No single simple model describes all nuclear phenomena. Nuclear physics therefore uses several models, each suited to a different range of nuclei and energies.
See also
Table of contents (84 articles)
Index
Full contents
References
- ↑ Iwanenko, D. (1932). "The Neutron Hypothesis". Nature 129 (3265): 798. doi:10.1038/129798d0. Bibcode: 1932Natur.129..798I.
- ↑ Heisenberg, W. (1932). "Über den Bau der Atomkerne. I". Z. Phys. 77 (1–2): 1–11. doi:10.1007/BF01342433. Bibcode: 1932ZPhy...77....1H.
- ↑ Angeli, I.; Marinova, K. P. (January 2013). "Table of experimental nuclear ground state charge radii: An update". Atomic Data and Nuclear Data Tables 99 (1): 69–95. doi:10.1016/j.adt.2011.12.006. Bibcode: 2013ADNDT..99...69A.
- ↑ Krane, Kenneth S. (1987). Introductory Nuclear Physics. Wiley. ISBN 978-0-471-80553-3.
- ↑ Gaffney, L. P.; Butler, P. A.; Scheck, M.; Hayes, A. B. (2013). "Studies of pear-shaped nuclei using accelerated radioactive beams". Nature 497 (7448): 199–204. doi:10.1038/nature12073. PMID 23657348. Bibcode: 2013Natur.497..199G.
Source attribution: Physics:Quantum atomic nucleus