nuclei

Nuclei are the central regions of atoms, composed of protons and neutrons, collectively known as nucleons. They contain most of an atom's mass and essentially all of its positive electric charge, while the surrounding electron cloud determines chemical behavior and size.

Nuclei vary in size approximately with radius R roughly equal to R0 times A to the one-third power, where R0 is about 1.2 femtometers and A the total number of nucleons. The nucleons are bound by the strong nuclear force, a short-range interaction that overcomes the electrostatic repulsion between protons. The binding energy per nucleon is roughly a few MeV, producing a small mass defect relative to the sum of free nucleons.

Nuclei are identified by Z, the number of protons, and N, the number of neutrons, together forming the isotope. Stable and unstable nuclei exist; unstable ones decay by processes such as alpha decay, beta decay, or gamma emission, with characteristic half-lives. Stability trends depend on the neutron-to-proton ratio and on closed-shell, or magic, numbers of nucleons.

Theoretical models include the nuclear shell model, which describes energy levels for nucleons in a mean field, and the liquid-drop model, which treats the nucleus as a finite drop of incompressible nuclear matter. Reactions among nuclei--fusion, fission, scattering--are studied in experiments with accelerators and detectors. Nuclei underpin applications in energy production, medicine, and basic science.