nuclides

Nuclides are distinct species of atomic nuclei, each characterized by its proton number Z and neutron number N. A nuclide may be stable for long times or radioactive, in which case it undergoes spontaneous decay. Isotopes are nuclides that share the same Z (the same element) but have different numbers of neutrons.

Nuclei are commonly described by the mass number A = Z + N and the chemical symbol, written as either 60Co for cobalt-60 or, in full notation, with Z and N. The term nuclide emphasizes the identity of a nucleus, whereas isotopes highlight the relationship between nuclides of the same element. There are about 7,000 known nuclides, far more than the 118 known elements.

Stability in nuclei is governed by the balance of nuclear binding energy and Coulomb repulsion among protons. The valley of stability is the region in a Z–N chart where nuclides have relatively long lifetimes; nuclides outside tend to decay toward it. Common decay modes include alpha decay (emission of a helium-4 nucleus), beta minus decay (a neutron converts to a proton with emission of an electron and an antineutrino), beta plus decay (proton to neutron with a positron and neutrino), electron capture, and gamma decay (emission of photons). The characteristic time scale is the half-life, related to the decay constant; activity is measured in becquerels or curies.

Nuclear reactions—fusion, fission, neutron capture, and spallation—change nuclide identities. Models such as the liquid drop model and the nuclear shell model help explain binding energies and stability. Properties of nuclides, including binding energy, spin, parity, and decay schemes, underpin many applications in medicine (diagnostic tracers and radiotherapy), dating (carbon-14), industry, and energy.