<physics> A species of atom characterised by the constitution of its nucleus, which is specified by the number of protons (Z), number of neutrons (N) and energy content or, alternatively, by the atomic number (Z), mass number (A = N + Z) and atomic mass, to be regarded as a distinct nuclide, an atom must be capable of existing for a measurable time, thus, nuclear isomers are separate nuclides, whereas promptly decaying excited nuclear states and unstable intermediates in nuclear reactions are not.

(16 Dec 1997)