nuclide

A nuclide is a distinct species of atomic nucleus defined by its numbers of protons and neutrons. The proton number is Z and the neutron number is N, and the mass number A equals Z plus N. Nuclide notation typically records A and Z and the chemical symbol, for example carbon-14 has A=14, Z=6, and symbol 14C.

An element can have many nuclides with differing N; those with the same Z but different N are called isotopes of that element. Conversely, nuclides with different Z are different elements. Examples include hydrogen-1, hydrogen-2 (deuterium), hydrogen-3 (tritium); carbon-12, carbon-13, carbon-14; uranium-235 and uranium-238.

Nuclides may be stable or radioactive. Stable nuclides persist indefinitely, while radioactive nuclides decay by emitting particles or radiation, transforming into other nuclides with different Z or N. Decay modes include alpha, beta, and gamma emission, each with characteristic half-lives ranging from fractions of a second to billions of years. The binding energy per nucleon and the mass defect influence stability; nuclides near iron-56 tend to be more tightly bound.

On a chart of nuclides (a two-dimensional plot of N vs Z), stable nuclides form the valley of stability. Nuclide data underpin applications in dating (carbon-14), medicine (technetium-99m), industry, and energy (fission of heavy nuclides, fusion of light ones). Nuclear reactions—whether in stars or accelerators—convert one nuclide into another.