Species of atom as characterized by the number of protons, neutrons, and the energy state of the nucleus. A nuclide is characterized by its mass number and its atomic number. To be regarded as distinct, a nuclide must have an energy content sufficient for a measurable lifetime, usually more than 1010 second. Nuclear isomers, which have the same number of protons and neutrons but differ in energy content and radioactivity, are also distinct nuclides. Nuclides are associated with radioactive decay and may be stable or unstable. There are about 1,700 known nuclides, of which about 300 are stable and the rest radioactive.