A species of atom that is characterized by the constitution of its nucleus, in particular by its atomic number Z and its neutron number A – Z, where A is the mass number. Whereas the terms isotope, isotone, and isobar refer to families of atomic species possessing common atomic number, neutron number, and mass number, respectively, the term nuclide refers to a particular atomic species. The total number of stable nuclides is approximately 275. About a dozen radioactive nuclides are found in nature, and in addition, hundreds of others have been created artificially.