Isotope

one of two or more nuclides that are chemically identical, having the same number of protons, yet differ in mass number, since their nuclei contain different numbers of neutrons

Description
Individual isotopes are named with the inclusion of their mass number in the superior position (e.g. 12C) and the atomic number (nuclear protons) in the inferior position (e.g. 6C). In former usage, the mass numbers follow the chemical symbol (e.g. C-12)