Atoms with the different numbers of neutrons and same atomic number are known as isotopes. The chemical properties of an element are determined by wildly the charge on the nucleus, and different isotopes of an element have very similar chemical properties. They are not quite identical, however, and in physical properties allow isotopes to be separated if desired and slight differences in chemistry.

Some elements have only one stable isotope (e.g. 19F, 27Al, 31P), others may have several (e.g. 1H and 2H, the latter also being defined deuterium, 12C and 13C); the record is held by tin (Sn), which has no fewer than 10. Natural samples of different elements therefore consist of mixtures of isotopes in nearly fixed proportions reflecting the ways in which these were made by nuclear synthesis. The molar mass (also known as relative atomic mass, RAM) of elements is choosing by these proportions. For many chemical purposes the existence of such isotopic mixtures can be ignored, although it is significant.