The presence of neutrons in atomic nuclei accounts for the occurrence of isotopes— samples of an element whose atoms contain different numbers of neutrons and hence exhibit different "nuclidic masses". The nuclidic mass is the mass of a "nuclide", where a nuclide is the term used for any atom whose nuclear composition (Number of protons and neutrons) is defined. For example, naturally occurring hydrogen has two stable nuclides, $$\ce{^{1}_{1}H}$$ and $$\ce{^{2}_{1}H}$$, which also are isotopes of one another. More than 99.98 percent is “light” hydrogen, 11H. This consists of atoms each of which has one proton, one electron, and zero neutrons. The rest is “heavy” hydrogen or deuterium, $$\ce{^{2}_{1}H}$$, which consists of atoms which contain one electron, one proton, and one neutron. Hence the nuclidic mass of deuterium is almost exactly twice as great as for light hydrogen. By transmutation of lithium, it is also possible to obtain a third isotope, tritium, $$\ce{^{3}_{1}H}$$. It consists of atoms whose nuclei contain two neutrons and one proton. Its nuclidic mass is about 3 times that of light hydrogen.