All elements have one or more isotopes—nuclear variations in which the number of protons, also the atomic number, are the same but the number of neutrons varies, resulting in different atomic masses.
Elemental hydrogen has three naturally occurring isotopes: The most abundant form is the hydrogen atom that contains no neutrons; the second is deuterium, containing one neutron; and the third is tritium, which contains two neutrons.
While the standard hydrogen atom and deuterium are stable under natural conditions, many other isotopes, such as tritium, “decay” into other substances by the emission of energy, and are known as radioactive isotopes. The tritium atom undergoes beta decay—one of its two neutrons is transformed into a proton by the emission of an electron. This results in a more stable, non-radioactive helium-3 isotope, containing two protons and one neutron.
The decay of radioactive isotopes occurs at a constant rate, which is the basis for radiometric dating—a technique used to determine geological age.