The element hydrogen has three different isotopes: protium, deuterium, and tritium. Each isotope has one proton but different numbers of neutrons. A protium nucleus has zero neutrons, a deuterium nucleus has one neutron and a tritium nucleus has two neutrons.
Protium is by far the most common isotope, with over 99 percent of all the hydrogen in the atmosphere being protium. Deuterium is the second most common. It is used as part of deuterium oxide in nuclear reactors. Tritium is the only radioactive isotope of hydrogen, and it is produced by nuclear reactors, as well as being part of the hydrogen bomb.