Tritium is an isotope of the element hydrogen.
Isotopes of hydrogen
Hydrogen has three main isotopes; protium ( 1H), deuterium ( 2H) and tritium ( 3H). The main isotopes of hydrogen are unique because they are the only isotopes that have a name. These names are still in use today. In addition, deuterium and tritium sometimes get their own symbols, D and T.
Protium is the most common isotope of hydrogen. It makes up more than 99.98% of all the hydrogen in the universe. It is named protium because its nucleus only has one proton. Protium has an atomic mass of 1.00782504(7) u.
Deuterium, or 2H or sometimes D, is another stable isotope of hydrogen. The isotope has one proton and one neutron. It makes up about 0.0026 – 0.0184% of all atoms of hydrogen on Earth.
Tritium (3H) is the most stable radioisotope of hydrogen. That is, of all radioactive isotopes of hydrogen, tritium is the least radioactive. It has a proton and 2 neutrons in its nucleus.