The isotope of hydrogen with one proton and two neutrons is tritium.
Tritium is also called hydrogen-3 and is commonly written as 3H or T. All hydrogen isotopes have one proton, which gives hydrogen its atomic identity. The number of neutrons distinguishes them: protium has none, deuterium has one, and tritium has two. Tritium therefore has a mass number of three.
Unlike the stable isotopes protium and deuterium, tritium is radioactive. It undergoes beta decay with a half-life of about 12.32 years, changing into helium-3. Naturally occurring tritium is extremely rare, although cosmic rays create tiny amounts in the atmosphere. It can also be produced by bombarding lithium with neutrons in nuclear reactors.
Tritium has practical uses in science and technology. Researchers use it as a radioactive tracer in chemical, biological, and environmental studies, while tritium-based devices can produce self-powered illumination. It is also important in deuterium–tritium fusion research. A common error is choosing helium-3 because tritium decays into it; helium-3 is the decay product, not the original hydrogen isotope.