Tritium (or, symbol or, also known as Hydrogen-3) is a radioactive isotope of hydrogen. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of protium (the most abundant hydrogen isotope) contains one proton and no neutrons. Tritium is extremely rare. The isotope name is formed from the Greek meaning "third". Decay While tritium has several different experimentally-determined values of its half-life, NIST recommends 4,500±8 days (app… (More on Tritium)