<P> H ( atomic mass 7000301604928199000 ♠ 3.016 049 281 99 ( 23 ) u ) is known as tritium and contains one proton and two neutrons in its nucleus . It is radioactive , decaying into helium - 3 through β − decay with a half - life of 12.32 years . Trace amounts of tritium occur naturally because of the interaction of cosmic rays with atmospheric gases . Tritium has also been released during nuclear weapons tests . It is used in thermonuclear fusion weapons , as a tracer in isotope geochemistry , and specialized in self - powered lighting devices . </P>
what is the mass number of hydrogen isotope that contains 2 neutrons
3.016 049 281 99 ( 23 ) u