Hydrogen (1H) has three naturally occurring isotopes, sometimes denoted 1 H , 2 H , and 3 H . 1 H and 2 H are stable, while 3 H has a half-life of 12.32(2) years.[3][nb 1] Heavier isotopes also exist, all of which are synthetic and have a half-life of less than one zeptosecond (10−21 s).[4][5]
Of these, 5 H is the least stable, while 7 H is the most.
Hydrogen is the only element whose isotopes have different names that remain in common use today: the 2 H (or hydrogen-2) isotope is deuterium[6] and the 3 H (or hydrogen-3) isotope is tritium.[7] The symbols D and T are sometimes used for deuterium and tritium. The IUPAC accepts the D and T symbols, but recommends using standard isotopic symbols (2 H and 3 H ) instead to avoid confusion in the alphabetic sorting of chemical formulas.[8] The isotope 1 H , with no neutrons, may be called protium to disambiguate.[9] (During the early study of radioactivity, some other heavy radioactive isotopes were given names, but such names are rarely used today.)