Hydrogen (H) (relative atomic mass: 1.00794) has three naturally occurring isotopes, sometimes denoted 1H, 2H, and 3H. The first two of these are stable while 3H has a half-life of 12.32 years. All heavier isotopes are synthetic and have a half-life less than one zeptosecond (10−21 second). Of these,5H is the most stable, and 7H is the least.[1][2]

Hydrogen is the only element whose isotopes have different names that are in common use today. The 2H (or hydrogen-2) isotope is usually calleddeuterium, while the 3H (or hydrogen-3) isotope is usually c