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.