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.