Hydrogen (H) (Standard atomic mass: 1.00782504(7) u) has three naturally occurring isotopes, sometimes denoted ¹H, ²H, and ³H. Other, highly unstable nuclei (⁴H to ⁷H) have been synthesized in the laboratory but not observed in nature. The most stable radioisotope is tritium, with a half-life of 12.32 years. All heavier isotopes are synthetic and have a half-life less than a zeptosecond (10^-21 second). Of these, ⁵H is the most stable, and the least stable isotope is ⁷H.

Hydrogen is the only element that has different names for its isotopes in common use today. The ²H (or H-2) isotope is usually called deuterium, while the ³H (or H-3) isotope is usually called tritium. The symbols D and T (instead of ²H and ³H) are sometimes used for deuterium and tritium. The IUPAC states that while this use is common it is not preferred. The ordinary isotope of hydrogen, with no neutrons, is sometimes called "protium". (During the early study of radioactivity, some other heavy radioactive isotopes were given names – but such names are rarely used today).