Hydrogen-1

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.

Hydrogen is the only element whose isotopes have different names that are in common use today. The ²H (or hydrogen-2) isotope is usually called deuterium, while the ³H (or hydrogen-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 in the 2005 Red Book that while the use of D and T is common, it is not preferred because it can cause problems in the alphabetic sorting of chemical formulae. 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.)

¹H (atomic mass 1.00782504(7) u) is the most common hydrogen isotope with an abundance of more than 99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium.

The proton has never been observed to decay and hydrogen-1 is therefore considered a stable isotope. Some grand unified theories proposed in the 1970s predict that proton decay can occur with a half-life between 10³¹ and 10³⁶ years. If this prediction is found to be true, then hydrogen-1 (and indeed all nuclei now believed to be stable) are only observationally stable. To date however, experiments have shown that the minimum proton half life is in excess of 10³⁴ years.

...
Wikipedia