Isotopes of hydrogen
Hydrogen (1H) has three naturally occurring isotopes, sometimes denoted 1
H
, 2
H
, and 3
H
. 1
H
and 2
H
are stable, while 3
H
has a half-life of 12.32(2) years. Heavier isotopes also exist, all of which are synthetic and have a half-life of less than one zeptosecond (10−21 s).
Of these, 5
H
is the least stable, while 7
H
is the most.
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| Standard atomic weight Ar°(H) | ||||||||||||||||||||||||||
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Hydrogen is the only element whose isotopes have different names that remain in common use today: the 2
H
(or hydrogen-2) isotope is deuterium and the 3
H
(or hydrogen-3) isotope is tritium. The symbols D and T are sometimes used for deuterium and tritium. The IUPAC accepts the D and T symbols, but recommends using standard isotopic symbols (2
H
and 3
H
) instead to avoid confusion in the alphabetic sorting of chemical formulas. The isotope 1
H
, with no neutrons, may be called protium to disambiguate. (During the early study of radioactivity, some other heavy radioactive isotopes were given names, but such names are rarely used today.)