Indium has 39 known
isotopes, ranging in
mass number from 97 to 135. Only two isotopes occur naturally as
primordial nuclides: indium-113, the only
stable isotope, and indium-115, which has a
half-life of 4.41×1014 years, four orders of magnitude greater than the
age of the Universe and nearly 30,000 times greater than that of
natural thorium.
[19] The half-life of 115In is very long because the
beta decay to 115
Sn is
spin-forbidden.
[20] Indium-115 makes up 95.7% of all indium. Indium is one of three known elements (the others being
tellurium and
rhenium) of which the stable isotope is less abundant in nature than the long-lived primordial radioisotopes.
[21]