Isotopes of antimony

Antimony (₅₁Sb) occurs naturally as two stable isotopes, ¹²¹Sb (57.21%) and ¹²³Sb (42.79%). There are 37 artificial radioactive isotopes known with mass numbers 104 to 142, the lightest two of which (^104-105Sb) are beyond the proton drip line. Isotopes that are lighter than the stable isotopes tend to decay by β⁺, and those that are heavier tend to decay by β⁻; the intermediate ¹²²Sb is observed to decay in both ways.

The most stable radioisotope of antimony is the minor fission product ¹²⁵Sb, with a half-life of 2.758 years; ¹²⁴Sb, with half-life 60.20 days; and ¹²⁶Sb, with half-life 12.35 days. All other isotopes have half-lives less than 4 days, most less than an hour. Of the numerous isomers reported, the longest-lived is ^120m1Sb with half-life 5.76 days; this nuclide has not been confirmed not to be the ground state.