 Antimony has two stable isotopes : 121Sb with a natural abundance of 57 @.@ 36 % and 123Sb with a natural abundance of 42 @.@ 64 % . It also has 35 radioisotopes , of which the longest @-@ lived is 125Sb with a half @-@ life of 2 @.@ 75 years . In addition , 29 metastable states have been characterized . The most stable of these is 120m1Sb with a half @-@ life of 5 @.@ 76 days . Isotopes that are lighter than the stable 123Sb tend to decay by β + decay , and those that are heavier tend to decay by β − decay , with some exceptions . 
