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