Naturally occurring rhenium (Re) is 37.4% 185Re, which is stable, and 62.6% 187Re, which is unstable but has a very long half-life (41.2×109 years). Among elements with a known stable isotope, only indium and tellurium similarly occur with a stable isotope in lower abundance than the long-lived radioactive isotope.
There are 33 other unstable isotopes recognized, the longest-lived of which are 183Re with a half-life of 70 days, 184Re with a half-life of 38 days, 186Re with a half-life of 3.7186 days, 182Re with a half-life of 64.0 hours, and 189Re with a half-life of 24.3 hours. There are also numerous isomers, the longest-lived of which are 186mRe with a half-life of 200,000 years and 184mRe with a half-life of 169 days. All others have half-lives less than a day.
Relative atomic mass: 186.207(1).