Isotopes of bismuth

Bismuth (₈₃Bi) has 41 known isotopes, ranging from Bi to Bi. Bismuth has no stable isotopes, but does have one naturally occurring, very long-lived isotope; thus, the standard atomic weight can be given from that isotope, bismuth-209. Though it is now known to be radioactive, it may still be considered practically stable because it has a half-life of 2.01×10 years, which is more than a billion times the age of the universe.

Besides Bi, the most stable bismuth radioisotopes are Bi with a half-life of 3.04 million years, Bi with a half-life of 368,000 years and Bi, with a half-life of 31.22 years, none of which occur in nature. All other isotopes have half-lives under 15 days, most under two hours. Of naturally occurring radioisotopes, the most stable is radiogenic Bi with a half-life of 5.012 days. Bi is unusual for being a nuclear isomer with a half-life many orders of magnitude longer than that of the ground state.