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Bismuth (83Bi) has 41 known isotpes, ranging from 184Bi to 224Bi. Bismuth has no stable isotopes, but does have one very long-lived isotope; thus, the standard atomic weight can be given as 208.98040(1). Although bismuth-209 is now known to be unstable, it has classically been considered to be a "stable" isotope because it has a half-life of approximately 2.01×1019 years, which is more than a billion times the age of the universe. Besides 209Bi, the most stable bismuth radioisotopes are 210mBi with a half-life of 3.04 million years, 208Bi with a half-life of 368,000 years and 207Bi, with a half-life of 32.9 years, none of which occur in nature. All other isotopes have half-lives under 1 year, most under a day. Of naturally occurring radioisotopes, the most stable is radiogenic 210Bi with a half-life of 5.012 days.
Commercially, the isotope bismuth-213 can be produced by bombarding radium with bremsstrahlung photons from a linear particle accelerator. In 1997, an antibody conjugate with 213Bi (which has a 45-minute half-life, and decays with the emission of an alpha particle) was used to treat patients with leukemia. This isotope has also been tried in targeted alpha therapy (TAT) program to treat a variety of cancers. Bismuth-213 is also found in the decay chain of uranium-233, which is the fuel "bred" by thorium reactors.

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