Bismuth (Bi) has no stable isotopes, but does have one very long-lived isotope; thus, the relative atomic mass can be given as 208.9803987(16). 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 over 1.9×1019 years, which is more than a billion (1000 million) 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 radioactive isotope bismuth-213 can be produced by bombarding radium with bremsstrahlung photons from a linear particle accelerator. In 1997 an antibody conjugate with Bi-213, 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 cancer treatment, e.g. in the Targeted Alpha Therapy (TAT) program. Bismuth-213 is also found on the decay chain of uranium-233.