bismuth is a chemical element with the symbolbi and atomic number 83. it is a pentavalentpost-transition metal and one of the pnictogens with chemical properties resembling its lighter homologs arsenic and antimony. elemental bismuth may occur naturally, although its sulfide and oxide form important commercial ores. the free element is 86% as dense as lead. it is a brittle metal with a silvery white color when freshly produced, but surface oxidation can give it an iridescent tinge in numerous colours. bismuth is the most naturally diamagnetic element, and has one of the lowest values of thermal conductivity among metals.
bismuth was long considered the element with the highest atomic mass that is stable, but in 2003 it was discovered to be extremely weakly radioactive: its only primordial isotope, bismuth-209, decays via alpha decay with a half-life more than a billion times the estimated age of the universe. because of its tremendously long half-life, bismuth may still be considered stable for almost all purposes.