Isotopes of gold
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Gold (79Au) has one stable isotope, 197Au, and known radioisotopes ranging from 169Au to 210Au, with 195Au being the most stable with a half-life of 186.01 days, followed by 196Au at 6.165 days. Isotopes heavier than the stable mass number 197 generally decay by beta emission to mercury isotopes, while those lighter decay by electron capture to platinum isotopes or alpha emission to iridium isotopes; 196 decays both to platinum and to mercury. Of the meta states the most stable is 198m2Au at 2.27 days.
Gold is currently the heaviest monoisotopic element (and is also mononuclidic). Bismuth formerly held that distinction until alpha decay of the 209Bi isotope was observed. All isotopes of gold are either radioactive or, in the case of 197Au, observationally stable, meaning that 197Au is predicted to be radioactive but no actual decay has been observed.