Arsenic

Arsenic occurs in nature as one stable isotope, 75As, and is therefore called a monoisotopic element.  As of 2024, at least 32 radioisotopes have also been synthesized, ranging in atomic mass from 64 to 95.  The most stable of these is 73As with a half-life of 80.30 days. The majority of the other isotopes have half-lives of under one day, with the exceptions being

71As (t½ = 65.30 hours),

72As (t½ = 26.0  hours),

74As (t½ = 17.77 days),

76As (t½ = 26.26 hours),

77As (t½ = 38.83 hours).

Isotopes that are lighter than the stable 75As tend to decay by β+ decay, and those that are heavier tend to decay by β− decay, with some exceptions.

At least 10 nuclear isomers have been described, ranging in atomic mass from 66 to 84. The most stable of arsenic's isomers is 68mAs with a half-life of 111 seconds.