Arsenic (As) has 33 known isotopes and at least 10 isomers. Only one of these isotopes, 75As, is stable; as such, it is considered a monoisotopic element. The longest-lived radioisotope is 73As has a half-life of 80 days. Arsenic has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of 75As, irradiated by the intense highenergy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope 76As with a half-life of 1.0778 days and produce approximately 1.13 MeV gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several hours. Such a weapon is not known to have ever been built, tested, or used.
Relative atomic mass: 74.92160(2)