Astatine

Astatine is the heaviest halogen, with an estimated density of about 6.35 g/cm³ and melting point around 302 °C. It exists only in trace amounts in the crust, every isotope is strongly radioactive, and the longest-lived half-life is only about 8.5 hours. It can be produced only in trace quantities by bombarding a bismuth target with alpha particles in an accelerator; it never forms an ore body. It therefore has no industrial or civilian use and enters no consumer product. Today astatine serves only as a target for nuclear physics and radiochemistry research; because its alpha particles are high in energy but short in range, At-211 is being explored for targeted alpha therapy, though it remains at the preclinical stage.

Astatine compounds such as hydrogen astatide (HAt) and silver astatide (AgAt) can only be synthesized at tracer levels in radiochemical experiments, have never been isolated in weighable amounts, and have no industrial or commercial use. They are studied only to probe bonding trends among the heavy halogens with hydrogen, metals and other halogens.