Astatine

Astatine is a radioactive chemical element with the chemical symbol At and atomic number 85. It occurs on Earth only as the result of the radioactive decay of certain heavier elements. All of its isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours. Accordingly, much less is known about astatine than most other elements. The observed properties are consistent with it being a heavier analog of iodine; many other properties have been estimated based on this resemblance.

Elemental astatine has never been viewed, because a mass large enough to be seen (by the naked human eye) would be immediately vaporized by the heat generated by its own radioactivity. Astatine may be dark, or it may have a metallic appearance and be a semiconductor, or it may even be a metal. It is likely to have a much higher melting point than does iodine, on par with those of bismuth and polonium. Chemically, astatine behaves more or less as a halogen, being expected to form ionic astatides with alkali or alkaline earth metals; it is known to form covalent compounds with nonmetals, including other halogens. It does, however, also have a notable cationic chemistry that distinguishes it from the lighter halogens. The second longest-lived isotope of astatine, astatine-211, is the only one currently having any commercial application, being employed in medicine to diagnose and treat some diseases via its emission of alpha particles (helium-4 nuclei). Only extremely small quantities are used, however, due to its intense radioactivity.

The element was first produced by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè at the University of California, Berkeley in 1940. They named the element "astatine", a name coming from the great instability of the synthesized matter (the source Greek word αστατος (astatos) means "unstable"). Three years later it was found in nature, although it is the least abundant element in the Earth's crust among the non-transuranic elements, with an estimated total amount of less than 28 grams (1 oz) at any given time. Six astatine isotopes, with mass numbers of 214 to 219, are present in nature as the products of various decay routes of heavier elements, but neither the most stable isotope of astatine (with mass number 210) nor astatine-211 (which is used in medicine) is produced naturally.