Astatine is a radioactive chemical element with the chemical symbol At and atomic number 85. It occurs on Earth 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 behaving as 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 eye would be immediately vaporized by the heat generated by its own radioactivity. Astatine may have a dark or metallic appearance and be a semiconductor, or it may be a metal. It is likely to have a higher melting point than iodine, on a par with those of bismuth and polonium. Chemically, astatine can behave as a halogen (the group including chlorine and fluorine), and could be expected to form ionic astatides with alkali or alkaline earth metals; it is known to form covalent compounds with nonmetals, including other halogens. It can also behave as a metal, with a cationic chemistry that distinguishes it from the lighter halogens. The second longest-lived isotope of astatine, astatine-211, is the only one with any commercial application, being used in medicine to diagnose and treat some diseases via its emission of alpha particles. Only extremely small quantities are used 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 it "astatine", after the Greek astatos meaning "unstable". Three years later it was found in nature, although it is the least abundant of the non-transuranic elements in the Earth's crust, with much less than one gram being present at any given time. Six astatine isotopes, with mass numbers of 214 to 219, occur naturally as the decay products of various heavier elements, but neither the most stable isotope astatine-210 nor the medically useful astatine-211 occurs naturally.