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出典:『Wikipedia』 (2011/07/04 18:58 UTC 版)

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Astatine ( /ˈæstəˌtiːn/ as-tə-teen or /ˈæstəˌtɪn/ as-tə-tin) is a radioactive chemical element with the symbol At and atomic number 85. It is the heaviest known halogen. Astatine is produced by radioactive decay in nature, but due to its short half life it is found only in minute amounts. Astatine was first produced by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè in 1940. Three years passed before traces of astatine were also found in natural minerals. Until recently most of the physical and chemical characteristics of astatine were inferred from comparison with other elements. Some astatine isotopes have been used as alpha-particle emitters in science, and medical applications for astatine-211 have been tested. Astatine is the rarest naturally occurring element, with less than 30 grams (1.058 ounces) estimated to be contained in the entire Earth's crust.