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Astatine is a very rare radioactive chemical element with the chemical symbol At and atomic number 85. It occurs on Earth as the decay product of various heavier elements. All its isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours. Elemental astatine has never been viewed because any macroscopic sample would be immediately vaporized by its radioactive heating. It has yet to be determined if this obstacle could be overcome with sufficient cooling. The bulk properties of astatine are not known with any certainty. Many of these have been estimated based on its periodic table position as a heavier analog of iodine, and a member of the halogens – the group of elements including fluorine, chlorine and bromine. It is likely to have a dark or lustrous appearance and may be a semiconductor or possibly a metal; it probably has a higher melting point than that of iodine. Chemically, several anionic species of astatine are known and most of its compounds resemble those of iodine. It also shows some metallic behavior, including being able to form a stable monatomic cation in aqueous solution (unlike the lighter halogens). Dale R. Corson, Kenneth Ross MacKenzie, and Emilio G. Segrè synthesized the element at the University of California, Berkeley in 1940, naming it after the Greek ''astatos'' (ἄστατος), "unstable". Four isotopes of astatine were subsequently found in nature, although it is the least abundant of all the naturally occurring elements, with much less than one gram being present at any given time in the Earth's crust. Neither the most stable isotope astatine-210 nor the medically useful astatine-211 occur naturally. They can only be produced synthetically, usually by bombarding bismuth-209 with alpha particles. == Characteristics == Astatine is an extremely radioactive element; all its isotopes have short half-lives of 8.1 hours or less, decaying into bismuth, polonium, radon, or other astatine isotopes. Most of its isotopes are very unstable with half-lives of one second or less. Of the first 101 elements in the periodic table, only francium is less stable. The bulk properties of astatine are not known with any certainty. Research is limited by its short half-life, which prevents the creation of weighable quantities. A visible piece of astatine would immediately vaporize itself because of the heat generated by its intense radioactivity.〔 It remains to be seen if, with sufficient cooling, a macroscopic quantity of astatine could be deposited as a thin film.〔 Astatine is usually classified as either a nonmetal or a metalloid; metal formation has also been predicted.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「astatine」の詳細全文を読む スポンサード リンク
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