Astatine is a radioactive chemical element with symbol At and atomic number 85. It is the rarest naturally occurring element on the Earth's crust. 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, bromine, and iodine. 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 occurs naturally. They can only be produced synthetically, usually by bombarding bismuth-209 with alpha particles....LESS