Wikipedia:Today's featured article/June 25, 2015
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, with half-lives of 8.1 hours or less. Elemental astatine has never been viewed because a mass large enough to be seen by the naked eye would be immediately vaporized by its radioactive heating. The bulk properties of astatine are not known with any certainty, but they have been predicted based on its similarity to the other halogens, the lighter elements directly above it in the periodic table: fluorine, chlorine, bromine and especially 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 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 the ability to form a stable monatomic cation in aqueous solution (unlike the lighter halogens). (Full article...)