Astatine.
Astatine is a very rare radioactive chemical element with the chemical symbol At and the atomic number 85. It occurs on Earth as the decay product of heavier elements. All its isotopes are short-lived, with half-lives of 8.1 hours or less. The element has never been viewed because a mass large enough to be seen would be immediately vaporized by the heat of its radioactivity. The bulk properties of astatine are not known with any certainty, but they have been predicted based on its similarity to the lighter halogens directly above it in the periodic table: fluorine, chlorine, bromine and especially iodine. It is likely to appear dark or lustrous and may be a semiconductor or a metal. Chemically, several anionic species of astatine are known and most of its compounds resemble those of iodine. It also shows some metallic behavior, and may be able to form a stable monatomic cation in aqueous solution, unlike the lighter halogens.
