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.