Astatine 砹

Astatine is a radioactive chemical element with the chemical symbol At and atomic number 85, and is the rarest natural 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.