Astatine, a radioactive chemical element. Astatine, a member of the halogen group of elements, is an extremely shortlived substance; its most stable isotope has a half-life period of only 8.3 hours. Astatine's properties, in many respects, are similar to those of the other, nonmetal members of the halogen family (fluorine, chlorine, bromine, and iodine), but it also has some of the properties of a metal.

Astatine occurs in nature as a result of radioactive decay but decays almost as soon as it is formed. It was first produced artificially, in 1940, at the University of California by D.R. Corson, K. R. Mackenzie, and Emilio Segr, who bombarded bismuth 209 with alpha particles. The name astatine is derived from a Greek word meaning "unstable."

Symbol: At. Atomic number: 85. Atomic weight: 210. Melting point: 576 F. (302 C.). Boiling point: 639 F. (337 C.). Three astatine isotopes, At-215, At-218, and At-219, occur in nature, and a number of others have been produced artificially. Astatine belongs to Group VIIA of the Periodic Table and has valences of +1, +3, +5, or +7.