astatine (ăsˈtətēn,–tĭn) [key] [Gr., = unstable], semimetallic radioactive chemical element; symbol At; at. no. 85; at. wt. of most stable isotope 210; m.p. 302°C (estimated); b.p. 337°C (estimated); density unknown; valence believed to be +1, +3, +5, or +7. Astatine is the heaviest known halogen (Group 17 of the periodic table). Its chemical properties are believed to be similar to those of iodine. The most stable isotope, astatine-210, has a half-life of about 8 hours. More than 30 isotopes of astatine have been identified. Small amounts of astatine exist in equilibrium with uranium and thorium in the earth's crust, but the total amount of astatine is probably less than 1 oz. Astatine-211 (half-life 7.21 hr) is sometimes used as a radioactive tracer; like iodine, it collects in the thyroid gland. The discovery of astatine (first called alabamine) was announced in 1931 by Fred Allison and E. J. Murphy. In 1940, Emilio Segré, D. R. Corson, and K. R. Mackenzie produced astatine-211 by bombarding bismuth-209 with alpha particles in the cyclotron at the Univ. of California.