Astatine is a semimetallic radioactive chemical element.  Its symbol is At, and it is 85th on the periodic table.  It has an atomic weight of 210 AMU.  The most stable isotope of Astatine has an unknown density, and its valence is believed to be +1, +3, +5, or +7. Astatine is the heaviest known halogen element.  It is in Group 17 of the periodic table.  Its chemical properties are believed to be similar to those of iodine, as a nonmetallic chemical element.  The most stable isotope, astatine-210, has a half-life of 8.3 hrs. 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 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. There are no uses known for Astatine.