ASTATINE

(At), radioactive chemical element, heaviest member of the halogen elements, or Group VIIa of the periodic table. Astatine, which has no stable isotopes, was first synthetically produced (1940) at the University of California by Dale R. Corson, K.R. MacKenzie, and Emilio Segr, who bombarded bismuth with accelerated alpha particles (helium nuclei) to yield astatine and neutrons. Naturally occurring astatine isotopes have subsequently been found in minute amounts in the three natural radioactive decay series in which they occur by minor branching (astatine-218 in the uranium series, astatine-216 in the thorium series, and astatine-215 and astatine-219 in the actinium series). About 20 isotopes are known; astatine-210, with a half-life of 8.3 hours, is the longest lived. Because of the short half-lives of astatine isotopes, only very small quantities have been available for study. By the use of astatine-210 and astatine-211 and the tracer methods of radiochemistry, some of the chemical properties of the element have been established. It generally resembles iodine (thus, like iodine, it concentrates in the thyroid gland of higher animals). It is somewhat soluble in water and much more soluble in benzene and carbon tetrachloride. Astatine can be reduced to the astatide ion, At-, and has been oxidized to positive oxidation states that appear to be +1 (AtO-) and +5 (AtO-3). atomic number 85 stablest isotope 210 oxidation states -1, +1, +3(?), +5, +7(?) electronic config. 2-8-18-32-18-7 or (Xe)4f 145d106s26p5