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.