A chemical element, At, atomic number 85. Astatine is the heaviest of the halogen groups, filling the place immediately below iodine in group 17 of the periodic table. Astatine is a highly unstable element existing only in short-lived radioactive forms. About 25 isotopes have been prepared by nuclear reactions of artificial transmutation. The longest-lived of these is 210At, which decays with a half-life of only 8.3 h. It is unlikely that a stable or long-lived form will be found in nature or prepared artificially. The most important isotope, used for tracer studies, is 211At. Astatine exists in nature in uranium minerals, but only in the form of trace amounts of shortlived isotopes, continuously replenished by the slow decay of uranium, The total amount of astatine in the Earth's crust is less than 1 oz (28 g).
In aqueous solution, astatine resembles iodine except for differences attributable to the fact that astatine solutions are of necessity extremely dilute. Like the halogen iodine, when astatine exists as a free element in solution, it is extracted by benzene. The element in solution is reduced by agents such as sulfur dioxide and is oxidized by bromine. It is more electropositive than the other halogens. It has oxidation states with coprecipitation characteristics similar to those of the iodide ion, free iodine, and the iodate ion. Powerful oxidizing agents produce an astatate ion, but not a perastatate ion. The free state is most readily obtained and is characterized by high volatility and high extractability into organic solvents.