Astatine

Astatine is the heaviest member of group 17 below fluorine, chlorine, bromine and iodine. Every isotope is radioactive, and the longest-lived, astatine-210, has a half-life of just 8.1 hours. No one has ever seen a visible sample: any amount large enough to see would be vaporised by the heat of its own radioactivity. Chemists study it in solutions containing fewer than a trillionth of a gram, tracking it by its radiation.

Those tracer experiments show astatine behaving partly like iodine and partly like a metal. It forms the astatide ion At-, which precipitates with silver like iodide, but it also forms positive species in solution, reflecting the trend toward metallic character down group 17. Calculations including relativistic effects predict that bulk astatine would be a metal rather than a dark solid like iodine. Astatine-211, with a 7.2-hour half-life, is being tested in cancer treatment because its alpha particles damage tumour cells over a very short range.

Discovery

Dale Corson, Kenneth MacKenzie and Emilio Segrè made astatine in 1940 at the University of California, Berkeley, by bombarding bismuth-209 with alpha particles in a cyclotron. Several earlier claims of finding element 85 in nature had been mistaken. The discoverers named it in 1947 from the Greek astatos, meaning 'unstable'. Tiny amounts were later found in natural uranium and thorium decay chains.