astatine

Astatine is a chemical element with the symbol At and atomic number 85. It is considered the heaviest known halogen, located in group 17 of the periodic table and in period 6. All of its isotopes are radioactive; there are no stable forms. The most stable isotope is typically cited as astatine-210, with a half-life of about 8 hours. In nature, astatine exists only in trace amounts and is produced as a decay product of heavier elements; it is also synthesized in particle accelerators and nuclear reactors for research purposes.

Astatine was first reported in 1940 by researchers at the University of California, Berkeley (Corson, MacFarlane, and Segrè) who produced it by bombarding bismuth with alpha particles. The name derives from the Greek astatos, meaning unstable. Because of its short half-lives and scarcity, astatine has not been studied extensively in bulk, and many of its properties are inferred from trends of the other halogens and from limited experimental data.

Chemically, astatine is expected to behave as a heavy halogen, forming negative ions and a variety of compounds such as interhalogens and halides. However, its extreme radioactivity and rarity mean that detailed experimental characterization is challenging, and its chemistry is not as well established as for lighter halogens. The element is predicted to be somewhat volatile and may exist in multiple oxidation states, though observations are limited.

Applications of astatine are primarily in basic research and potential medical uses. Astatine-211, in particular, has been studied for targeted alpha-particle therapy in cancer research. Production of astatine isotopes typically involves irradiation of bismuth-209 with alpha particles or other nuclear reactions, and handling requires specialized radiochemical facilities and safety protocols.