astatines

Astatines, or astatine isotopes, are the radioactive nuclides of the element astatine (symbol At, atomic number 85). Astatine is the rarest naturally occurring halogen and is highly radioactive; there are no stable isotopes. The name derives from the Greek astatos, meaning unstable.

Natural occurrence and production: In nature, astatine appears only in trace amounts as a decay product of heavier elements such as uranium and thorium. Most astatine used in research is produced synthetically in nuclear reactors or particle accelerators by reactions involving bismuth or other heavy elements.

Isotopes and chemistry: Dozens of astatine isotopes have been identified; they are all radioactive and have half-lives ranging from fractions of a second to hours. Because of their short lifetimes, direct chemical studies are limited; the element is predicted to behave as a heavy halogen with a tendency toward covalent bonding and forms halide compounds such as AtF, AtCl, AtBr, and AtI under appropriate conditions, though experiments are challenging. The chemistry is complicated by relativistic effects and strong radiolysis.

Applications and research: Astatine isotopes have potential in medicine and research. In particular, astatine-211 has been studied for targeted alpha therapy, where alpha-emitting atoms are attached to tumor-targeting molecules.

History and nomenclature: Astatine was discovered in 1940 by Corson, Mackenzie, and Seaborg through the bombardment of bismuth with alpha particles. The name derives from astatos, meaning unstable. Astatine remains among the rarest elements to handle due to its scarcity and radioactivity.