astatate

Astatine is a highly radioactive chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element on Earth and belongs to the halogen group in the periodic table, which includes fluorine, chlorine, bromine, and iodine. Due to its extreme scarcity and radioactivity, astatine is not found in significant quantities in nature and is primarily produced synthetically in laboratories.

The element was first discovered in 1940 by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè at the University of California, Berkeley. They identified astatine by bombarding bismuth-209 with alpha particles, producing the isotope astatine-211. The name "astatine" derives from the Greek word *astatos*, meaning "unstable," reflecting its radioactive nature.

Astatine exhibits properties intermediate between those of iodine and the noble gases, though its chemical behavior is poorly understood due to its scarcity and short half-lives. Its isotopes range from astatine-205 to astatine-217, with half-lives varying from fractions of a second to several hours. The most stable isotope, astatine-210, has a half-life of about 8.3 hours.

Due to its radioactivity and rarity, astatine has no significant commercial applications. Research on astatine is limited to nuclear physics and theoretical chemistry, as its chemical properties are difficult to study. The element is predicted to form compounds similar to those of iodine, such as astatine chloride (AtCl) and astatine iodide (AtI), but these have not been isolated in macroscopic quantities.

Astatine is primarily of interest in scientific research, particularly in the study of nuclear decay and the behavior of heavy elements. Its extreme instability and scarcity make it one of the least understood elements in the periodic table.