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A chemical element, in the periodic table astatine has the symbol At and atomic number 85. It was discovered in 1940 by Dale Corson, K.R. MacKenzie, and Emilio Segre at the University of California, USA. Its name comes from the Greek word "astatos," meaning "unstable."

Astatine has about 20 known isotopes, all of which are radioactive; the longest-lived isotope is 210At which has a half-life of only 8.3 hours. Astatine is a halogen and possibly accumulates in the thyroid like iodine.