From the Greek astatos meaning unstable. Synthesized in 1940 by D.R. Corson, K.R. MacKenzie, and E. Segre at the University of California by bombarding bismuth with alpha particles. The longest-lived isotopes, with naturally occurring uranium and thorium isotopes, and traces of 217At are equilibrium with 233U and 239Np resulting from integration of thorium and uranium with naturally produced neutrons. The total amount of astatine present in the earth's crust, however, is less than 1 oz.