(Gr. astatos, 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. Thelongest-lived isotopes, with naturally occurring uranium and thorium isotopes, and tracesof 217At are equilibrium with 233U and 239Np resulting from integration of thorium anduranium with naturally produced neutrons. The total amount of astatine present in theearth's crust, however, is less than 1 oz.