Astatine (At). Diagram of the nuclear composition and electron configuration of an atom of astatine-210 (atomic number: 85), a synthetic isotope of this element. The nucleus consists of 85 protons (red) and 125 neutrons (yellow). 85 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Astatine is a radioactive halogen in group 17, period 6, and the p-block of the periodic table. Astatine-211, used in nuclear medicine, is produced by bombarding bismuth-209 with alpha particles. It has a half-life of less than 8 hours, its unstable nucleus undergoing electron capture or alpha particle decay.