Berkelium, chemical element, symbol Bk; for physical constants see Periodic Table. Berkelium was discovered in Berkeley, Calif., in 1949 by S.G. Thompson, Albert Ghiorso, and Glenn Seaborg. It was synthesized initially by bombarding americium-241 with helium ions. It is produced from plutonium by multiple neutron capture in a high-flux nuclear reactor. Berkelium is a metallic element and a member of the actinide series. Several compounds of berkelium have been made, and its chemistry is analogous to that of curium. Berkelium-247 is the longest-lived isotope. It is an alpha-emitter with a half-life of 1,400 years. Ten isotopes of the element are known.