< berkelium (bûr'klēəm) [from Berkeley], artificially produced radioactive chemical element; symbol Bk; at. no. 97; mass no. of most stable isotope 247; m.p. about 1,050°C; b.p. about 2,590°C; sp. gr. 14 (estimated); valence +3, +4. Berkelium is believed to be similar to the other members of the actinide series and to terbium, its homolog in the lanthanide series. It is found in group IIIb of the periodic table. The 10 isotopes of berkelium that are known are all radioactive; the element has not been found in the earth's crust. Berkelium-247, the most stable isotope (half-life about 1,400 years), is difficult to produce; berkelium-249 (half-life 314 days) is more easily produced in weighable quantities and is used in studies of berkelium chemistry. Berkelium metal exists in two crystal modifications (see allotropy) and is chemically reactive; the chloride, fluoride, sulfide, nitrate, sulfate, perchlorate, oxide, and dioxide have been produced. Berkelium was the fifth transuranium element to be synthesized. It was discovered late in 1949 by Glenn T. Seaborg, Stanley G. Thompson, and Albert Ghiorso, who produced it by bombarding americium-241 with alpha particles in the cyclotron of the Univ. of California at Berkeley. Weighable quantities of the pure element were first isolated by Thompson and B. B. Cunningham in 1958.