Element number 97, symbol Bk, the eighth member of the actinide series of elements. In this series the Sf electron shell is being filled, just as the 4f shell is being filled in the lanthanide (rare-earth) elements. These two series of elements are very similar in their chemical properties, and berkelium, aside from small differences in ionic radius, is especially similar to its homolog terbium. Berkelium does not occur in the Earth's crust because it has no stable isotopes.
It must be prepared by means of nuclear reactions using more abundant target elements. These reactions usually involve bombardments with charged particles, irradiations with neutrons from high-flux reactors, or production in a thermonuclear device.
Berkelium metal is chemically reactive, exists in two crystal modifications, and melts at 986°C (1806°F). Berkelium was discovered in 1949 by S. G. Thompson, A. Ghiorso, and G. T. Seaborg at the University of California in Berkeley and was named in honor of that city. Nine isotopes of berkelium are known, ranging in mass from 243 to 251 and in half-life from 1 hour to 1380 years. The most easily produced isotope is ²⁴⁹Bk, which undergoes beta decay with a half-life of 314 days and is therefore a valuable source for the preparation of the isotope ²⁴⁹Cf. The berkelium isotope with the longest half-life is ²⁴⁷Bk (1380 years), but it is difficult to produce in sufficient amounts to be applied to berkelium chemistry studies.