Berkelium (₉₇Bk) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was ²⁴³Bk in 1949. There are nineteen known radioisotopes, from ²³³Bk to ²⁵³Bk (except ²³⁵Bk and ²³⁷Bk), and seven nuclear isomers. The longest-lived isotope known is ²⁴⁷Bk with a half-life of 1,380 years; however ²⁴⁸Bk, which has not been observed to decay, may live longer.

The isotope commonly used in study, though, is ²⁴⁹Bk as is it the only that can be usefully extracted from reactor actinides and the only ever available in weighable quantity.