Isotopes of berkelium
Berkelium 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, 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.