dubnium

Synthesized, radioactive, metallic element of the transactinide series, atomic number 105, relative atomic mass 261. Six isotopes have been synthesized, each with very short (fractions of a second) half-lives. Two institutions claim to have been the first to produce it: the Joint Institute for Nuclear Research in Dubna, Russia, in 1967; and the University of California at Berkeley, USA, who disputed the Soviet claim in 1970. It was known by the temporary name unnilpentium until 1997.