Superactinide

A superactinide is any one of 32 hypothetical chemical elements from atomic numbers 121 (unbiunium) through 153 (unpenttrium), at which the 5g and 6f electron shells are filled up. They may be referred to using IUPAC systematic element names. None of these elements have been synthesized, and it is possible that none have isotopes with stable enough nuclei to receive significant attention in the near future. Synthesis has only been attempted for elements 122, 124, 126, and 127. It is also probable that, due to drip instabilities, only the lighter superactinides are physically possible and the periodic table may end soon after the island of stability expected to be centered at element 126. The theoretical existence of the series was proposed by Glenn T. Seaborg, winner of the 1951 Nobel Prize in Chemistry.