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The 'actinide concept' in nuclear chemistry was first theorized by Glenn T. Seaborg in 1944, resulting in the correction of Dmitri Mendeleev's periodic table of the elements by placing a new actinide series below the lanthanide series. In later theoretical work, Seaborg proposed the transactinide series spanning elements 104-121 and the superactinide series containing elements 122-153.