The actinide concept in nuclear chemistry was first theorized by Glenn T. Seaborg in 1944, resulting in the extension of Dmitri Mendeleev's periodic table of the elements by placing a new actinide series, for elements 89–103, 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.