Actinide concept

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.