The Nobel Prize in Chemistry 1951 was awarded jointly to Edwin Mattison McMillan and Glenn Theodore Seaborg "for their discoveries in the chemistry of the transuranium elements".
 

Transuranium elements are the chemical elements with atomic numbers greater than 92. None of these elements are stable; they decay radioactively into other elements. Dr. McMillan, in 1934 Fermi showed that nuclear transmutations could be brought about by irradiating the heaviest elements with neutrons. Research into the reactions thus produced has, however, met with certain difficulties, and it took longer than was expected for the existence of the transuranium elements to be proved. Dr. Seaborg, at a time when the possibilities of finding new elements appeared to be exhausted, has produced a whole row of them and thus extended the Periodic System beyond the limits which, one might say, Nature seemed to have established.