In addition to his theoretical work in the development of the actinide concept, which placed the actinide series beneath the lanthanide series on the periodic table, he postulated the existence of super-heavy elements in the transactinide and superactinide series.
Udover sit teoretiske arbejde i udviklingen af konceptet om actinider, der blev placeret under lanthaniderne i det periodiske system, postulerede han eksistensen af supertunge grundstoffer i transactinid- og superactinid-serierne.