Actinide concept

Glenn Theodore Seaborg, one of the researchers who synthesized transuranic elements, proposed the actinide concept in 1944 as an explanation for observed deviations and a hypothesis to guide future experiments.

They were believed to form a fourth series of transition metals, characterized by the filling of 6d orbitals, in which thorium, protactinium, and uranium were respective homologs of hafnium, tantalum, and tungsten.

[2] This view was widely accepted as chemical investigations of these elements revealed various high oxidation states and characteristics that closely resembled the 5d transition metals.

[3] Nevertheless, research into quantum theory by Niels Bohr and subsequent publications proposed that these elements should constitute a 5f series analogous to the lanthanides, with calculations that the first 5f electron should appear in the range from atomic number 90 (thorium) to 99 (einsteinium).

Seaborg et al. failed to identify these elements under the premise that they were transition metals, but they were successfully separated and discovered in 1944, following the assumption that they would be chemically similar to the lanthanides.

Periodic table illustrating the placement of thorium and uranium as transition metals . Although hafnium was not known at the time, thorium was assumed to follow periodicity of group 4 and thus was placed there, and uranium was placed in group 6 below tungsten .