In the late 1930s, the first four actinides (actinium, thorium, protactinium, and uranium) were known. 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.[1] 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.[2] 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). Inconsistencies between theoretical models and known chemical properties thus made it difficult to place these elements in the periodic table.[2]