text: The discoveries of americium and curium under the hypothesis that they resembled the lanthanides prompted Seaborg to propose the concept of an actinide series to his colleagues in 1944 – with the central premise being similarity to the lanthanides and filling of f orbitals. Despite its apparent correctness, they did not recommend Seaborg to submit a communication to Chemical and Engineering News, fearing that it was a radical idea that would ruin his reputation. He nevertheless submitted it and it gained widespread acceptance; new periodic tables thus placed the actinides below the lanthanides. Following its acceptance, the actinide concept proved pivotal in the groundwork for discoveries of heavier elements, such as berkelium in 1949. The actinide concept explained some of the observed properties of the first few actinides, namely the presence of +4 to +6 oxidation states, and proposed hybridization of the 5f and 6d orbitals, whose electrons were shown to be loosely bound in these elements. It also supported experimental results for a trend towards +3 oxidation states in the elements beyond americium.
  Further elaborations on the actinide concept led Seaborg to propose two more series of elements continuing the established periodicity. He proposed a transactinide series from atomic number 104 to 121 and a superactinide series from atomic number 122 to 153.