In nuclear chemistry, theactinide concept (also known as actinide hypothesis) proposed that theactinides form a second inner transition series homologous to thelanthanides. Its origins stem from observation of lanthanide-like properties intransuranic elements in contrast to the distinct complex chemistry of previously known actinides.Glenn Theodore Seaborg, one of the researchers whosynthesized transuranic elements, proposed the actinide concept in 1944 as an explanation for observed deviations and a hypothesis to guide future experiments. It was accepted shortly thereafter, resulting in the placement of a new actinide series comprising elements 89 (actinium) to 103 (lawrencium) below the lanthanides inDmitri Mendeleev'speriodic table of the elements.[1]
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 respectivehomologs of hafnium,tantalum, and tungsten.[2] This view was widely accepted as chemical investigations of these elements revealed various highoxidation states and characteristics that closely resembled the 5d transition metals.[3] Nevertheless, research into quantum theory byNiels Bohr and subsequent publications proposed that these elements should constitute a 5f series analogous to the lanthanides, with calculations that the first 5felectron should appear in the range fromatomic number 90 (thorium) to 99 (einsteinium). Inconsistencies between theoretical models and known chemical properties thus made it difficult to place these elements in theperiodic table.[2]
The first appearance of the actinide concept may have been in a 32-column periodic table constructed byAlfred Werner in 1905. Upon determining the arrangement of the lanthanides in the periodic table, he placed thorium as a heavier homolog of cerium, and left spaces for hypothetical radioelements in the seventh period, though he did not establish the correct order of the known actinides.[4]
Following the discoveries oftransuranic elementsneptunium andplutonium in 1940 and preliminary investigations of their chemistry, their placement as a fourth transition metal series was challenged. These new elements exhibited various properties that suggested a close chemical similarity to uranium rather than their supposed transition metal homologs.[3] Subsequent experiments targeting the then-unknown elementsamericium andcurium raised further questions. 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 thelanthanides.[5] Further experiments corroborated the hypothesis of an actinide (then referred to as "thorides" or "uranides")[2] series. A spectroscopic study at theLos Alamos National Laboratory byMcMillan, Wahl, and Zachariasen indicated that 5f orbitals, rather than 6dorbitals, were being filled. However, these studies could not unambiguously determine the first element with 5f electrons and therefore the first element in the actinide series.[2][3]
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.[3] Despite its apparent correctness, they did not recommend Seaborg to submit a communication toChemical and Engineering News, fearing that it was a radical idea that would ruin his reputation.[5] He nevertheless submitted it and it gained widespread acceptance; new periodic tables thus placed the actinides below the lanthanides.[5] Following its acceptance, the actinide concept proved pivotal in the groundwork for discoveries of heavier elements, such asberkelium in 1949.[6] The actinide concept explained some of the observed properties of the first few actinides, namely the presence of +4 to +6 oxidation states, and proposedhybridization 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.[2]
Further elaborations on the actinide concept led Seaborg to propose two more series of elements continuing the established periodicity. He proposed atransactinide series from atomic number104 to121 and asuperactinide series from atomic number122 to 153.[3]