actinide concept
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In nuclear chemistry, the actinide concept proposed that the
actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The info ...
s form a second inner transition series homologous to the
lanthanides The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and ytt ...
. Its origins stem from observation of lanthanide-like properties in
transuranic element The transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium. All of these elements are unstable and decay radioactively into other elements. ...
s in contrast to the distinct complex chemistry of previously known actinides. Glenn T. 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. It was accepted shortly thereafter, resulting in the placement of a new actinide series comprising elements 89 (
actinium Actinium is a chemical element with the symbol Ac and atomic number 89. It was first isolated by Friedrich Oskar Giesel in 1902, who gave it the name ''emanium''; the element got its name by being wrongly identified with a substance An ...
) to 103 (
lawrencium Lawrencium is a synthetic chemical element with the symbol Lr (formerly Lw) and atomic number 103. It is named in honor of Ernest Lawrence, inventor of the cyclotron, a device that was used to discover many artificial radioactive elements. A radio ...
) below the lanthanides in Dmitri Mendeleev's periodic table of the elements.


Origin

In the late 1930s, the first four actinides (actinium, thorium,
protactinium Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds ...
, and uranium) were known. They were believed to form a fourth series of transition metals, characterized by the filling of 6 d orbitals, in which thorium, protactinium, and uranium were respective
homolog In biology, homology is similarity due to shared ancestry between a pair of structures or genes in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats and birds, the arms of prima ...
s of hafnium,
tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...
, and tungsten. This view was widely accepted as chemical investigations of these elements revealed various high
oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
s and characteristics that closely resembled the 5d transition metals. Nevertheless, research into quantum theory by
Niels Bohr Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922 ...
and subsequent publications proposed that these elements should constitute a 5 f series analogous to the lanthanides, with calculations that the first 5f
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
should appear in the range from
atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
90 (thorium) to 99 (
einsteinium Einsteinium is a synthetic element with the symbol Es and atomic number 99. Einsteinium is a member of the actinide series and it is the seventh transuranium element. It was named in honor of Albert Einstein. Einsteinium was discovered as a com ...
). Inconsistencies between theoretical models and known chemical properties thus made it difficult to place these elements in the periodic table. The first appearance of the actinide concept may have been in a 32-column periodic table constructed by Alfred 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. Following the discoveries of
transuranic elements The transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium. All of these elements are unstable and decay radioactively into other elements. ...
neptunium Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after the planet Uranus, led to it bein ...
and
plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...
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. Subsequent experiments targeting the then-unknown elements americium and curium 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 the lanthanides. Further experiments corroborated the hypothesis of an actinide (then referred to as "thorides" or "uranides") series. A spectroscopic study at the
Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development laboratories of the United States Department of Energy (DOE), located a short distance northwest of Santa Fe, New Mexico, ...
by
McMillan MacMillan, Macmillan, McMillen or McMillan may refer to: People * McMillan (surname) * Clan MacMillan, a Highland Scottish clan * Harold Macmillan, British statesman and politician * James MacMillan, Scottish composer * William Duncan MacMill ...
, Wahl, and Zachariasen indicated that 5f orbitals, rather than 6d orbitals, 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.


Acceptance

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 ''Chemical & Engineering News'' (''C&EN'') is a weekly news magazine published by the American Chemical Society, providing professional and technical news and analysis in the fields of chemistry and chemical engineering.berkelium Berkelium is a transuranic radioactive chemical element with the symbol Bk and atomic number 97. It is a member of the actinide and transuranium element series. It is named after the city of Berkeley, California, the location of the Lawrence Ber ...
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
104 104 may refer to: *104 (number), a natural number *AD 104, a year in the 2nd century AD * 104 BC, a year in the 2nd century BC * 104 (MBTA bus), Massachusetts Bay Transportation Authority bus route * Hundred and Four (or Council of 104), a Carthagin ...
to 121 and a
superactinide An extended periodic table theorises about chemical elements beyond those currently known in the periodic table and proven. , the element with the highest atomic number known is oganesson (''Z'' = 118), which completes the seventh period (row ...
series from atomic number
122 122 may refer to: *122 (number), a natural number *AD 122, a year in the 2nd century AD *122 BC, a year in the 2nd century BC *122 (film), ''122'' (film), a 2019 Egyptian psychological horror film *"One Twenty Two", a 2022 single by the American roc ...
to 153.


See also

*
Actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The info ...
* History of the periodic table *
Mendeleev's predicted elements Dmitri Mendeleev published a periodic table of the chemical elements in 1869 based on properties that appeared with some regularity as he laid out the elements from lightest to heaviest. When Mendeleev proposed his periodic table, he noted gaps ...


References

{{cite book , author=Glenn Seaborg , author-link=Glenn Seaborg , title=Lanthanides/Actinides: Chemistry , chapter=Origin of the Actinide Concept , series=Handbook on the Physics and Chemistry of Rare Earths , volume=18 , edition=1 , date=1994 , isbn=9780444536648 , id=LBL-31179 , chapter-url=https://cloudfront.escholarship.org/dist/prd/content/qt7cz371hx/qt7cz371hx.pdf Actinides Glenn T. Seaborg Nuclear chemistry Periodic table