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Chapter 13
Actinoid and Post-Actinoid Elements
The location of the series of radioactive elements that we now call the actinoids was once a question in itself. These elements were first believed to be the commencement of a new d-block row, as a result of similarities to the chemistry of the corresponding elements above them. Then it was realized that they formed a new f-series of elements. However, it is sometimes overlooked that the earlier actinoids do indeed have strong resemblances to transition metals in their chemistry. This chapter also includes discussion of the post-actinoid elements as they fit better in this context.
In this chapter, the actinoids will be considered as encompassing the elements from 89 to 103. As even the longest lived isotopes of the later actinoids are highly radioactive, most of the discussions will be about the earlier actinoids that have very long-lived isotopes. In addition, the limited known chemistry of the post-actinoid elements will be contextualized.
The Actinoid Elements
Just as the lanthanoids were defined in Chapter 12, so it is necessary to define the actinoids. The commonly accepted definition of an actinoid is therefore:
An actinoid is any of the series of 15 consecutive chemical elements in the Periodic Table from actinium to lawrencium (Figure 13.1).
Actinide Hypothesis
But first, a step back in time to consider the dispute on the location of the actinoid elements in the Periodic Table. Two of the heavy radioactive elements had been discovered by the mid-19th century: uranium (1798) and thorium (1829). These featured as the sole members of the Series 10 in Mendeléev’s 1871 Periodic Table, in Groups IV and VI respectively. By the 1905 version of his Periodic Table, radium (then Rd) had been discovered (1899). This element was placed in the Group II location in what had become Series 12 (Figure 13.2) [1].
Figure 13.1 The actinoid elements as defined in this chapter.
Figure 13.2 The 8th through 12th Periods of Mendeléev’s 1905 version of the Periodic Table.
By the early 20th century, two more elements were added to the series: actinium and protactinium, the “missing” elements exhibiting +3 and +5 oxidation-state chemistry. Cotton has commented [2]:
So on its appearance in 1938, Emeléus and Anderson’s “Modern Aspects of Inorganic Chemistry” (which was to become the leading inorganic chemistry textbook of the day) [3] printed a Periodic Table on page 2 which showed the four known actinides (although they did not refer to them as that) Ac, Th, Pa, and U in groups III-VI respectively.
It was in 1942 that Villar queried this assignment. He first made a general proposal for a revision of the 7th Period [4]:
If the sixth and seventh periods are made up of an equal number of elements, they should have identical configurations and therefore there must exist in the seventh period an array of 15 elements similar to the rare earths which, by analogy, should occupy the place reserved so far for actinium alone (Z = 89).
In a follow-up article, Villar focused upon the similarities of thorium to the rare earth elements, especially cerium. One potential problem that he posed was the dominance of the +4 oxidation state for thorium compared with +3 for the rare earths. He answered his own question [5]:
It is important to note that the element which occupies the second place in the rare earth series and which would be the homolog of thorium in the actinium series is cerium, which is characterized by being tri- and tetravalent. … besides, cerium compounds in general have the same empirical formulas as the corresponding thorium compounds.
Unfortunately, Villar’s contribution was totally overlooked, as was the Actinide Hypothesis championed in French journal articles by Janet in 1928 [6].
During the 1940s, four more elements were synthesized: neptunium, plutonium, americium, and curium. However, their respective chemistries did not correspond at all with those of the matching transition metal series. In fact, with the predominant oxidation state of +3, these, too, more resembled the lanthanoids. It was Seaborg who gained fame for pronouncing that the elements had been assigned the wrong location in the Periodic Table. In an article in Science, he displayed a version of the Periodic Table (Figure 13.3) with the post-radium elements located both as the traditional continuation of the d-series and as a new “actinide” set [7].
Figure 13.3 Seaborg’s 1946 version of the Periodic Table (from Ref. [7]).
Seaborg commented [7]:
… I do want to say that the evidence strongly indicates that we are dealing here with a transition series of elements in which the 5f electron shell is being filled in a manner similar to the filling of the 4f electron shell in the well-known rare earth series. Apparently this new transition series begins with actinium in the same sense that the rare earth series begins with lanthanum, and, although the first elements in the heavy series exhibit the property of undergoing oxidation to higher oxidation states up to a maximum oxidation state of
