Periodic Table, The: Past, Present, And Future

by Geoffrey Rayner-canham;

  Table 12.2 Schematic of predominant species with pH under oxidizing conditions

  Commentary

  It is time for the lanthanoids to have their “day in the Sun” and not be relegated to a passing mention — if anything at all — in chemistry courses. Apart from the fact that they do have interesting chemistry, their many applications in the real world require students to be aware of them.

  References

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  2.H. Liu et al., “One-Pot Solvothermal Synthesis of Singly Doped Eu3+ and Codoped Er3+,Yb3+ Heavy Rare Earth Oxysulfide Y2O2S Nano-Aggregates and their Luminescence Study,” RSC Adv. 4(100), 57048–57053 (2014).

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  21.M. Laing, “A Revised Periodic Table with the Lanthanides Repositioned,” Found. Chem. 7, 203–233 (2005).

  22.P. F. Lang, “Is a Metal ‘Ions in a Sea of Delocalized Electrons’?” J. Chem. Educ. 95, 1787–1793 (2018).

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  31.C. T. Palumbo et al., “Molecular Complex of Tb in the +4 Oxidation State,” J. Am. Chem. Soc. 141(25), 9827–9831 (2019).

  32.P. F. Lang and B. C. Smith, “Ionization Energies of the Lanthanides,” J. Chem. Educ. 87(8), 875–881 (2010).

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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 th
rough 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 VI, the tendency in the later members is to have a stable lower oxidation state, such as the III state.

  Laing showed that, during 1945, Sacks had drawn the same conclusion from studying trends, not in orbital occupancy, but in physical properties [8]:

  Uranium seemed like a transition metal, seemed like eka-tungsten−and yet, I felt somehow uncomfortable about this, and decided to do a little exploring, to examine the densities and melting points of all the transition metals. As soon as I did this I discovered an anomaly, for where the densities of the metals steadily increased through Periods 4, 5, and 6, they unexpectedly declined when one came to the elements in Period 7. Uranium was actually less dense than tungsten, though one would have expected it to have been more so (thorium, similarly, was less dense than hafnium, not more so, as one would have expected). It was precisely the same with their melting points, these reached a maximum in Period 6, then declined. … Could it be that these elements were instead the beginning of a second rare-earth series precisely analogous to the first one in Period 6?

  Figure 13.4 Part of Coryell’s Periodic Table showing linkages of actinoids to earlier periods (adapted from Ref. [9]).

  The problem remained, as will be discussed in the following, that the early actinoids did indeed have high oxidation-state chemistry, which resembled that of the corresponding heavy transition metals. To recognize this fact, Coryell devised a Periodic Table, part of which is shown in Figure 13.4, which showed “tie-lines” to indicate the relevant linkages (the circled symbols are natural radioactive elements, while the boxed ones are synthetic).

  Oxidation States of the Actinoids

  The pattern of the highest common oxidation states of the early actinoids reflects the loss of all outer electrons, and this pattern parallels that of the transition metals more than that of the lanthanoids. For the later actinoids, +3 oxidation state is usually dominant, as with the lanthanoids. However, an additional oxidation state that results in an f7 or f14 configuration can be found for some of the actinoids.

  Kaltsoyannis and Kerridge have commented [10]:

  … the 5f orbitals of the early actinide elements have a larger radial extension than their 4f counterparts, and 5f involvement in the bonding of compounds of the first few actinides is not uncommon. Hence the bonding in compounds of, for example, uranium, is often found to be more covalent than in analogous compounds of the lanthanide series. … It is generally accepted that as the actinide series is crossed, the chemistry increasingly resembles that of the lanthanides; ionic bonding and a single (trivalent) oxidation state dominate.

  The first five actinoids definitely have the highest oxidation state as corresponding to the noble gas electron configuration (see Table 13.1). Naturally, there has been much interest in whether the series can be completed with plutonium(VIII) compounds but as of the date of writing, no stable solid plutonium(VIII) compound had been isolated. There is evidence from spectroscopic measurements of solution species [11] and also from computational studies [12] that Pu(VIII) species should be isolable.

  Table 13.1 Atom configurations and highest oxidation states for the early actinoids

  Element Atom

  Configuration Charge on Noble Gas

  Configuration Ion

  Actinium [Rn]7s25f06d1 +3

  Thorium [Rn]7s25f06d2 +4

  Protactinium [Rn]7s25f26d1 +5

  Uranium [Rn]7s25f36d1 +6

  Neptunium [Rn]7s25f46d1 +7

  Plutonium [Rn]7s25f6 +8(?)

  Table 13.2 Atom configurations and highest oxidation states for elements 101–103

  Element Atom

  Configuration Charge on [Rn]5f14

  Configuration Ion

  Mendelevium [Rn]7s25f13 +1

  Nobelium [Rn]7s25f 14 +2

  Lawrencium [Rn]7s25f147p1 +3

  Beyond, plutonium, americium has common oxidation states up to +7; curium, +4; berkelium, +4; while for the later actinoids, +3 predominates. There are two exceptions. First, the most stable oxidation state of nobelium is +2 [13]. Second, surprisingly, the +1 oxidation state is found for mendelevium [14]. It is stated that the +1 ionic radius is about 120 pm, in the range of the sodium and potassium ions. Both of these oxidation states can be accounted for by considering that they correspond to the full 5d14 set with the other outer orbitals empty (Table 13.2). For lawrencium, the 7p1 electron has an extremely low ionization energy [15], yet its chemistry is dominated by the +3 oxidation state.

  The Early Actinoid Relationships with Transition Metals

  As discussed earlier in this chapter, there are significant resemblances of the early actinoids to transition metals. We can state this similarity as:

  The actinoid relationship relates to similarities in chemical formulas and chemical properties between early members of the actinoid series and the corresponding members of the transition metal series.

  Hoffman and Lee reconstructed the Periodic Table to illustrate the closer links of the early actinoids, than lanthanoids with the corresponding transition metal group. They
placed the actinoids above the lanthanoids. By stepping the early actinoids, Hoffman and Lee hoped to indicate the degree of resemblance [17]. These similarities diminished from thorium to plutonium, the only link from the lanthanoids to the transition metals being cerium (Figure 13.5).

  To illustrate some of the transition metal/actinoid similarities, Table 13.3 shows comparative formulas of some compounds and polyatomic ions of uranium(VI) with those of the Group 6 metals in the +6 oxidation state. However, similarity in formula does not necessarily mean similarity in bonding and structure. For example, the molybdenyl cation, [MoO2]2+ has a bent geometry, while the uranyl cation [UO2]2+ is linear [16].

  Figure 13.5 Relationship of early actinoid (and early lanthanoid) elements to the transition metals (adapted from Ref. [17]).

  Table 13.3 A comparison of some species of uranium(VI) with those of the (VI) oxidation state of Group 6 members

  Similarities of Thorium(IV), Cerium(IV), and Hafnium(IV)

  Hoffman and Lee specifically noted (as had Villar, decades earlier) that not only did thorium(IV) chemistry strongly resemble that of zirconium(IV) and hafnium(IV) of the Group 4 elements, it also resembled that of cerium(IV) [17].

  As examples, all four of these ions form insoluble fluorides and phosphates. The similarity can also be noted from a comparison of acid–base behavior under strongly oxidizing conditions. The insoluble MO2 oxides (M = Th4+, Ce4+, Zr4+, Hf4+) being the only species over most of the pH range, the soluble M4+ ion only existing under very acidic conditions. The similarity in the electron configurations of their 4+ ions can be seen in Table 13.4.

  There are particularly strong similarities in the chemistry of cerium(IV) and thorium(IV). Cerium(IV) oxide and thorium(IV) oxide both adopt the fluorite structure. They form isostructural nitrates, M(NO3)4⋅5H2O, where M is Ce or Th, and both form hexanitrato-complex ions [M(NO3)6]2–. The major difference between the two elements in this oxidation state is that thorium(IV) is the thermodynamically stable form of that element while cerium(IV) is strongly oxidizing (Eθ = +1.44 V). There is a mineralogical link between cerium(IV) and thorium(IV). Thorium and the lanthanoids — particularly cerium — are found together in two minerals, monazite and xenotime [18].