Periodic Table, The: Past, Present, And Future

by Geoffrey Rayner-canham;

  Table 13.4 The 4+ ions and their corresponding electron configurations

  Ion Noble Gas Core Electron Configuration

  Zirconium(IV) [Kr]

  Cerium(IV) [Xe]

  Hafnium(IV) [Xe]4f14

  Thorium(IV) [Rn]

  Similarities of the Later Actinoids with the Lanthanoids

  There is a generic similarity of the later actinoids with the lanthanoids as they share the common oxidation state of +3. As an example of a close parallel, Xu and Pyykkö have shown that each of the first three ionization potentials of lawrencium is very close to those of the lanthanoid analogue, lutetium [19].

  One specific correlation was demonstrated by Thompson et al. in the context of chromatographic elution curves (Figure 13.6). As can be seen there is a remarkable match in peaks for the corresponding pairs of terbium–berkelium, gadolinium–curium, and europium–americium [20].

  According to computational studies, one difference between a lanthanoid(III) ion and the corresponding actinoid ion is the degree of bond covalency involving f orbitals. In this investigation, a comparison of the bonding in valence-isoelectronic [EuCl6]3− and [AmCl6]3− was made. Cross et al. concluded that the involvement of the americium 5f electrons in bonding with the 3d electrons of chlorine is far more significant than any involvement of the europium 4f electrons with the 3d orbitals of chlorine [21].

  Figure 13.6 A comparison of chromatographic elution curves for three lanthanoids and three actinoids (adapted from Ref. [20]).

  Similarity of Nobelium(II) and Group 2 Elements

  As was mentioned earlier, nobelium favors the +2 oxidation state. This preference is far more than that of ytterbium, the corresponding lanthanoid, for which the +2 oxidation state is readily oxidized.

  Table 13.5 Atom and 2+ ion configurations for radium and nobelium

  Element Atom

  Configuration +2 Ion

  Configuration

  Radium [Rn]7s2 [Rn]

  Nobelium [Rn]7s25f 14 [Rn]5f 14

  However, somewhat surprisingly, there is a much greater similarity to the chemistry of the lower alkaline earth metal ion as Maly et al. have stated [22]:

  In the absence of oxidizing or reducing agents the chromatographic and coprecipitation behavior of element 102 is similar to that of the alkaline earth elements. … Nobelium is the first actinide for which the +2 oxidation state is the most stable species in aqueous solution.

  The similarity in electron configuration with radium can be seen from Table 13.5.

  Post-Actinoid Elements

  The post-actinoid elements or, more correctly, the super-heavy elements (see Chapter 5), are those from element 104 (rutherfordium) up to yet-to-be-discovered element 126. From rutherfordium to copernicium, these are the 6d members of the transition metal series, then nihonium to oganesson correspond to the filling of the 7p orbitals.

  With ever shorter half-lives leading to the ephemeral elements (see Chapter 5), the knowledge of their actual chemistry is very limited. Computational studies of what their chemistry should be increasingly fill the literature, but these unsubstantiated claims will be given only limited space here.

  The 6d Elements

  With isotope half-lives up to 1.3 hr, aspects of the chemistry of rutherfordium are well established. In particular, the +4 ion seems to be the sole common oxidation state, corresponding to an [Rn]5f14 electron configuration. The compounds characterized to the date of writing this book match those of the heavier Group 4 elements (zirconium and hafnium), specifically: RfCl4, RfBr4, RfOCl2, and K2RfCl6 [23].

  Proceeding along the 6d series, the chemistry is based on every more limited data. Dubnium seems to behave like its Group 5 “relatives” niobium and tantalum, and also “pseudo-homologue” protactinium [24]. Likewise, seaborgium appears to form SgO2Cl2 analogous to MoO2Cl2 and WO2Cl2 [25]. Synthesis of Sg(CO)6 matching with Mo(CO)6 and W(CO)6, has also been claimed [26]. Lougheed has commented that, despite predictions of relativistic effects causing dramatic changes in the chemical behavior of these elements, the chemistry of the 6d elements established so far seems to be that of their corresponding earlier group members. Seaborgium, in particular, from its limited chemistry, seems to be just a “normal” Group 6 element [27].

  Bohrium, too, follows the pattern of having a matching chemical compound to the other heavy transition metals of Group 7. That is, it forms the compound BhO3Cl, analogous to TcO3Cl and ReO3Cl [28]. Similarly, hassium has been shown to form the characteristic species of Group 8, that is, HsO4 [29].

  With such short-lived isotopes, meitnerium seems to mark the current limit of practical chemistry. The chemistry of this element would be of particular interest. Assuming that it did behave chemically as a member of Group 9, it has been proposed from theoretical studies that it might form a compound of meitnerium(IX), that is, [MtO4]+, isoelectronic to HsO4 [30].

  A theoretical study of the properties of darmstadtium indicated that it should resemble platinum of Group 10 in its chemistry, in particular in forming a strong bond to the carbonyl ligand [31]. Similarly, roentgenium is predicted to resemble silver of Group 11 in readily forming a +1 species in the [Rg(OH2)2]+ ion [32]. Likewise, according to computational analysis, copernicium is likely to have physical and chemical properties resembling those of mercury [33].

  The 7p Elements

  All of the presumptions of the chemistry of these elements come from theoretical computations. As such, and as some are contradictory, only a few selected cases will be described. Nihonium is expected to be a typical Group 13 element with a predominant +3 oxidation state [34] while it has been proposed that flerovium is a volatile metal [35]. The most attention has focused on oganesson, the Group 18 member of the period. It has been suggested that this element would be a liquid at room temperature [36]. It has also been proposed that cooled to its solid phase, oganesson would be a semiconductor [37].

  And Beyond . . .

  With the synthesis of element 117, the 7th Period has been completed. In Chapter 1, the issue of the future was addressed from the nucleosynthesis aspect. From the chemistry perspective, it is not about synthesizing a few fleeting atoms, but about finding long-lived isotopes — if they exist. Is there an “island of stability” yet to be found? If so, can super-neutron-rich projectiles be synthesized to bombard superheavy nuclei and reach the “island” [38]? Though such claims have been made before, it really does seem that the limit of actual chemistry of any new element has been reached. Yet there is the tantalization that such elements would open new possibilities in electron structure. Beyond the 8s orbitals loom the possibility of the 5g set. What are the properties of elements with g electrons? Will the orbitals fill in sequence or will things become “messy” as Pyykkö has suggested, in what is now called the Pyykkö Model [39].

  Gilead has commented upon the “eka-elements” — those which are not actually known [40]:

  There is no guarantee that they [eka-elements] will eventually be discovered, synthesized, or isolated as actual.

  Perhaps this is indeed the completion of elemental chemistry as chemists know it.

  Commentary

  The actinoids exhibit a wider variety of chemical behavior than do the lanthanoids. Even though the first members of the actinoid series are no longer placed with the transition metals, nevertheless, in the highest oxidation states, there are strong chemical similarities to them. In addition, toward the end of the series, the preference for a 5f14 electron configuration marks results in two surprisingly low stable oxidation states. Unfortunately, the short half-lives of these later actinoids decrease the ability to fully explore their chemistry. Despite the strong influence of relativistic effects, even for the post-actinoid elements, the element properties so far seem to be mostly those expected for the appropriate group membership.

  Will this continue into the 8th Period? As Haba has commented [41]:

  Owing to the predicted strong influence of relativistic effect, any experimental investigation of their properties
is fascinating.

  Is attaining stable enough atoms of elements of the 8th Period a feasible goal or simply fantasy? Only time will tell.

  References

  1.P. J. Stewart, “Mendeleev’s Predictions: Success and Failure,” Found. Chem. 21, 3–9 (2019).

  2.S. A. Cotton, Personal Communication, 06 October 2019.

  3.H. J. Emeléus and J. S. Anderson, Modern Aspects of Inorganic Chemistry, Routledge & Sons Ltd., London, England (1938).

  4.G. E. Villar, “On a Suggested Revision of the Seventh Period of the Periodic Table,” J. Chem. Educ. 19(6), 286 (1942).

  5.G. E. Villar, “A Suggested Revision of the Position of Thorium in the Fourth Period of the Periodic Table,” J. Chem. Educ. 19(7), 329–330 (1942).

  6.P. J. Stewart, “Charles Janet: Unrecognized Genius of the Periodic System,” Found. Chem. 12, 5–15 (2010).

  7.G. T. Seaborg, “The Transuranium Elements,” Science 104, 379–386 (1946).

  8.M. J. Laing, “The Question Mark at Uranium,” Found. Chem. 12, 27–30 (2010).

  9.C. D. Coryell, “The Periodic Table: The 6d-5f Mixed Transition Group,” J. Chem. Educ. 29, 62–64 (1952).

  10.N. Kaltsoyannis and A. Kerridge, “11: Chemical Bonding of Lanthanides and Actinides,” in G. Frenking and S. Shaik (Eds.), The Chemical Bond: Chemical Bonding Across the Periodic Table, Wiley-VCH, Weinheim, Germany, 342–343 (2014).

  11.Yu. M. Kiselev et al., “On Existence and Properties of Plutonium (VIII) Derivatives,” Radiochim. Acta 102(3), 227–237 (2014).

  12.W. Huang et al., “Is Octavalent Pu(VIII) Possible? Mapping the Plutonium Oxyfluoride Series PuOnF8–2n (n = 0–4),” Inorg. Chem. 54(17), 8825–8831 (2015).

  13.R. J. Silva et al., “Comparative Solution Chemistry, Ionic Radius, and Single Ion Hydration Energy of Nobelium,” Inorg. Chem. 13(9), 2233–2237 (1974).

  14.N. B. Mikheev, “Lower Oxidation States of Actinides,” Radiochim. Acta 32, 69–80 (1983).

  15.A. Türler, “Lawrencium Bridges a Knowledge Gap,” Nature 520, 166–167 (2015).

  16.K. Tatsumi and R. Hoffmann, “Bent cis d0 MoO22+ vs. Linear trans d0f0 UO22+: A Significant Role for Nonvalence 6p Orbitals in Uranyl,” Inorg. Chem. 19(9), 2656–2658 (1980).

  17.D. C. Hoffman and D. M. Lee, “Chemistry of the Heaviest Elements — One Atom at a Time,” J. Chem. Educ. 76(3), 332–347 (1999).

  18.H. E. Kremers, “Technology of the Rare Earths,” J. Chem. Educ. 62(8), 665–667 (1985).

  19.W.-H. Xu and P. Pyykkö, “Is the Chemistry of Lawrencium Peculiar?” Phys. Chem. Chem. Phys. 18, 17351–17355 (2016).

  20.S. G. Thompson, A. Ghiorso, and G. T. Seaborg, “The New Element Berkelium (Atomic Number 97),” Phys. Revs. 80, 781–789 (1950).

  21.J. N. Cross et al., “Covalency in Americium(III) Hexachloride,” J. Am. Chem. Soc. 139, 8667–8677 (2017).

  22.J. Maly et al., “Nobelium: Tracer Chemistry of the Divalent and Trivalent Ions,” Science 160, 1114–1115 (1968).

  23.B. Kadkhodayan et al., “On-Line Gas Chromatographic Studies of Chlorides of Rutherfordium and Homologs Zr and Hf,” Radiochim. Acta 72(4), 169–178 (1996).

  24.L. Öhrström, “Brief Encounters with Dubnium,” Nat. Chem. 8, 986 (2016).

  25.M. Schädel et al., “Chemical Properties of Element 106 (Seaborgium),” Nature 388, 55–57 (1997).

  26.J. Even et al., “Synthesis and Detection of a Seaborgium Carbonyl Complex,” Science 345(6203), 1491–1493 (2014).

  27.R. Lougheed, “Oddly Ordinary Seaborgium,” Nature 388, 21–22 (1997).

  28.R. Eichler et al., “Chemical Characterization of Bohrium (Element 107),” Nature 407, 63–65 (2000).

  29.C. E. Düllman et al., “On the Stability and Volatility of Group 8 Tetroxides, MO4 (M = Ruthenium, Osmium, and Hassium (Z = 108)),” J. Phys. Chem. B 106(26), 6679–6684 (2002).

  30.D. Himmel et al., “How Far Can We Go? Quantum-Mechanical Investigations of Oxidation State +IX,” ChemPhysChem. 11(4), 865–869 (2010).

  31.M. Patzschke and P. Pyykkö, “Darmstadtium Carbonyl and Carbide Resemble Platinum Carbonyl and Carbide,” Chem. Commun. 17, 1982–1983 (2004).

  32.R. D. Hancock, L. J. Bartolotti, and N. Kaltsoyannis, “Density Functional Theory-Based Prediction of Some Aqueous-Phase Chemistry of Element 111. Roentgenium(I) Is the ‘Softest’ Metal Ion,” Inorg. Chem. 45(26), 10780–10785 (2006).

  33.R. Eichler et al., “Thermochemical and Physical Properties of Element 112,” Angew. Chem. Int. Ed. Engl. 47(17), 3262–3266 (2008).

  34.M. Seth and P. Schwerdtfeger, “The Chemistry of Superheavy Elements. III. Theoretical Studies on Element 113 Compounds,” J. Chem. Phys. 111, 6422–6433 (1999).

  35.A. Yakushev et al., “Superheavy Element Flerovium (Element 114) Is a Volatile Metal,” Inorg. Chem. 53(3), 1624–1629 (2014).

  36.R. M. MacRae and T. J. Kemp, “Oganesson: A Most Unusual ‘Inert Gas’,” Sci. Prog. 101(2), 101–120 (2018).

  37.J.-M. Mewes et al., “Oganesson Is a Semiconductor: On the Relativistic Band-Gap Narrowing in the Heaviest Noble-Gas Solids,” Angew. Chem. Int. Ed. Engl. 58(40), 14260–14264 (2019).

  38.J. Reedijk, “Row 7 of the Periodic Table Complete: Can We Expect More New Elements; and If So, When?” Polyhedron 141, 1–4 (2018).

  39.P. Pyykkö, “A Suggested Periodic Table up to Z ≤ 172, Based on Dirac-Fock Calculations on Atoms and Ions,” Phys. Chem. Chem. Phys. 13, 161–168 (2011).

  40.A. Gilead, “Eka-Elements as Chemical Pure Possibilities,” Found. Chem. 18, 183–194 (2016).

  41.H. Haba, “A New Period in Superheavy-Element Hunting,” Nat. Chem. 11, 10–13 (2019).

  Chapter 14

  Pseudo-Elements

  Most chemists regard the ammonium ion as behaving much like an alkali metal cation. Likewise, cyanide ion shows similarities to halide ions. In this chapter, the similarities and differences of these polyatomic ions to their group analogues will be explored. In addition, some less-common “pseudo-element” ions will be introduced.

  … and so, Gentle Reader, the end of this particular voyage has been reached. Rest assured, Gentle Reader, there are other voyages out there to cross hitherto uncharted seas of patterns and trends in the Periodic Table. The Periodic Table is not, and never has been “set in stone.” The Author has endeavored to show that, contrary to public belief and to the passing references to the Periodic Table in chemistry textbooks, it is a living evolving organism. The White Queen in Alice Through the Looking Glass had it correct (Figure 14.1).

  Yes, Alice, there are indeed fantastical — and once thought impossible — aspects to the Periodic Table. For example, until 1962, it was known that it was impossible to make any compound of the noble gas elements. There was the “proof” of the octet-rule limit — so how could they? In some chapters, it has been proposed that elements belonged in more than one place in the Periodic Table — or in a place other than their atomic number/electron configuration mandated location — heresy in the past. The awesomeness of the Periodic Table continues in this final chapter: compounds and polyatomic ions that “behave” like elements and element ions.

  Figure 14.1 Alice and the White Queen, from Alice Through the Looking Glass [1].

  Pseudo-Elements

  Some polyatomic ions resemble element ions in their behavior, and, in a few cases, there is a molecule that corresponds to the matching element. We can define this unusual category as:

  A pseudo-element is the parent of a polyatomic ion whose behavior in many ways mimics that of an ion of an element or of a group of elements.

  In this chapter, the focus will be on the ammonium ion as a pseudo-alkali metal ion, and on the cyanide ion as a pseudo-halide ion (Figure 14.2).

  The Ammonium Ion as a Pseudo-Alkali Metal Ion

  Even though the ammonium ion is a polyatomic cation containing two nonmetals, it behaves in many respects like an alkali metal ion. The similarity results from the ammonium ion being a large low-charge cation just like the cations of the alkali metals. In
fact, the radius of the ammonium ion (151 pm) is very close to that of the potassium ion (152 pm). However, the chemistry of ammonium salts more resembles that of rubidium or cesium ions, perhaps because the ammonium ion is not spherical, and its realistic radius is larger than its measured value. The similarity to the heavier alkali metals is particularly true of the crystal structures. Ammonium chloride, like rubidium chloride and cesium chloride, has a CsCl crystal lattice at high temperatures and a NaCl crystal lattice at low temperatures.

  Figure 14.2 Relationship of ammonium to the Group 1 elements and of cyanide to the Group 17 elements.

  The ammonium ion also resembles an alkali metal ion in its precipitation reactions. Although all simple sodium compounds are water soluble, there are insoluble compounds of the heavier alkali metal ions with very large anions. The ammonium ion gives precipitates with solutions of these same anions. A good example is the hexanitritocobaltate(III) ion, [Co(NO2)6]3−, which is commonly used as a test in qualitative analysis for the heavier alkali metals. With ammonium ion, a bright yellow precipitate of (NH4)3[Co(NO2)6] is obtained analogous to that of K3[Co(NO2)6] with potassium ion.

  However, the similarity does not extend to all chemical reactions that these ions undergo. For example, gentle heating of alkali metal nitrates typically gives the corresponding nitrite and oxygen gas, but heating ammonium nitrate results in decomposition of the cation and anion to give dinitrogen oxide and water.