Table 9.2 A comparison of standard reduction potentials for the Group 3 and 13 elements
In terms of the electron configuration of the tripositive ions, one would indeed expect that Al3+ (electron configuration, [Ne]) would resemble Sc3+ (electron configuration, [Ar]) more than Ga3+ (electron configuration, [Ar]3d10). Also of note, the standard reduction potential for aluminum fits better with those of the Group 3 elements than the Group 13 elements (Table 9.2) — as does its melting point.
Table 9.3 A comparison of aluminum, scandium, and gallium species under oxidizing conditions
In terms of their comparative solution behavior, aluminum resembles both scandium(III) and gallium(III). For each ion, the free hydrated cation exists only in acidic solution. On addition of hydroxide ion to the respective cation, the hydroxides are produced as gelatinous precipitates. Each of the hydroxides redissolve in excess base to give an anionic hydroxo-complex, [M(OH)4]−. The similarities are summarized in Table 9.3 (data for this, and later tables, from Ref. [16]).
There does seem to be a triangular relationship between these three elements. However, aluminum does more closely resemble scandium rather than gallium in its chemistry. If hydrogen sulfide is bubbled through a solution of the respective cation, scandium ion gives a precipitate of scandium hydroxide, and aluminum ion gives a corresponding precipitate of aluminum hydroxide. By contrast, gallium ion gives a precipitate of gallium(III) sulfide. Also, scandium and aluminum both form carbides, while gallium does not.
Not previously identified, there are similarities in the chemistry of yttrium and indium. For example, their aqueous chemistry is dominated by the soluble 3+ cation in acid and by the insoluble hydroxide at neutral and basic pH (Table 9.4).
As a final note, the bottom member of Group 13, thallium, has very different chemistry to either yttrium or indium. The chemistry of thallium is more appropriately linked to that of silver through the “knight’s move” relationship (see Chapter 10).
Table 9.4 A comparison of yttrium and indium species under oxidizing conditions
Group 4 and Group 14
This Group–pair (Figure 9.5) seems to be unique in that, although there are similarities between titanium(IV) and silicon(IV), there is a much greater resemblance of titanium(IV) with tin(IV), farther down Group 14.
In some ways, the chemistry of titanium(IV) resembles that of all the Group 14 elements in their +4 oxidation state. In particular, all five form tetrahedrally coordinated chlorides that are hydrolyzed to give the dioxide and hydrogen chloride.
Interestingly, even though they have significantly different molar masses, the chlorides of titanium(IV) and tin(IV) have remarkably similar melting and boiling points (Table 9.5). By contrast, both zirconium(IV) chloride and hafnium(IV) chloride are high-melting solids with a polymeric, six-coordinate structure.
As another example of the similarity of titanium(IV) and tin(IV), the most common form of crystal structure of titanium(IV) oxide is rutile, and tin(IV) oxide adopts the same structure. Also, titanium(IV) oxide and tin(IV) oxide share the rare attribute of thermochromism by turning from white to yellow reversibly on heating.
Figure 9.5 Members of Group 3 and Group 13.
Table 9.5 A comparison of melting points for the Group 4 and 14 chlorides
Although the emphasis in this chapter is on links near the top of the respective groups, there are also relationships among the lower members. For example, the zirconates, M2Zr2O7, and the stannates, M2Sn2O7, adopt similar structures [17].
Group 5 and Group 15
In this Group–pair (Figure 9.6), the major resemblance seems to be between vanadium(V), phosphorus(V), and arsenic(V). There is also a similarity in aqueous species between niobium and antimony.
Figure 9.6 Members of Group 5 and Group 15.
Table 9.6 A comparison of aqueous vanadium, phosphorus, and arsenic species in dilute solution under oxidizing conditions
In terms of the simple oxo-anions, vanadium resembles both phosphorus and arsenic. Vanadate, phosphate, and arsenate are all strong bases with similar pKa values. The only significant difference is that at low pH, vanadium forms the vanadyl ion, not the undissociated acid as do phosphorus and arsenic (Table 9.6).
There are a significant number of parallel compounds between vanadium, phosphorus, and arsenic, as can be seen from Table 9.7. Of the two, vanadium more closely resembles phosphorus as there are several examples, two of which are listed in the following, for which there is no known arsenic analogue.
Table 9.7 Some parallel compounds and ions of vanadium(V) with phosphorus(V) and arsenic(V)
Table 9.8 A comparison of aqueous antimony and niobium species under oxidizing conditions
There is also an interesting parallel in aqueous species between antimony and niobium as is shown in Table 9.8. Though the aqueous antimonate ion is usually represented as [Sb(OH)6]−(aq) and the aqueous niobate ion as [NbO3]− (aq), Greenwood and Earnshaw [18] have pointed out that for both of them, isopolymeric species predominate over most of the soluble range. By contrast, tantalum forms insoluble tantalum(V) oxide across the full pH range, while bismuth(III) dominates that element’s aqueous chemistry.
The 4th Period Anomaly Revisited
In Chapter 7, the concept of the 4th Period anomaly was introduced. This anomaly was characterized by some aspects of the chemistry of the 4th Period member of a specific group differing from the pattern for the other group members [19]. Dasent listed some of the 4th Period examples [20]. In this context, he noted that while PCl5, NbCl5 (of Group 5), and SbCl5 (of Group 15) are stable and well characterized, members of the 5th Period, both VCl5 (Group 5) and AsCl5 (Group 15) were elusive. They are now known, AsCl5 decomposing above −50°C [21] and VCl5 decomposing above −40°C [22], but certainly not “stable” species like the other matching chlorides.
Group 6 and Group 16
Just as vanadium(V) resembles phosphorus(V) and arsenic(V), so chromium(VI) resembles both sulfur(VI) and selenium(VI) (Figure 9.7).
Again there are parallels in the acid–base behavior of the oxo-anions, the only difference in this case being that chromic acid is a weaker acid than either sulfuric acid or selenic acid (Table 9.9).
There are also several formula similarities between chromium(VI) and both sulfur(VI) and selenium(VI). A few examples are given in Table 9.10.
Figure 9.7 Members of Group 6 and Group 16.
Table 9.9 A comparison of aqueous chromium, sulfur, and selenium species under oxidizing conditions
Table 9.10 Some parallel compounds and ions of chromium(VI) with sulfur(VI) and selenium(VI)
Group 7 and Group 17
Once again, there seems to be a triangular relationship, this time between manganese(VII), chlorine(VII), and bromine(VII) (Figure 9.8). There are also parallels in formulas between rhenium(VII) and iodine(VII).
The most obvious similarity between the three at the top of their Groups are the strongly oxidizing oxo-anions: permanganate, perchlorate, and perbromate. All three elements form corresponding trioxofluorides: MnO3F, ClO3F, and BrO3F. There seems to be a slightly greater similarity between manganese(VII) and chlorine(VII) in that only those two form oxides in the +7 oxidation state: Cl2O7, and Mn2O7, both of which are highly explosive liquids at room temperature.
Figure 9.8 Members of Group 7 and Group 17.
Table 9.11 Some parallel compounds and ions of rhenium(VII) and iodine(VII)
Equally interesting in this group–pair are the similarities of rhenium with iodine.
Some of the parallel compounds are shown in Table 9.11. Of note, ReOF5 has a melting point of 44°C while that of IOF5 is 45°C.
Group 8 and Group 18
Somewhat surprisingly, there are two elements near the bottom of each group that share several similarities in chemical formulas: “noble metal” osmium(VIII) and “noble gas” xenon(VIII) (Figure 9.9).
Figure 9.9 Members of Group 8 and Group 18.
Table 9.12 Some parallel compounds of osmium(VIII) and
xenon(VIII)
Group 8 Group 18
OsO4 XeO4
OsO3F2 XeO3F2
OsO2F4 XeO2F4
Some parallels in formula are shown in Table 9.12. Similarities even extend to chemical behavior: osmium(VIII) oxide, OsO4, is a yellow solid and strongly oxidizing; while xenon tetraoxide, XeO4, is a pale yellow explosive compound.
Group 1 and Group 11
Group 1 and Group 11 are the most problematic of the group–pairs. In Chapter 8, it was argued that, in fact, there is no “Group 11” as the three elements: copper, silver, and gold, have so little in common. In the context of this chapter, it is the parallel in the +1 oxidation state, which must be considered. It is only for silver that this oxidation state dominates, thus it will be the focus of the comparison here.
There is one similarity: several of the sodium and silver(I) compounds are isostructural. These pairs include NaNO3 and AgNO3; Na2SO4 and Ag2SO4; and Na2S2O6·2H2O and Ag2S2O6·2H2O.
This brief list provides meager evidence of linkage between these two Groups. Thompson alluded to the lack of any correlations [23]:
Because of these differences in electronic structure comparison of silver with the alkali metals is fruitless even though each possesses a single s electron in the outermost shell. Probably the only similarity between the two is their diamagnetism and lack of colour.
Group 2 and Group 12
Based upon chemical similarities, in 1905, Werner designed a Periodic Table that showed beryllium and magnesium to belong to the zinc group (Figure 9.10) [5]. This was not mere speculation. Over 100 years later, Restrepo has shown on chemical topological grounds, by means of a hypergraph, that the chemistry of beryllium and magnesium fits more closely with that of zinc, cadmium, and mercury, than the lower members of Group 2 [24].
Figure 9.10 Part of Werner’s Periodic Table showing beryllium and magnesium as part of the zinc Group (from Ref. [5]).
Figure 9.11 Members of Group 2 and Group 12.
There are certainly grounds to consider this assignment. All of these elements have +2 as the sole common oxidation state, except for mercury for which it is the higher oxidation state. Thus there are some resemblances for all of the Group 2 elements with zinc and cadmium (Figure 9.11). For example, all six of these elements form hygroscopic anhydrous metal chlorides.
The closer link seems to be between cadmium and calcium. Cadmium oxide has the NaCl structure, as do the Group 2 oxides. Of specific biochemical relevance, high levels of calcium ion inhibit the toxicity of cadmium(II) ion, suggesting that calcium and cadmium ions share the same cellular pathway [25]. Thus overall, cadmium and calcium seem to have the closer resemblance.
A Curious (n + 5) and (n + 10) Case
In this chapter, it has been shown how there are resemblances between scandium in Group 3 and aluminum in Group 13. Curiously, the chemistry of aluminum also resembles that of the iron(III) ion (Figure 9.12). These similarities may be ascribed to the same 3+ charge and near-identical ion radii (and hence charge density). As a result of the high charge density, the [M(OH2)6]3+ ions of both metals are very strongly acidic through hydrolysis.
Figure 9.12 Iron of Group 8 and aluminum of Group 13.
There are some very specific similarities. For example, in the vapor phase, both ions form covalent chlorides of the form M2Cl6. These (anhydrous) chlorides can be used as Friedel–Crafts catalysts in organic chemistry, where they function by the formation of the [MCl4]– ion [26]. In addition, another result of their high charge densities.
There are, however, some significant differences. For example, the amphoteric aluminum oxide reacts with hydroxide ion to give the soluble tetrahydroxoaluminate ion, [Al(OH)4]–, whereas the basic iron(III) oxide remains in the solid phase. It is this difference that enables aluminum oxide to be separated from iron(III) oxide in the commercial Beyer process, prior to the aluminum smelting step [27].
Commentary
Thanks to the work by Laing, the forgotten links between main group elements and transition metals have been rediscovered. Even though the eight-Group Table has long since been consigned to ancient history, these links in formula of compounds and polyatomic ions of elements of Group (n) with pseudo-isoelectronic species of elements of the corresponding Group (n + 10) provide chemists with a different perspective. Who would ever have imagined, for example, that there would be chemical similarities of osmium(VIII) and xenon(VIII)? What other unusual matching pairs are yet to be synthesized?
References
1.C. J. Giunta, “J. A. R. Newland’s Classification of the Elements: Periodicity, but No System,” Bull. Hist. Chem. 24, 24–31 (1999).
2.E. R. Scerri, “A Philosophical Commentary of Giunta’s Critique of Newlands’ Classification of the Elements,” Bull. Hist. Chem. 26(2), 124–129 (2001).
3.G. N. Quam and M. B. Quam, “Types of Graphic Classifications of the Elements: Introduction and Short Tables: Introduction and Short Tables,” J. Chem. Educ. 11, 27–32 (1934).
4.P. J. F. Rang, “The Periodic Arrangement of the Elements,” Chem. News 178 (14 April 1893).
5.G. N. Quam and M. B. Quam, “Types of Graphic Classifications of the Elements: Long Charts,” J. Chem. Educ. 11, 217–223 (1934).
6.W. C. Fernelius and W. H. Powell, “Confusion in the Periodic Table of Elements,” J. Chem. Educ. 59(6), 504–508 (1982).
7.E. Fluck, “New Notations in the Periodic Table,” Pure Appl. Chem. 60(3), 431–436 (1988).
8.R. T. Sanderson, “One More Periodic Table,” J. Chem. Educ. 31(9), 481 (1954).
9.M. Laing, “The Periodic Table: A New Arrangement,” J. Chem. Educ. 66, 746 (1989).
10.M. Laing, “The Periodic Table — Again,” Educ. Chem. 26, 177–178 (1989).
11.R. L. Rich, “Are Some Elements More Equal than Others?” J. Chem. Educ. 82, 1761–1763 (1991).
12.D. M. P. Mingos, Essential Trends in Inorganic Chemistry, Oxford University Press, Oxford, 196–202 (1998).
13.G. Rayner-Canham, “Periodic Patterns: The Group (n) and Group (n + 10) Linkage,” Found. Chem. 15, 229–237 (2013).
14.N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, 946–948 (1997).
15.F. Habashi, “Metals: Typical and Less Typical, Transition and Inner Transition,” Found. Chem. 12, 31–39 (2010).
16.G. K. Schweizer and L. L. Pesterfield, Aqueous Chemistry of the Elements, Oxford University Press, Oxford (2010).
17.R. A. Chapman, D. B. Meadowcroft, and A. J. Walkden, “Some Properties of Zirconates and Stannates with the Pyrochlore Structure,” J. Phys. D. Appl. Phys. 3(3), 307–319 (1970).
18.Ref. 14, Greenwood and Earnshaw, 577, 987.
19.R. T. Sanderson, “An Explanation of Chemical Variations within Periodic Major Groups,” J. Am. Chem. Soc. 74(19), 4792–4794 (1952).
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21.K. Seppelt, “Arsenic Pentachloride, AsCl5,” Angew. Chem. Int. Ed. 15(6), 377–378.
22.F. Tamadon and K. Seppelt, “The Elusive Halides VCl5, MoCl6, and ReCl6,” Angew. Chem. Int. Ed. 15(6), 377–378.
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27.F. Habashi, “A Short History of Hydrometallurgy,” Hydrometallurgy 79(1–2), 15–22 (2005).
Chapter 10
Chemical
“Knight’s Move” Relationship
In 1999, Laing reported on a previously unrecognized relationship between elements in the lower right quadrant of the Periodic Table. He named these linkages the chemical “Knight’s Move” Relationship. The name was chosen as the linkages were between an element and the element one Period below and two Groups to the right, the classic knight’s move in the game of chess. In this chapter, we will look at some of these connections, focusing especially upon the double pairs.
The discovery of all the relationships covered in the previous chapters date back to the 19th and early 20th centuries. This chapter is devoted to a correlation that was not spotted until late in the 20th century.
The Group (n)/Period (m) and Group (n + 2)/Period (m + 1) Linkages
It was in the pages of Education in Chemistry that Laing first reported the knight’s move linkages in the Periodic Table [1]. The article described similarities that he had spotted between an element in the lower right quadrant of the Periodic Table and the element in the next lower Period and two Groups to the right. Thus, the linkages can be defined as:
The knight’s move relationship represents a pattern of similarities in the lower right quadrant of the Periodic Table between an element of Group (n) and Period (m) with an element of Group (n + 2) and Period (m + 1).
Periodic Table, The: Past, Present, And Future Page 14
