Periodic Table, The: Past, Present, And Future

subsequently adopted worldwide [7]. A disadvantage has been that the similarities that caused Mendeleev and others to put, for example, silicon and titanium in the same Group, became largely forgotten or overlooked. Only Sanderson in 1954 bravely continued to expound the pedagogical benefits of the eight-column table [8]. For example, in his opinion, that it made much more sense, in teaching general chemistry to have a unitary Group 3 consisting of B–Al–Sc–Ga–Y– In–La–Tl–Ac, all of which have a common oxidation state of +3. The common oxidation states of the other Groups: 0 to VII can be readily identified in a similar manner.

The Rediscovery of the A and B Links

It was Laing who, in 1989, first reminded the modern generations of chemists of these similarities [9, 10]. He noted the resemblances between silicon and titanium compounds (such as the pair SiCl₄ and TiCl₄); phosphorus and vanadium compounds (such as POCl₃ and VOCl₃); sulfur and chromium polyatomic ions (such as and ); and chlorine and manganese compounds (such as Cl₂O₇ and Mn₂O₇). To emphasize the linkage, Laing proposed that the element “boxes” of lithium to fluorine and sodium to chlorine be repeated above the corresponding transition metal column.

Rich [11] proposed a modification to Laing’s diagram: that oxygen and fluorine be deleted from the duplication as there are no similarities between those elements and the corresponding transition metals of chromium and manganese. In fact, there is little similarity between any of the 2nd Period main group elements and the corresponding d-block elements. It is of note that all of Laing’s examples compare 3rd Period main group elements with the 4th Period d-group elements. Thus, the segment of the Periodic Table in the following, derived from Rich and Lang, simply show the addition of the respective Period 3 main group elements to the top of the respective transition metal columns (Figure 9.2).

Laing’s study focused on the formula similarities of the 3rd Period main group elements with the corresponding Groups of the transition series. However, Mingos showed that such parallels in formula existed in other (n) and (n + 10) pairs [12]. Here, a wide exploration of such connections will be made.

Figure 9.2 A segment of the Periodic Table showing the proposed additional 3rd Period members of Groups 3–7 and 12.

Definition of the Group (n) and Group (n + 10) Relationship

The linkage in chemical formulas and chemical behavior between each Group (n) member and the corresponding Group (n + 10) member is quite specific. The relationship is between compounds and polyatomic ions of the highest oxidation state of the main group elements and those of the same oxidation state of the matching transition elements. A general definition is [13]:

The (n) and (n + 10) relationship identifies some similarities in some of Group (n) members with those of the corresponding Group (n + 10). This resemblance is usually in the highest oxidation state. Such similarities can be in chemical formulas and structures of compounds and polyatomic ions, and of their aqueous behavior.

The (n) and (n + 10) linkage for highest oxidation states comes about through electronic structural similarities. That is, the (n) element in its highest oxidation state has a noble gas electron configuration while the corresponding (n + 10) element in its highest oxidation state has, in addition, a filled d¹⁰ set. For the elements lower in the respective groups, there is also a filled f¹⁴ electron set. As the metal is in a high oxidation state, the bonding in each compound is predominantly covalent. As examples, Table 9.1 shows oxo-anions of the 4th Period of Group 5, Group 6, and Group 7, together with the corresponding pseudo-isoelectronic oxo-anions of the 3rd Period and 4th Period of Group 15, Group 16, and Group 17.

Table 9.1 Similarities in formula of some oxo-anions to illustrate the (n) and (n + 10) relationship

Group 3 and Group 13

It was Rang in 1893 who seems to have been the first, on the basis of chemical similarity, to place boron and aluminum in Group 3 (see Figure 9.3) [4].

Such an assignment seems to have been forgotten until more recent times. Greenwood and Earnshaw [14] have discussed the way in which aluminum can be considered as belonging to Group 3 as much as to Group 13 (Figure 9.4), particularly in its physical properties. Habashi has suggested that there are so many similarities between aluminum and scandium that aluminum’s place in the Periodic Table should actually be shifted to Group 3 [15].

Figure 9.3 The first section of Rang’s Periodic Table showing the location of boron and aluminum (from Ref. [4]).

Figure 9.4 Members of Group 3 and Group 13.

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