Periodic Table, The: Past, Present, And Future

this element is at least as volatile as simultaneously investigated elements Hg, At, and element 112. This behaviour is rather unexpected for a typical metal of group 14.

Laing’s prediction of a similarity to mercury seems to have been confirmed.

Laing’s Knight’s Move Legacy

Laing’s many contributions to the discussions on the Periodic Table are sprinkled throughout this book. On the basis of that one article, the knight’s move has become accepted as a genuine periodic relationship, having been included in such resources as: a conference proceedings on the Periodic Table [6]; and a definitive work on the history and structure of the Periodic Table [7]. And now, in this work, a whole K-M chapter. It will be through the K-M that Laing’s name will live on [8].

Reevaluation of the Knight’s Move Relationship

An overriding and oft-forgotten point about the chemical elements is that each element is unique. It is this individuality that makes inorganic chemistry such an interesting but, at the same time, gargantuan field of study. Thus, in looking for relationships, one should not expect total congruence among the elemental behaviors; on the other hand, one should be hoping to find consistent patterns that are more than simple probability. For this reason, the knight’s move concept needs to be tested by looking systematically and comprehensively at one or more pairs of elements.

The Significance of Oxidation Number

In the opinion of this Author, it is the compounds of the same oxidation states that provide the knight’s move with its main validity. In fact, this matching of oxidation states — generally the lower one — seems to be the key feature of the linkages (Figure 10.3).

The K-M Silver(I) — Thallium(I) Similarities

The most intriguing example of the K-M relationship is that of silver(I) and thallium(I). Some of the similarities are as follows:

Figure 10.3 The common oxidation states of the “Knight’s Move” elements with the less common oxidation state indicated in parentheses.

•For silver and thallium, unique in their respective Groups, the +1 state is stable and preferred in aqueous solution.

•Silver(I) and thallium(I) halides are whitish except for the iodides that are yellow.

•Silver(I) fluoride and thallium(I) fluoride are water soluble and all other silver and thallium(I) halides are insoluble.

•Unique among chromates, insoluble silver(I) chromate, Ag₂CrO₄, and thallium(I) chromate, Tl₂CrO₄ are both brick red in color.

•In the mineral, crookesite, Cu₇(Ag,Tl)Se₄, silver(I) and thallium(I) occupy the same lattice sites.

•Thallium(I) tetrafluoroborate, TlBF₄, has been proposed as a substitute reagent for silver(I) perchlorate, AgClO₄ [9].

A Thallium Detour

Earlier, the knight’s move linkage of thallium(I) with silver(I) was explored. In Chapter 9, the limited (n) and (n + 10) relationship of silver with the alkali metals was briefly mentioned. Combining these two links, a most curious connection is that of thallium(I) with the heavier alkali metals. For example, thallium, like potassium, forms a hydroxide, TlOH, which is very water soluble to produce a very basic solution. Thallium(I) is also one of the cations that fits the large monopositive ion site in an alum as a substitute for an alkali metal ion [10].

Thallium(I) ion is highly poisonous. It enters the body through the potassium ion uptake pathways. Once absorbed, it is the attraction to sulfur ligands that provide thallium(I) with its toxicity (and difference from the alkali metal ions). In this way, the thallium(I) ion disrupts many cellular processes by interfering with the function of proteins that incorporate the sulfur-containing amino acid, cysteine [11].

Knight’s Move Relationships among “Double Pairs”

In addition to simple pairs, there are also “double pairs” of K-M related elements. These are copper–indium/indium–bismuth and zinc–tin/tin–polonium, in which each central element has two other elements linked by potential Knight’s Move relationships (Figure 10.4).

To summarize Laing’s claims, he proposed that the following specific features indicate the existence of a knight’s move pattern:

•Similarities of metal melting points.

•Patterns in compound formulas and structures.

•Parallels in melting and boiling points of compounds of the same formulation.

Figure 10.4 The two sets of “double pair” K-M related elements, one double pair is in italic.

Here the three aspects will be examined in the context of the two “double pairs.”

Are Metal Melting Points Irrelevant to K-M?

Laing noted in his paper [1] the similarity in melting points between tin (232°C) and polonium (254°C). Looking at the melting and boiling points of the first double pair of zinc–tin–polonium (see Table 10.2), there does seem to be a similarity of melting points (tin–polonium) and boiling points (zinc–polonium), though there is no systematic pattern involving all three metals.

The matching table for the copper–indium–bismuth double pair indicates that low melting points are characteristic of all the lower p-block metallic elements (see Table 10.3). In fact, the closest match in melting and boiling points does not come from knight’s move pairs. Instead, by comparing Tables 10.2 and 10.3, the major similarity for the main group elements is controlled by Period. Thus, tin and indium have similar melting and boiling points; as do bismuth and polonium. Therefore, similarities in metal melting points and boiling points do not seem to be a defining K-M feature [12].

Table 10.2 The phase change temperatures for the Zn–Sn–Po double pair

Table 10.3 The phase change temperatures for the Cu–In–Bi double pair

Copper(I)–Indium(I) and Indium(III)–Bismuth(III) Double T-M Links

It is always the lower oxidation state of the 5th Period element that matches an oxidation state of the 4th Period element. Then it is the higher oxidation state of the 5th Period element that matches the lower oxidation state of the 6th Period element. For the double T-M links here, the pairs are compounds of copper(I) and indium(I); and then corresponding compounds of indium(III) and bismuth(III). Four main sources of information have been used [13–16].

The usual aqueous oxidation state for copper is +2 while that for indium is +3. For both copper and indium, the +1 oxidation state is a comparative rarity. It is found mostly in insoluble solid-state species, though in itself, the existence of this matching oxidation state is notable in the context of