Variable valence main group

The exhibition of variable valency is indeed a characteristic of transition metals. Main group metal ions such as those of groups 1 or 2 exhibit a single valence state. Other main group metals may show a number of valencies (usually two) which are related by a change in oxidation state of two units. This is typified by the occurrence of lead(iv) and lead(ii) or thallium(iii) and thallium(i). However, all the transition metals exhibit a range of valencies that is generally not limited in this manner. 

Figure 4.27. Valence electron count and vertex count in main group clusters. Notice, according to McGrady (2004), that the different classes of clusters (electron-rich, electron-precise, etc.) simply occupy different domains in a continuum defined by the two variables (electron and vertex counts).

The electronic configurations of transition metal ions, like those of main group ions, are determined by removal of the electrons from the shell of highest n value first. Next, electrons may be lost from the d subshell next to the valence shell. The capability of removing a variable number of electrons makes it possible for most transition metals to have ions of different charges. 

Figure 4.3 summarises this Main Group variable valence in a graphical form, emphasising the variation of the s"p " configuration against the oxidation number, for the respective first and second row elements. 

In the variable valence of the transition metals, with a [Ar]4s 3d" configuration, the distribution of the oxidation states (Figure 4.4) is not as systematic as in the Main Group elements (Figure 4.3). Thus, for the first-row transition metal ions the following generalisations may be made ... 

Figure 4.3 A summary of Main Group variable valence...

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