Group Variable Valence

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. 

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

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. 

Chung [34] concluded that the semiconducting properties of a metal species influence discoloration. In contrast to metals belonging to the insulator group, metals belonging to the semiconductor group promote yellowing, perhaps due to catalysis of the polymerization of vinyl esters. The formation of chromophores is enhanced when the metal has a variable valency with a reduction potential near to zero. 

Zinc, cadmium and mercury are at the end of the transition series and have electron configurations ndw(n + l)s2 with filled d shells. They do not form any compound in which the d shell is other than full (unlike the metals Cu, Ag and Au of the preceding group) these metals therefore do not show the variable valence which is one of the characteristics of the transition metals. In this respect these metals are regarded as non-transition elements. They show, however, some resemblance to the d-metals for instance in their ability to form complexes (with NH3, amines, cyanide, halide ions, etc.). 

The (/-block elements tend to lose their valence s-electrons when they form compounds. Most of them can also lose a variable number of d-electrons and show variable valence. The only elements of the block that do not use their (/-electrons in compound formation are the members of Group 12 (zinc, cadmium, and mercury). The ability to exist in different oxidation states is responsible for many of the special properties of these elements and plays a role in the action of many vital biomolecules (Box 16.1). 

The covalency falls with group member and variable valency is observed in a number of these non-metals. The maximum possible oxidation state increases from +5 in group 15 to +8 in group 18. 

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 ... 

ChemProp Group Oxidation Number - Inert Gas Core - 8-electron configuration - V-type Diagrams - Group Oxidation State - Variable Valence. Ligands - [M(OH2)6] - Oxyanions. 

This is particularly apparent in the sections on introductory inorganic chemistry, where the underlying electron configuration of the elements of the Periodic Table not only determines the Long Form of the Periodic Table, but also determines the physical properties of the elements, atom size, ionisation energies and electron affinities (electron attachment enthalpies), and the chemical properties, characteristic or group oxidation numbers, variable valence and the formation of ionic... 

Chapter 4 describes how the Chemical Properties of the Elements are related to their valence shell configuration, i.e. characteristic or group oxidation number, variable valence, ionic and covalent bonding. This chapter includes a section on the volumetric calculations used in an introductory inorganic practical course, including the calculation of the stoichiometry factors for chemical reactions. 

The NASICON structure was chosen because it can be readily synthesized, is thermally very stable, and can accommodate a large fraction of vacancies and cation substitutions [9-12], In addition, this structure possesses two features which should be important for the catalyst design as envisioned above. First, it is a phosphate and hence expected, owing to its acidic nature, to stabilize the lower oxidation states of transition metals, e.g., V second, owing to its structure, layered octahedral metal centers with variable valence are separated from each other by redox inactive tetrahedral phosphate groups, i.e., the structure provides for isolation of descrete layers. 

The zinc group does not display the variable valence associated with transition metal ions. However the group does have the transition-like property of forming many complexes or coordination compounds, such as [Zn(NH3)4] and [Hg(CN]]4]2-. 

If the valency is equated to the number of unpaired electrons, then nitrogen is tervalent and oxygen bivalent as required by their normal chemical behaviour. These prescriptions are not rigid and this circumstance corresponds to the fact of variable valency. In carbon there might -well be paired s electrons, and the group (2s)2 would leave unpaired 2p Y only. This would signify a bivalent... 

Many investigators have studied the phase relationship and the formation of the carbides in both the Eu-C and the Yb-C systems, but no complete phase diagram was reported although some data are available on carbides that are formed in the systems. Although europium and ytterbium exhibit variable-valence tendencies, the properties and lattice parameters of their carbides do not follow the systematic variation encountered between the individual lanthanide-carbon system. In addition, an additional new phase, RCs (R = Eu and Yb) forms in the two systems, which is reported to be hexagonal with a Pbj/mmc space group (Guerard and Herold 1975, El-Makrini et al. 1980). 

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