Some simple covalent structures

The characteristic properties of the covalent bond discussed above impose severe restrictions on the possible types of crystal structure in which such bonds may occur. Since the number of neighbours to which a given atom may be bound by covalent bonds is limited to the covalency of that atom, and since this covalency is usually small, the vast majority of structures containing the covalent bond are molecular structures in which covalent forces occur within discrete molecules but in which the molecules are bound to one another by forces of a different kind. There are, however, a number of structures in which the binding throughout is due to covalent forces, and of these the simplest is that of carbon in the form of diamond. 

It immediately follows from this discussion that the interatomic forces throughout the diamond structure are covalent or chemical5 in nature, exactly analogous to those responsible for the formation of, say, the molecule of fluorine, F2. Thus, if we regard F2 as a molecule of fluorine 

Among the covalent AX compounds which crystallize with either the zincblende or wurtzite structure are ZnS itself, ZnO, AIN, A1P, HgS, CuCl and many more. At first sight it would seem that such an arrangement is inconsistent with the known covalencies of the atoms involved. In ZnS, for example, the zinc and sulphur atoms, with two and six electrons, respectively, in the outermost shell, would be expected to be limited to a covalency of 2 and therefore to be capable of covalent 

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This exercise will familiarise you with (he structures of some simple covalent compounds and the methods we have for representing the structure and shape of their molecules. 

Simple metals like alkalis, or ones with only s and p valence electrons, can often be described by a free electron gas model, whereas transition metals and rare earth metals which have d and f valence electrons camiot. Transition metal and rare earth metals do not have energy band structures which resemble free electron models. The fonned bonds from d and f states often have some strong covalent character. This character strongly modulates the free-electron-like bands. 

Figures 9.1-9.3 illustrate these interconnected relationships.13 Figure 9.1 defines some of the terms used in this chapter. Small molecules are species with molecular weights below about 1,000. They are volatile at temperatures below say 200 100 °C. Clusters are oligomers derived from covalently linked small molecules. They have a lower volatility than small molecules and, if large enough, can be shaped by melting or by solvent evaporation methods. Linear polymers can be simple chain structures or may consist of rings linked together. In either case they are usually non-volatile and easily fabricated. Cross-linked systems can be produced from polymers or from clusters. The final ceramic may be amorphous or crystalline.

Here we consider the factors which determine whether a given compound prefers an ionic structure or a covalent one. We may imagine that for any binary compound - e.g. a halide or an oxide - either an ionic or a covalent structure can be envisaged, and these alternatives are in thermochemical competition. Bear in mind that there may be appreciable covalency in ionic substances, and that there may be some ionic contribution to the bonding in covalent substances. Since there is no simple means - short of a rigorous MO treatment - of calculating covalent bond energies, and since quantitative calculations based upon the ionic model are subject to some uncertainties, the question of whether an ionic or a covalent structure is the more favourable thermodynamically cannot be answered in absolute terms. We can, however, rationalise the situation to some extent. 

We overview our valence bond (VB) approach to the ir-electron Pariser-Parr-Pople (PPP) model Hamiltonians referred to sis the PPP-VB method. It is based on the concept of overlap enhanced atomic orbitals (OEAOs) that characterizes modern ab initio VB methods and employs the techniques afforded by the Clifford algebra unitary group approach (CAUGA) to carry out actual computations. We present a sample of previous results, sis well sis some new ones, to illustrate the ability of the PPP-VB method to provide a highly correlated description of the ir-electron PPP model systems, while relying on conceptusilly very simple wave functions that involve only a few covalent structures. 

Binary compounds are ones with two elements present. Simple crystal structures may be classed as ones in which each atom (or ion) is surrounded in a regular way by atoms (or ions) of the other kind. Even with this limited scope many structures are possible. Figure 1 shows a selection of simple ones that exemplify some important principles. Although many are found with ionic compounds, some of these structures are shown by compounds with covalent bonding, and a discussion of the bonding factors involved in favoring one structure rather than another is deferred to Topic D4. 

When a compound has strong covalent character then we expect it to be molecular with all the typical properties associated with simple molecular structures, such as relatively low melting and boiling points and non-electrolyte behaviour in water. This means that the solution will contain molecules and be non-conducting. Simple molecular substances are also non-conducting in the solid and liquid states. However, some molecular substances react with water to release ions. This is known as hydrolysis. An example is anhydrous aluminium chloride, which reacts with water in a hydrolysis reaction to form aluminium hydroxide and hydrochloric acid. 

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