Covalent bonds localized electron model

Carbon occurs in the allotropes (different forms) diamond, graphite, and the fullerenes. The fullerenes are molecular solids (see Section 16.6), but diamond and graphite are typically network solids. In diamond, the hardest naturally occurring substance, each carbon atom is surrounded by a tetrahedral arrangement of other carbon atoms, as shown in Fig. 16.26(a). This structure is stabilized by covalent bonds, which, in terms of the localized electron model, are formed by the overlap of sp3 hybridized atomic orbitals on each carbon atom. 

All the Group 4A elements can form four covalent bonds to nonmetals—for example, CFI4, Sip4, GeBr4, SnC, and PbC. In each of these tetrahedral molecules the central atom is described as sp hybridized by the localized electron model. 

All the Group 4A elements can form four covalent bonds to nonmetals—for example, CH4, Sip4, GeBr4, SnClj, and PbCl4. In each of these tetrahedral molecules, the central atom is described as sp hybridized by the localized electron model. AU these compounds, except those of carbon, can react with Lewis bases to form two additional covalent bonds. For example, SnClj, which is a fuming liquid (bp = 114°C), can add two chloride ions ... 

A description of covalent bond formation in terms of atomic orbital overlap is called the valence bond method. The creation of a covalent bond in the valence bond method is normally based on the overlap of half-filled orbitals, but sometimes such an overlap involves a filled orbital on one atom and an empty orbital on another. The valence bond method gives a localized electron model of bonding Core electrons and lone-pair valence electrons retain the same orbital locations as in the separated atoms, but the bonding electrons do not. Instead, they are described by an electron probability density that includes the region of orbital overlap and both nuclei. 

The qualitative interpretation of these results in terms of conduction —> covalent electronic transformation model is based on the following principles (1) covalent electrons are localized and therefore are identifiable with a group of ions, whereas conduction ( free ) electrons are delocalized and are simultaneously shared by all ions. (2) thus, covalent electrons having no Fermi surface whereas conduction electrons (because of the Pauli exclusion principle) having well defined Fermi surface, and (3) electrons are needed in forming covalent bonds, (i.e., under no circumstances can holes be substituted for electrons in forming bonds) in sharp contrast, holes behave in much the same way as electrons in band structure. 

A striking analogy exists between localized molecular orbital, electron-domain models of organic and other covalently bonded molecules (Figs. 3—8) and ion-packing models of inorganic compounds 47). 

The width of the band formed is an indication of the degree of delocalization of the band states and a comparison of Figure 8.5a and b shows the biggest difference in the UVB region. In the ionic model of MgO, the localization of electron density on the 0 ions leads to a relatively narrow UVB, while the covalent bonding in Ti02 gives rise to more delocalized states and so to a broader UVB. 

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