Hybridization and the Localized Electron Model

5 Combining the Localized Electron and Molecular Orbital Models 

Recall that the LE model views a molecule as a collection of atoms bound together by sharing electrons between atomic orbitals. The arrangement of valence electrons is represented by the Lewis structure (or structures, where resonance occurs), and the approximate molecular geometry can be predicted using the VSEPR model. In this section we will describe what types of atomic orbitals are used by this model to share the electrons and hence to form the bonds. 

Hybridization is a modification of the LE model to account for the observation that atoms often seem to use special atomic orbitals in forming molecules, 

Let s reconsider the bonding in methane, which has the Lewis structure and molecular geometry shown in Fig. 14.1. In general, we assume that bonding involves only the valence orbitals. This means that the hydrogen atoms in methane use Is orbitals. The valence orbitals of a carbon atom are the 2s and 2p orbitals shown in Fig. 14.2. Thinking about how carbon can use these orbitals to bond to the hydrogen atoms reveals two related problems  

Using the 2p and 2s atomic orbitals will lead to two different types of C—H bonds (a) those from the overlap of a 2p orbital of carbon and a Is orbital of hydrogen (there will be three of these) and (b) those from the overlap of a 2s orbital of carbon and a Is orbital of hydrogen (there will be one of these). This presents a problem because the experimental evidence indicates that methane has four identical C—H bonds. 

The valence orbitals are the orbitals associated with the highest principal quantum level that contains electrons on a given atom. 

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The native 2s and three 2p atomic orbitais characteristic of a free carbon atom are combined to form a new set of four sp orbitais. The smaii iobes of the orbitais are usuaiiy omitted from diagrams for ciarity. 

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