Introduction to the Valence Bond Method

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. 

only 3p orbitals are shown. The phases of the lobes of the sulfur 3p orbitals are shown in blue and red for positive and negative, respectively. The choice of which lobe is positive is arbitrary. Flowever, once this choice is made, the other lobe is necessarily negative. Bond formation can be represented diagrammatically by the overlap of orbitals that are in phase (same color), although the hydrogen Is orbital is colored yellow here, not blue, for clarity. 

EXAMPLE 11-1 Using the Valence Bond Method to Describe a Molecular Structure 

Describe the phosphine molecule, PFfs. by the valence bond method. 

Step 1. Draw valence-shell orbital diagrams for the separate atoms. 

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