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Valence Bond Theory

The valence bond theory emphasizes that all the orbitals in bonding are the same. The atomic orbitals, which are different, mix or hybridize into orbitals that are all the same. The key is that the number of orbitals before and after hybridization are the same. The name of each hybrid orbital is the atomic orbitals that were mixed. Orbitals are added to the mix in the following order: s, p, p, p, d, d, and so on. The Lewis structure is used to determine the number of orbitals that are mixed. One orbital is needed for each lone pair and each bond.

For resonance structures, if an atom has a double bond in any of the resonance structures, the hybridization of that atom is determined from the structure when that atom has a double bond.

>> Example 1

What is the hybridization of each atom in the following?

1. CO₂
2. NO₃^–
3. SiCl₄
4. SF₆

Solution:

1. The Lewis structure of CO₂ is

   The carbon has two bonds, therefore sp hybridization. The oxygen atoms have the same bonding, one bond and two lone pairs, therefore the hybridization is sp².

2. The Lewis structure of nitrate ion is

   The nitrate ion has three resonance structures, with the double bond on a different oxygen for each. In all resonance structures the nitrogen has three bonds, therefore its hybridization is sp² . Each of the oxygen atoms also has sp² hybridization, since the hybridization is determined by the resonance structure with the double bond, where the oxygen has a double bond and two lone pairs.

3. The Lewis structure of silicon tetrachloride is

   Silicon has four bonds and therefore an sp³ hybridization. Each chlorine has three lone pairs and one bond, so an sp³ hybridization.

4. The Lewis structure of SF₆ has all six fluorines bonded to the sulfur. Each fluorine has three lone pairs and a single bond. Therefore the hybridization of sulfur is d²sp³ and the hybridization of each fluorine is sp³.

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