Valence Bond Theory

Introduction

Valence bond theory: a model of bonding that places electron pairs between adjacent atoms to create bonds
- Bonds are created by the overlap of atomic orbitals on adjacent atoms
- Causes bonds to be localized between adjacent atoms
Linus Pauling proposed that atomic orbitals for each atom should be thought of as combining to form new atomic orbitals, called hybrid orbitals, which then interact to form bonds by overlapping with orbitals from other atoms.
Hybrid orbital: an atomic orbital formed by the combination of two or more atomic orbitals.
Hybridization: the combination of atomic orbitals of different types.
- The number of hybrid orbitals formed is equal to the number of atomic orbitals combined.
- Elements of the second period form three types of hybrid orbitals, designated sp3, sp2, and sp, each of which can contain up to two electrons.

s + 3 p = 4 sp3
Each sp3 hybrid orbital consists of a larger lobe pointing in one direction and a smaller lobe of opposite sign pointing in the opposite direction.
sp3 hybridization results in bond angles of approximately 109.5
Each sp3 orbital has 25% s-character and 75% p-character because those are the percentages of the orbitals combined when constructing them (one 2s orbital, three 2p orbitals)

s + 2 p = 3 sp2
Always occurs in sets of 3
Consists of 2 lobes, one larger than the other
Angle is 120
The third p orbital isn't involved in hybridization and remains as two lobes lying perpendicular to the plane of the sp2 hybrid orbitals
Each sp2 orbital has 33% s-character and 67% p-character (one 2s orbital, two 2p orbitals).

s + p = 2 sp
Always occur in sets of 2
Angle is 180
The axes of the 2 unhybridized p orbitals are perpendicular to the axes of the hybridized orbitals
Each sp orbital has 50% s-character and 50% p-character because those are the percentages of the orbitals combined when constructing them (one 2s orbital, one 2p orbital).