Valence Bond Theory and Hybrid Orbitals

The following pages have been developed to help you understand and visualize hybrid orbitals.  There are a number of theories in chemistry that attempt to rationalize the electronic structure and / or geometry observed in molecules. Two you should already be familiar with are Lewis and VSEPR, but as a quick review...

• Lewis: attempts to explain the valence shell electron structure of atoms and molecules based on the concept of the "octet rule" or noble gas electron configurations, but it does not attempt to explain the observed geometry of molecules.
• VSEPR: (Valence Shell Electron Pair Repulsion) attempts to rationalize the shapes (geometries) of molecules based on the simple idea that since electrons all have the same charge (-1) they will repel each other.  Thus the pairs of electrons, both bonding and non-bonding (i.e. lone pair electrons) or groups of electrons for multiple bonds, will arrange themselves in the lowest energy configuration based on the number of electron pairs.

Neither of these theory's explanations are based on the concept of orbitals nor do they try to explain how or why bonds form, a major shortfall in both, therefore new theories have arisen.

Valence Bond Theory

Valence Bond Theory (VB) is one of the theories which attempts to explain bonding in molecules and their geometries based on orbital interactions.  In contrast to Molecular Orbital Theory (MO) which mixes atomic orbitals from all atoms in the molecule to generate a set of new molecular orbitals, VB mixes the valence orbitals on single atoms first, to generate the correct orbital geometry, and then builds molecules from the new "hybrid" orbitals.  The hybrid orbitals are constructed by adding or subtracting standard (s, p and d) atomic orbitals in fixed proportions.  Both theories allow for bonding between atoms in the same manor, the combination of orbitals to produce bonding and anti-bonding molecular orbitals.  The major difference is that VB can be viewed as a "localized" model of bonding i.e. between pairs of atoms, while MO allows for molecular orbitals to span multiple atoms, or even the entire molecule.  The success in VB theory is that it correctly predicts and explains the experimentally observed geometry of molecules based on their (hybridized) atomic orbitals.  The advantage is that it is a "simple theory" that works well for most molecules.  Unfortunately VB theory fails for some cases (i.e. O₂) and does not adequately explain the type of very delocalized bonding observed in metals and aromatic compounds.

In either case the σ or π bonds that form between atoms arise from two (or more) atoms sharing a pair of electrons in an orbital that spans both atoms (review bonding?).

Valence Shell Electron Pair Repulsion (VSEPR) theory states that there are five basic molecular geometries possible:

• linear including an animation of the formation of an sp hybrid orbital
• trigonal planar
• tetrahedral
• trigonal bipyramidal
• octahedral

All of these molecular geometries require "mixing" different combinations of atomic orbitals on an atom to produce the correct geometry of the hybrid orbitals.  The subsequent pages will deal with each case individually.