valence bond theory - chemistry.caddell.org

Valence Bond Theory

The overlap of atomic orbitals results in lower potential energy for the atoms.

Directionality is predicted by valence bond theory.

Sometimes valence bond theory gives incorrect predictions.

Hybridization Theory

V.B. theory can not predict the bond lengths/angles.

We know from experiment that methane has 4 identical C-H bonds, all 109.47o apart.

So, we use hybridization theory to explain the bonding.

Methane (CH4)

Whenever an atom requires a set of equivalent tetrahedral atomic orbitals the atom becomes

sp3 hybridized.

The carbon atoms in ethylene (H

2C=CH

2) are sp2 hybridized.

The p orbital is perpendicular to the sp2 orbitals.

A double bond consists of a sigma (s) bond and a pi (p) bond.

A s bond is the head-on overlap of any 2 atomic orbitals.

A p bond is the sideways overlap of p (or d) orbitals.

When an atom has three effective pairs (lone pairs or bonds) it is sp2

hybridized.

With 2 effective pairs, an atom is sp hybridized. Look at N

2.

Carbon dioxide also has sp hybridized carbon atoms.

Predict hybridization by molecular geometry.