VSEPR

1. What is VSEPR?

The Valence Shell Electron Pair Repulsion (VSEPR) model:

• is based on the number of regions of high electron density around a central atom.

• can be used to predict structures of molecules or ions that contain only non-metals by minimizing the electrostatic repulsion between the regions of high electron density.

• can also be used to predict structures of molecules or ions that contain multiple bonds or unpaired electrons.

• does fail in some cases.

2. VSEPR Rules

1. Draw the Lewis structure for the molecule or ion.
2. Count the total number of regions of high electron density (bonding and unshared electron pairs) around the central atom.

• Double and triple bonds count as ONE REGION OF HIGH ELECTRON DENSITY.
• An unpaired electron counts as ONE REGION OF HIGH ELECTRON DENSITY.
• For molecules or ions that have resonance structures, you may use any one of the resonance structures.

1. Draw the Lewis structure for the molecule or ion.

2. Count the total number of regions of high electron density (bonding and unshared electron pairs) around the central atom.

• Double and triple bonds count as ONE REGION OF HIGH ELECTRON DENSITY.
• An unpaired electron counts as ONE REGION OF HIGH ELECTRON DENSITY.
• For molecules or ions that have resonance structures, you may use any one of the resonance structures.

Example :

molecule	Lewis structure	# regions of high electron density	molecule	Lewis structure	# regions of high electron density
BeCl2		2	BF3		3
HCN		2	SO3		3
CO2		2	NO2		3
CH4		4	NH3		4
PCl5		5	SF6		6