Valence Shell Electron Pair Repulsion (VSEPR) Theory
VSEPR theory builds on Lewis’s bonding theory and Pauling’s valence bond and
hybridization theories to explain the shape of molecules. The VSEPR theory is believed
to be a simpler system of explaining and predicting compound shapes. The basic idea
is that when molecules form the atoms move as far away from each other as possible
due to their electron repulsion. The repulsion, as the name of the theory suggests, of
electron pairs is the main factor into the molecular shape of different molecules.
When predicting or explaining the shape of molecules the following steps must be
followed:
1. Draw the Lewis structure of the molecule including the bonding pairs and lone
pairs of electrons for the central atom.
2. Treat all double and triple bonds as single bonds for the purpose of molecular
shape.
3. Add the number of atoms surrounding the central atom to the number of lone
pairs of electrons (remember that 2 electrons makes one pair). This number is
known as the total electron pairs.
4. Using the table below predict the shape of the molecule based on the bonding
pairs with surrounding atoms and the number of lone pairs (lone pairs repel the
bonding pairs forcing the atoms closer together).
Example: H2O
1. The Lewis Structure
2. No double or triple bonds to deal with.
3. There are 2 H atoms plus the 2 pairs of lone electrons. Total electron pairs = 4.
4. The geometry falls under tetrahedral shape. Because there are 2 bonding pairs
and 2 lone pairs the shape is considered bent. The molecular shape is only
based on the HOH atoms angle and is predicted to be less than the 109.5 o
tetrahedral angle because the lone pairs repel the bonding pairs forcing the
hydrogens closer. The actual bond angle for water is 104.5o.
SCH4UVALENCESHELLELECTRONPAIRREPULSIONTHEORY.docx
Molecular Shapes Explained by VSEPR Theory
Electron
Groups
Electron
Geometry
Shape
SCH4UVALENCESHELLELECTRONPAIRREPULSIONTHEORY.docx