The Valence Shell Electron Pair Repulsion
Theory is a way of predicting the shape of a molecule based
on the number of bonding and lone pairs of electrons in a polyatomic
It is based on the fact that these electrons
pairs interact and repel each other due the electrostatic repulsion.
In doing so, they adopt a spatial arrangement such that they are
as far apart as possible and the electrostatic repulsion is minimized.
The result of this is that the electron pairs
around a given atom are arranged at the vertices of a regular polyhedron,
with the number of vertices of the polyhedron being the number of
electron pairs. These polyhedra are given in the table.
|Number of electron pairs
The arrangements of the electron pairs are therefore
as shown below, but it should be noted that this is the arrangement
of all electron pairs, both bonding and lone pairs. The shape
of the molecule is, however, determined by the positions of the
atoms, and not the electron pairs, and so the shape of the molecule
may be different.
molecules for tetrahedrally arranged electron pairs
||Number of Lone pairs
The VSEPR can be extended to molecules with multiple
bonds by noting that the multiple bond is treated the same way
as a single bond. Hence, the sulphate ion, SO42-,
which has two S=O bonds and two S-O bonds, is tetrahedral as the
two types of bond are treated the same.
Modifications to the basic shape
The VSEPR model gives the basic arrangements of the
electron pairs. However, it is important to distinguish between
the lone and bonding pair electrons.
The electrons in a lone pair experience attraction
to only one atom, as they are unshared, and so are considered to
be closer to the nucleus than the electrons in a bonding pair.
The degree of the repulsion between electron pairs therefore changes
with the nature of the pairs being considered. It lies in the order:
Lone Pair - Lone Pair > Lone Pair - Bond Pair
> Bond Pair - Bond Pair
This means that the presence of lone pairs will change
the shape of a molecule. In the series above, methane has a H-C-H
bond angle of 109.5o. In Ammonia, the greater repulsion
from the lone pair means that the three bonding pairs are pushed
closer together, and the H-N-H bond angle is 107o. In
Water, there are two lone pairs, and hence the two remaining bonding
pairs are pushed closer together still, and the H-O-H bond angle
Structure prediction by VSEPR
|The central S atom has 10 electrons
around it (6 from the S and 1 from each of the 4 F atoms).
These 10 electrons form 5 pairs, and so the basic structure
is a trigonal bipyramid. The 5 pairs consist of 4 bonding
pairs and 1 lone pair, and the lone pair occupies one of
the trigonal positions, where repulsion is the lowest. The
extra repulsion between the lone pair and the bonding pairs
means that the axial bonds are pushed away from the lone
pair, and the resulting shape is the see-saw shape
|Other examples of structure prediction
by VSEPR are shown in the structures of the interhalogen
These are accounted for in structure prediction by VSEPR by treating
an unpaired electron in the same way as a pair of electrons. The
repulsion between an electron pair and a single electron is lower
than that between electron pairs, and this will affect the shape
in a similar way to the presence of the different repulsion between
bonding and lone pairs.