1
2.3. VSEPR continued
In order to determine the shapes of molecules we
need to consider the arrangement of bonding
pairs of electrons and lone pairs of electrons (if
present).
The repulsion between lone pairs is bigger than
between bonding pairs. This means that lone
pairs will favour more space.
If there are two or more lone pairs, then the lone
pairs will be arranged to minimise repulsions
between lone pairs.
The final shape of the molecule depends on the
arrangement of the bonding and lone pairs of
electrons – see handout. (table 9.2 p. 351; table
9.3, p. 354)
Note
If lone pairs are present, the shape of the
molecule is not the same as the shape of the
electron domains.
2
We can predict the three-dimensional structure or
molecular geometry of any main group molecule
using the following procedure:
1. Determine
molecule.
the
Lewis
structure
of
the
2. Count the number of electron domains around
the central atom and arrange these to minimise
repulsion i.e. according to one of the 5 basic
shapes.
This gives the electron domain
geometry.
Note that double or triple bonds count as one
electron domain because they occupy the same
general region of space.
3. From this electron domain geometry, identify
the molecular geometry (how the atoms are
arranged) by considering the bonded atoms.
3
Example:
H2CO
1.
Determine the Lewis structure.
2.
Total number of electron pairs around the
central atom is 4.
However, there is one  bond, which means
the number of electron domains is 3.
3.
There are no lone pairs on the central atom,
so the electron domain geometry is the
same as the molecular geometry. H2CO is
therefore trigonal planar.
4
More examples.
A. PCl5
1. Determine Lewis structure:
Cl
Cl
Cl
Cl
P
Cl
2. Count number of electron domains around
central atom.
P is central atom. 5 electron domains,
therefore electron domain geometry is
trigonal bipyramidal.
5
3. From electron domain geometry, identify
molecular geometry by considering bonded
atoms.
5 bonding domains, no lone pairs, therefore
molecular geometry is also trigonal
bipyramidal.
Cl
Cl
P
Cl
Cl
Cl
Exercise: H2S