X. VSEPR Theory –
Valence Shell Electron Pair Repulsion theory.
[Remember: Like charges repel!]
 A. A theory to predict the 3-dimensional
geometry, ie. the“shape” of a molecule

1. The theory is based on “electrostatic
repulsion”: Molecules will adjust their
shape to keep the negatively-charged pairs
of valence electrons as far apart as possible
from each other.
 B. When NOT to use VSEPR theory: When
there are only 2 atoms in a molecule. These
molecule’s shapes are called linear – it
doesn’t matter if there are single bonds,
double bonds, triple bonds, or unshared
electron pairs.

C. Using VSEPR theory:
1. Draw the Lewis dot structure for the
molecule.
 2. Identify its central atom.
 3. Identify the sets of valence electrons as one
of two possibilities:
 A. Those connecting two atoms.
 B. Those that do not connect two atoms.
These are called “unshared pairs”.

4. The unshared pairs found on a central atom
strongly repel each other; and molecules that
would otherwise be linear, will be forced into a
bent (or angular) shape.
 5. Unshared pairs also cause a molecule
that would be shaped like a flat triangle
(trigonal planar), to be forced into a not flat
( trigonal pyramidal) shape.

6.Count the number of connections separately
from the number of unshared pairs.
 1 single bond counts as 1 connection.
 1 double bond counts as 1 connection.
 1 triple bond counts as 1 connection.
 Each unshared set of 2 dots counts as 1 unshared pair.

D Predicting Shapes Using VSEPR
Table

Read horizontally across the table.
# of
Connections To
total
the Central Atom
atoms
Unshared Pairs of
Molecular Shape
Electrons Around Central
Around Central Atom
Atom
3
2
0
Linear
4
3
0
Trigonal Planar
5
4
0
Tetrahedral, 109.5o
3
2
1 or 2
Bent
3
1
Pyramidal
4
Shapes:
,
Linear diatomic
E. Shapes:
Trigonal planar
Trigonal Planar
Bent
Linear diatomic
Linear triatomic
Linear triatomic
Pyramidal
Tetrahedral
Trigonal pyramidal
tetrahedral
bent