VSEPR Theory
In Valence Shell Electron Pair Repulsion (VSEPR) theory, pairs of electrons that surround
the central atom of a molecule or ion are arranged as far apart as possible to minimise
electron-electron repulsion. This simple idea can be used to predict the shapes of
molecules by following a simple procedure:
1. Decide which is the central atom in a molecule.
In cases of ambiguity, pick the least electronegative atom as this atom will be better able
to share its electrons with the other atoms in the molecule.
2. Count up the valence (outer shell) electrons on the central atom.
3. Count up the electrons used by the outer atoms to make bonds with the cental atom.
4. The sum of (2) + (3) divided by two gives the Valence Shell Electron Pair (VSEP)
count.
5. The predicted geometry of the molecule is based on the number of VSEP.
VSEP Shape
2
Linear
3
Trigonal Planar
4
Tetrahedral
5
Trigonal Bipyramidal
6
Octahedral
VSEPR Theory - Lone Pairs
Sometimes the central atom will have more VSEP than are needed to make bonds to the
outer atoms. The extra pairs of electrons on the central atom are called 'lone-pairs'.
Bond angles will deviate from their ideal values according to the rule that lone pairs repel
other electrons more strongly than bonding pairs.
Although lone pairs are clearly smaller than atoms, they need to be closer to the nucleus
of an atom than a bonding pair. Being closer to the central atom causes lone-pairs take up
more of the available 'bonding space'.
1
Examples:
1. NH3
Central atom:
N
N contributes:
5 e−
3 x H contibute: 3 e−
Total VSE:
8
Total VSEP:
4
Geometry:
Trigonal pyramidal (based on tetrahedral)

H–N–H bond angle = 107° (less than 109° due to lone-pair:bonding-pair repulsion
being greater than bonding-pair:bonding-pair repulsion)
2. H2O
Central atom:
O
O contributes:
6 e−
2 x H contibute: 2 e−
Total VSE:
8
Total VSEP:
4
Geometry:
V-shaped (based on tetrahedral)

H–O–H bond angle = 104.5° (less than 109° due to lone-pair:bonding-pair repulsion
being greater than bonding-pair:bonding-pair repulsion)
2
3. ClF3
Central atom:
Cl
Cl contributes:
7 e−
3 x F contibute: 3 e−
Total VSE:
10
Total VSEP:
5
Geometry:
T-shaped (based on trigonal-bipyramidal)



The lone pairs occupy equatorial positions on the trigonal bipyramid. This minimises
the number of 90° lone-pair:bonding-pair interactions
The equatorial-axial F–Cl–F bond angles are less than 90° due to lone-pair:bondingpair repulsion being greater than bonding-pair:bonding-pair repulsion
The axial-axial F–Cl–F bond angle is less than 180° for the same reason
4. HCl
Central atom:
Cl
Cl contributes:
7 e−
1 x H contibutes: 1 e−
Total VSE:
8
Total VSEP:
4
Geometry:
Linear (based on tetrahedral)
3

The geometry of a species containing only two atoms is, of course, linear. However
the four electron pairs around the chlorine atom will be arranged in a tetrahedral
manner.
5.PCl3
Central atom:
P
P contributes:
5 e−
3 x Cl contibute: 3 e−
Total VSE:
8
Total VSEP:
4
Geometry:
Trigonal pyramidal (based on tetrahedral)
Note
 Cl–P–Cl bond angle is less than 109° due to lone-pair:bonding-pair repulsion being
greater than bonding-pair:bonding-pair repulsion
6. XeF4
Central atom:
Xe
Xe contributes: 8 e−
4 x F contibute: 4 e−
Total VSE:
12
Total VSEP:
6
Geometry:
Square planar (based on octahedral)
4
Notes
 F–Xe–F bond angles = 90 or 180°
 Lone pairs are on opposite sides of the molecule (180° from each other) to minimise
lone-pair:lone-pair interactions.
5
VSEPR Theory – Lewis Structure of Polyatomic Ions







Place the Central Atom
Add all the Valance electrons
Add electrons if it is negatively charged
Subtract electron if the ion is positively charged
Calculate total VSE
Calculate total VSEP
Decide the geometrical shape based on VSEP.
Examples:
-
1. Ammonium NH4+
5. Hydroxide OH
2. Nitrate NO3-
6. Carbonate CO3
3. Bicarbonate HCO3 -
7.
-
2
Sulfate SO42-
4. Cyanide CN-
6
7