VSEPR THEORY
(Valence Shell Electron
Pair Repulsion Theory)
Adapted by Mr. M. McIsaac
Carleton North High School, Bristol, NB
From Mr. James Montgomery
SCH4U Grade 12 Chemistry
Sir John A. MacDonald Secondary School
Waterloo, ON
sjam.wrdsb.on.ca/files/Lesson14vsepr.ppt
What Is The VSEPR Theory?
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VSEPR Theory is used to predict the shapes of
molecules.
Think of bonded pairs (shared) or lone pairs
(nonbonded, unshared) of e-’s as negatively
charged clouds that repel each other.
To achieve the most stable condition the clouds
must be as far apart as possible in 3-D, thereby
decreasing repulsion.
The amount of repulsion can be ordered:
LP-LP > LP-BP > BP-BP
In order to determine the shape, the Lewis
diagram must be drawn first.
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2 Bond Pairs/Electron Groups
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Molecules that only have 2 bonding pairs on the
central atom will have a LINEAR SHAPE with a
bond angle of 180°
e.g. BeF2, CO2, CS2
General Formula: AX2
Central atom A from group 2; 2 BP 0 LP
F
Be
F
180°
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3 Bond Pairs/Electron Groups
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Molecules that have 3 bonding pairs on the
central atom will have a TRIGONAL
PLANAR SHAPE with bond angles of 120°.
e.g. BF3, BH3
General Formula: AX3
Central atom A from group 13; 3 BP 0 LP
F
F
B
120°
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4 Bonding Pairs/Electron Groups
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If the central atom is placed at the center of a
sphere, than each of the four pairs of electrons will
occupy a position to be as far apart as possible.
This will result in the electron pairs being at the
corners of a regular tetrahedron, therefore these
molecules are said to have a TETRAHEDRAL
SHAPE.
The angle between each bond will be 109.5°
e.g. CCl4, CH4, SiH4
General Formula: AX4
Central atom A from group 14; 4 BP 0 LP
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Example CCl4
Cl
C
Cl
Cl
Cl
Cl
C
Cl
109.5 °
Cl
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3 Bonding Pairs & 1 Non-bonding Pair
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Four pairs of electrons will always arrange themselves
tetrahedrally around the central atom.
The shape of the molecule is determined by the
arrangement of the atoms not the electrons.
As a result such molecules will have a TRIGONAL
PYRAMIDAL shape.
Due to the repulsion, a non-bonding electron pair
requires more space than a bonding pair, the angles
in these molecules are 107° not 109.5° as in the
tetrahedral molecules.
e.g. NH3, PCl3
General Formula: AX3E
Central atom A from group 15; 3 BP 1 LP
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Example NH3
N
H
H
H
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2 Bonding Pairs & 2 Non-bonding Pairs
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The four pairs of electrons will be arranged
tetrahedrally but since only 2 pairs are bonding
electrons, the surrounding atoms are at 2 corners of
the tetrahedron.
As a result these molecules will have a V-SHAPE or
BENT.
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The repulsion between the non-bonding pairs will
result in a bond angle of 104.5°.
For each pair of non-bonding electrons, the bond
angle decreases by 2.5°
e.g. H2O, H2S, OCl2
General Formula: AX2E2
Central atom A from group 16; 2 BP 2 LP
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Example H2O
O
H
H
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Things to Remember
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In order to predict the shape of a molecule
you must draw the Lewis Dot Diagram for
the molecule, determine the number of
bonding and non-bonding electron pairs and
compare this with the chart you have been
given (the shapes must be memorised).
When determining the shape of a molecule
with multiple bonds, treat the multiple
bonds as if they were single bonds (i.e. one
bonding pair)
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5 Molecular
Shapes
From
Single Bonds
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