VSEPR THEORY
(Valence Shell Electron Pair Repulsion Theory)
Take notes on the slides
Mrs Jacobus
Adapted from Mr. M. McIsaac
Carleton North High School, Bristol, NB
What Is The VSEPR Theory?
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VSEPR Theory is used to predict the shapes of
molecules.
Think of bonded pairs or BP (shared) or lone pairs
or LP (nonbonded, unshared)
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To achieve the most stable condition:
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e-’s are negatively charged clouds that repel each other.
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 dot
structures must be drawn first.
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2 Bond Pairs/Electron Groups
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Molecules that only have 2 bonding pairs(BP) 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
F
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.
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results in the electron pairs being at the corners of a
regular tetrahedron
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
Cl
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3 Bonding Pairs(BP) & 1 Non-bonding
(LP) 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
(LP) requires more space than a bonding pair (BP),
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 (BP)& 2 Non-bonding
Pairs (LP)
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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 (LP)
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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Go to the following website:
http://www.chemmybear.com/shapes.html
Complete the attached worksheet and be sure to
click on each of the animated shapes
 ..\..\Honors Chemistry 2007 and on\CH 7Bonding and Molecular Geometry\Molecular
Geometry Activity.doc
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