Molecular Geometry and
VSEPR Theory
Chapter 4 Pages 106-111
Mrs. Weston
Advanced Chemistry
1
Molecular Shapes
• The shape of a molecule plays a large part in
determining its properties and reactivities.
• We can predict shapes by examining the
Lewis structure for orientation of electron
pairs.
• Electron pairs arrange themselves to
minimize electrical repulsion.
2
VSEPR Theory
 In order to predict molecular shape, we
assume the valence electrons repel each other.
Therefore, the molecule adopts whichever 3D
geometry minimized this repulsion.
 Electron pairs arrange themselves as far as
possible from each other.
 We call this process Valence Shell Electron
Pair Repulsion (VSEPR) theory.
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Why is VSEPR Theory Important?
• Gives a specific shape due to the number
of bonded and non-bonded electron pairs
in a molecule
• Tells us the actual 3-D structure of a
molecule
• Again in bonding, electron pairs want to
be as far away from each other as
possible.
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The VSEPR Model
Predicting Molecular Geometries
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How does VSEPR THEORY work?
We can use VSEPR theory using 4 steps.
1. Draw the Lewis Structure for the
molecule.
Example: SiF4
F
F-Si-F
F
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How does VSEPR THEORY work?
We can use VSEPR theory using 4 steps
1. Draw the Lewis Structure for the
molecule.
2. Tally the number of bonding pairs and
lone (non-bonding) pairs on the center
atom.
F
Bonding pairs: 4
F-Si-F
Lone pairs on central atom: 0
F
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How does VSEPR THEORY work?
We can use VSEPR theory using 4 steps
1. Draw the Lewis Structure for the
molecule
2. Tally the number of bonding pairs and
lone pairs on the center atom.
3. Arrange the rest of the atoms so that
they are as far away from each other as
possible.
F
Si
F
F
F
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How does VSEPR THEORY work?
We can use VSEPR theory using 4 steps
1. Draw the Lewis Structure for the
molecule
2. Tally the number of bonding pairs and
lone pairs on the center atom. (Double
and triple bonds only count as one pair.)
3. Arrange the rest of the atoms so that
they are as far away from each other as
possible
4. Give the type of geometry the molecule
has: Tetrahedral
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The VSEPR Model
Difference between geometry and shape
Electron Pair Geometry:
We determine the geometry only looking at electrons.
All the atoms that obey the octet rule and have single
bonds have the same tetrahedral-like geometry.
Shape or Molecular Structure:
We name the shape by the positions of atoms.
We ignore lone pairs in the shape.
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VSEPR and Resulting Shapes
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The VSEPR Model
Predicting Shape
Shape
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The VSEPR Model
Predicting Shape
Shape
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The VSEPR Model
The Effect of Nonbonding Electrons and
Multiple Bonds on Bond Angles
By experiment, the H-X-H bond angle decreases on
moving from C to N to O:
H
H C H
H
109.5O
H N H
H
107O
O
H
H
104.5O
Since electrons in a bond are attracted by two nuclei, they do
not repel as much as lone pairs.
Therefore, the bond angle decreases as the number of lone
pairs increase.
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The VSEPR Model
The Effect of Nonbonding Electrons and
Multiple Bonds on Bond Angles
Similarly, electrons in multiple bonds repel more than
electrons in single bonds.
Cl
111.4o
Cl
C O
124.3o
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The VSEPR Model
Molecules with More than One Central Atom
In acetic acid, CH3COOH, there are three central
atoms.
We assign the geometry about each central atom
separately.
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Polarity of Molecules
A molecule is POLAR if its centers of positive and
negative charges do not coincide.
The molecule behaves as a dipole (Having two ends of
opposite charge)
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Polarity of Molecules
Dipole: def: two charges, equal in magnitude and
opposite in sign, are separated by a distance
Polarity of Polyatomic Molecules
•Each bond can be polar.
•The orientation of these polar bonds determines
whether the molecule is polar overall.
•It is possible for a molecule with polar bonds to be
either polar or non-polar.
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Polarity of Molecules
Dipole Moments of Polyatomic Molecules
Example: in CO2, each C-O dipole is canceled because
the molecule is linear. In H2O, the H-O dipoles do not
cancel because the molecule is bent.
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Polarity of Molecules
Dipole Moments of Polyatomic Molecules
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Hybrid Orbitals
• s and p orbitals are used in bonding. It is
easy to tell which ones are used by
looking at our molecule. (2s and 2p)
• For example, CH4. Looking again at the
Lewis structure, we see that there are 4
bonds.
• The s orbital and three p orbitals can
combine and form 4 sp3 bonds.
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Hybrid Orbitals
• Regions of electron density-EACH BOND
AND LONE PAIR OF ELECTRONS ON
THE CENTRAL ATOM IS KNOWN AS A
REGION OF ELECTRON DENSITY.
• 4 regions of electron density-sp3 hybridized
• 3 regions of electron density-sp2 hybridized
• 2 regions of electron density-sp hybridized
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Hybrid Orbitals
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Hybrid Orbitals
Summary
To assign hybridization:
1. Draw a Lewis structure.
2. Assign the electron pair geometry using VSEPR
theory.
3. Use the electron pair geometry to determine the
hybridization.
4. Name the shape by the positions of the atoms.
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