Molecular Geometry and VSEPR Theory

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Molecular Geometry and VSEPR Theory
Chapter 4 Pages 106-111
Mrs. Weston Advanced Chemistry
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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.

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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.
• Draw the Lewis Structure for the molecule.
• Example SiF4

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How does VSEPR THEORY work?

• We can use VSEPR theory using 4 steps
• Draw the Lewis Structure for the molecule.
• Tally the number of bonding pairs and lone
  (non-bonding) pairs on the center atom.

Bonding pairs 4 Lone pairs on central atom 0
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How does VSEPR THEORY work?

• We can use VSEPR theory using 4 steps
• Draw the Lewis Structure for the molecule
• Tally the number of bonding pairs and lone pairs
  on the center atom.
• Arrange the rest of the atoms so that they are as
  far away from each other as possible.

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How does VSEPR THEORY work?

• We can use VSEPR theory using 4 steps
• Draw the Lewis Structure for the molecule
• Tally the number of bonding pairs and lone pairs
  on the center atom. (Double and triple bonds only
  count as one pair.)
• Arrange the rest of the atoms so that they are as
  far away from each other as possible
• 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 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.
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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
• Draw a Lewis structure.
• Assign the electron pair geometry using VSEPR
  theory.
• Use the electron pair geometry to determine the
  hybridization.
• Name the shape by the positions of the atoms.