From 2-D to 3-D
Chapter 9
Molecular Geometry
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Moving on to Shapes
•
A chemical formula tells us
The identity of atoms
The number of atoms
•
The Lewis structure tells us
The identity of atoms
The number of atoms
How the atoms and electrons are arranged.
•
The molecular geometry tells us
The identity of atoms
The number of atoms
How the atoms and electrons are arranged
The 3-d shape of that atom arrangement.
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VSEPR Model
• Valence Shell Electron Pair Repulsion
• A fancy name for a simple model.
• A model used to sort out how the bonding and
nonbonding electrons orient themselves in
three dimensions
In the model, pairs of electrons arrange
themselves to get as far away from each other as
possible in order to minimize their negative
repulsions.
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Vocabulary
• Electron Domain
 a region around a central atom where electrons are
likely to be located.
Bonding pair - a pair of electrons between two nuclei.
Unshared pair (nonbonding pair, lone pair) - a pair of electrons
Multiple bonds - double or triple bonds make up a single
domain.
• Consider O3
 three domains around the central oxygen.
Bonding pair
Unshared pair
Multiple bond
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Electron Domain Geometry
• 2 domains
• Build it with a styro-ball and
2 toothpicks
• Linear
• Angle
 180º
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Electron Domain Geometry
•
•
•
•
3 domains
Build it with styro-ball and 3 toothpicks
Trigonal Planar
Angles
 120º
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Electron Domain Geometry
• 4 domains
• Build it with styro-ball and 4 toothpicks
• Build it with the model kit. Find a ball with 4
holes.
• Tetrahedral
• Angles
 109.5º
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Electron Domain Geometry
• 5 domains
• Build it with your model kit.
Use the ball with 5 holes.
• Trigonal bipyramidal
• Angles
 120º
 90º
 180º
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Electron Domain Geometry
• 6 domains
• Build it with the model kit. Find the ball
with 6 holes.
• Octahedral
• Angles
 90º
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Molecular Geometry
What is the shape of the atoms?
• AB2 Example:
 CO2
 SCN−
 C 2H 2
• 2 electron domains
 Linear domains
• 0 nonbonding domains
• Linear molecule
• Bond Angles
 180º
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Molecular Geometry
What is the shape of the atoms?
• AB3 Examples
 BF3
 CH2O
 NO3−
 (CH3)2CO
• 3 electron domains
 Trigonal planar
• 0 nonbonding domains
• Trigonal planar
molecule
• Bond Angles
 120º
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Molecular Geometry
What is the shape of the atoms?
• AB2 Examples:
 O3
 NO2−
• 3 electron domains
Trigonal planar
• 1 nonbonding
domain
• Bent molecule
• Bond Angles
< 120º
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Ozone
• O3
• 3 domains = trigonal planar domains
• 1 unshared pair = bent molecule
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Molecular Geometry
What is the shape of the atoms?
• AB4 Examples
 CH4
 CF2H2
• 4 electron domains
Tetrahedral
• 0 nonbonding domain
• Tetrahedral molecule
• Bond Angles
 109.5º
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Molecular Geometry
What is the shape of the atoms?
• AB3 Examples
 NH3
 PCl3
• 4 electron domains
 Tetrahedral
• 1 nonbonding domain
• Trigonal pyramidal
molecule
• Bond Angles
 < 109.5º
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Molecular Geometry
What is the shape of the atoms?
• AB2 Examples
 H 2O
 CH3OH
• 4 electron domains
 Tetrahedral
• 2 nonbonding domain
• Bent molecule
• Bond Angles
 << 109.5º
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Bond Angles
• Resketch the molecular shapes of CH4, NH3,
and H2O.
• Consider the bond angles.
• Unshared pairs take up more space than
bonding pairs.
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More on Bond Angles
• Sketch the Lewis structure for CCl2O.
• Consider the bond angles.
• Multiple bonds bulge and take up more space
that single bonds.
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Build a Trigonal Bipyramidal Structure
• Consider the five positions different or the same?
• There are two different positions in
this expanded octet molecule
 2 Axial positions
 3 Equatorial positions
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Molecular Geometry
What is the shape of the atoms?
• AB5 Example
 PCl5
• 5 electron domains
 Trigonal bipyramidal
• 0 nonbonding domain
• Name?
 Trigonal bipyramidal
molecule
• Bond Angles
90º, 120º, 180º
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Molecular Geometry
What is the shape of the atoms?
• AB4 Example
 SF4
• 5 electron domains
 Trigonal bipyramidal
• 1 nonbonding domain
• Name?
 See saw molecule
• Bond Angles
< 90º, < 120º, < 180º
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Molecular Geometry
What is the shape of the atoms?
• AB2 Example
 ClF3
• 5 electron domains
 Trigonal bipyramidal
• 2 nonbonding domains
• Name?
 T-shape molecule
• Bond Angles
<< 90º, << 180º
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Molecular Geometry
What is the shape of the atoms?
• AB2 Examples
 XeF2
 I3-1
• 5 electron domains
 Trigonal bipyramidal
• 3 nonbonding domains
• Name?
 Linear molecule
• Bond Angles
180º
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Molecular Geometry
What is the shape of the atoms?
• AB6 Example
 SF6
• 6 electron domains
Octahedral
• 0 nonbonding
domain
• Name?
Octahedral
molecule
• Bond Angles
90º, 180º
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Molecular Geometry
What is the shape of the atoms?
• AB5 Example
 BrF5
• 6 electron domains
 Octahedral
• 1 nonbonding domain
• Name?
 Square pyramidal
molecule
• Bond Angles
 < 90º
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Molecular Geometry
What is the shape of the atoms?
• AB4 Example
 XeF4
• 6 electron domains
 Octahedral
• 2 nonbonding domains
• Name?
 Square planar
molecule
• Bond Angles
 90º, 180º
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