1. Valence Shell Electron Pair Repulsion Theory
Planar triangular
Valence Shell Electron Pair Repulsion Theory
Tetrahedral
Trigonal bipyramidal
Octahedral

2. 2-D vs 3-D Structures Draw the Lewis Dot structure for CH4
This molecule exists as a 3-D structure
Lets Draw NH3 and H2O
Look at the molecule made using the molecule kit
How can we explain/describe the 3-D structure of molecules?

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Molecular Shapes
The shape of a molecule plays an important role in its reactivity.
By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.

4. What Determines the Shape of a Molecule?
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What Determines the Shape of a Molecule?
Simply put, electron pairs, whether they be bonding or nonbonding, repel each other.
By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule.

5. Nonbonding Pairs and Bond Angle
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Nonbonding Pairs and Bond Angle
Nonbonding pairs are physically larger than bonding pairs.
Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

6. Valence Shell Electron Pair Repulsion Theory (VSEPR)
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Valence Shell Electron Pair Repulsion Theory (VSEPR)
“The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”

7. The VSEPR Model
The valence shell electron pair repulsion model is useful to predict molecular geometries using the assumption that the structure around an atom is determined by minimizing electron repulsion.
Geometric shapes include: linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral
There are variations of these shapes depending on lone-pair repulsions.
Lone-pairs require more room than bonding pairs and tend to compress the angles between bonds.

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Planar triangular
Tetrahedral
Trigonal bipyramidal
Octahedral

9. VSEPR: Lone Pairs

10. Lone pairs
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Thus far we have considered only structures where there are no free electrons around the central atom H C H C
Vs. H N H N or or
These electrons that are not involved in bonds are called “lone pairs”
Essentially, they have the same influence on molecular structure as electron pairs in bonds
The result is some weird shapes and names…

11. Variations on Tetrahedral Molecule
The tetrahedral molecule is AX4
Lone pairs can be indicated with AXYEZ, where Z is the number of lone pairs
By replacing 1 bond with a lone pair the tetrahedral shape becomes “trigonal pyramidal”
AX3E
By replacing two bonds with lone pairs we get a “bent” (non-linear) shape (AX2E2 )

12. Variations on Trigonal Bipyramidal
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AX5 is trigonal bipyramidal
AX4E is unsymmetrical tetrahedron
AX2E3 is linear
AX3E2 is T-shaped

13. Variations on Octahedral Shape
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AX6 is octahedral
AX5E is square pyramidal
AX4E2 is square planar
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14. Multiple Bonds and Bond Angles
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Multiple Bonds and Bond Angles
Double and triple bonds place greater electron density on one side of the central atom than do single bonds.
Therefore, they also affect bond angles.
Multiple bonds are counted as one electron pair of electrons.
Any resonance structure can be used to predict molecular geometries.

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Larger Molecules
In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.

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Larger Molecules
This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.

17. Lets Review Single Bonds
Examples
Shape
Angle
4 Single Bonds
3 Single
& 1 unshared pair
2 bonds & 2 unshared pairs
1 single bond & 3 Unshared pairs

18. Multiple Bonds Bonds Examples Shape Angle
1 double bond & 2 single bonds
1 double bond, 1 single bond, and 1 unshared pair
2 double bonds
1 triple bond and 1 single bond