1. Valence Shell Electron Pair Repulsion Theory
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Planar triangular
Valence Shell Electron Pair Repulsion Theory
Tetrahedral
Trigonal pyramidal
Bent

2. Molecular Shape
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VSEPR theory assumes that the shape of a molecule is determined by the repulsion of electron pairs.

3. VSEPR Theory Based on Lewis structures
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Based on Lewis structures
Theory predicts shapes of compounds
VSEPR (pronounced “vesper”) stands for Valence Shell Electron Pair Repulsion
VSEPR predicts shapes based on electron pairs repelling (in bonds or by themselves)
Electrons around central nucleus repel each other. So, structures have atoms maximally spread out

4. VSEPR overview Names of Shapes: tetrahedral trigonal pyramidal Bent
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Each shape has a name
Names of Shapes:
tetrahedral
trigonal pyramidal
Bent
Linear
trigonal planar

5. Models Tetrahedral Triangular Planar Bent or V Linear
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Models
Tetrahedral
Triangular Planar
Bent or V
Linear
Trigonal pyramidal

6. methane, CH4 Tetrahedral Bonds are all evenly spaced electrons 109.5°
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methane, CH4
Tetrahedral
109.5°
Bonds are all evenly spaced electrons

7. ammonia NH3 .. .. .. .. Trigonal Pyramidal
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ammonia
NH3 .. .. ..
Trigonal Pyramidal
Less repulsion between the bonding pairs of electrons

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water, H2O
109.5° (109.5°)
109.5° (107°)
109.5° (104.5°)

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Bent or V
2 unshared pairs of e’s at top of O repel bonds and force them to bend

10. Number of electron pairs
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Molecule
Lewis Structure
Number of electron pairs
CH4
NH3
SHAPE
Tetrahedral 4
Trigonal Pyramidal 4
(3 shared
1 lone pair)

11. Number of electron pairs
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Molecule
Lewis Structure
Number of electron pairs
H2O
SHAPE
Bent 4
(2 shared
2 lone pairs)
Linear
CO2 2

12. Number of electron pairs
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Molecule
Lewis Structure
Number of electron pairs
BeCl2
BF3
SHAPE
Linear 2
Trigonal Planar 3

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Hybrid Orbitals
VSEPR works well for shapes/geometry, but not for describing the types of bonds formed
Hybridization: the mixture of atomic orbitals to form the same number of new orbitals
Carbon is the most common element that undergoes hybridization

14. Methane Hybrid Orbitals
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Methane Hybrid Orbitals
Electron configuration of methane: [He]2s22p2
One s and three p orbitals hybridize to form four sp3 orbitals

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Molecular Shape/Hybrid Orbitals: Linear/sp Trigonal planar/sp2 Tetrahedral/sp3 Trigonal pyramidal/sp3 Bent/sp3

16. Phosphorus trihydride
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Phosphorus trihydride
Total number of valence electrons: 8
Lewis structure: three single bonds and one lone pair
Shape: trigonal pyramidal
Four bonding positions = sp3 hybrid

17. Intermolecular Forces
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Intermolecular Forces
Forces that hold together identical particles such as water molecules in a drop of water
Three such forces are: dispersion forces, dipole-dipole forces, and hydrogen bonds

18. Dispersion Forces Also known as London forces
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Also known as London forces
Weak forces that result from a temporary shift in the density of electrons in electron clouds
For example, if two nonpolar molecules collide, the electron clouds of one molecule repels the electron cloud of the other molecule, creating a greater electron density in on region of each electron cloud

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Dipole-Dipole Forces
Attractions between oppositely charged regions of polar molecules
Since dipoles in polar molecules are permanent, dipole-dipole forces are stronger than dispersion as long as the molecules are similar in mass.

20. Hydrogen Bonds Type of dipole-dipole attraction
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Hydrogen Bonds
Type of dipole-dipole attraction
Occurs between molecules containing a hydrogen atom bonded to a small, highly electronegative atom with at least one lone pair (i.e., fluorine, oxygen, or nitrogen)
F, O, and N are electronegative enough to cause a large partial positive charge on the H, but small enough to allow their lone pairs to come close to H atoms