1. Valence Shell Electron-pair Repulsion model
VSEPR
Valence Shell Electron-pair Repulsion model

2. VSEPR This model is useful in predicting the geometry of molecules.
Remember: molecules are nonmetal & nonmetal.
The geometry around a given atom in a molecule is determined by minimizing electron pair repulsions.

3. Linear Electron Pair Repulsion.
Note there are two electron pairs surrounding Be.
The charge of an electron is negative.
The position that reduces that repulsion is 180 ° or linear.

4. Three pairs or Trigonal Planar
Consider three bonds such as BH3.
What arrangement will minimize electron pair repulsion?
120 ° or trigonal planar.

5. What about 4 pairs?
With 4 pairs in a plane, the largest angle available is 90°.
Is that the largest angle possible?
What if you go to three dimensions?

6. Tetrahedral
By arranging the 4 bonds around the central atom in three dimensions you can increase the bond angle to 109.5° and reduce the electron pair repulsion.

7. Trigonal Bipyramidal 5 bonds
This is the only electron arrangement in which there are two different angles between electron pairs to reduce repulsion.
Example: PCl5

8. Octahedral
6 electron pairs.
Each angle is 90°
Examples include PCl6

9. Steps to apply VSEPR Draw the Lewis Dot structure for the molecule.
Count the electron pairs and arrange them in the way that minimizes repulsions.
Determine the positions of the atoms from the way the electron pairs are shared.
Determine the name of the shape from the position of the atoms.

10. Lone pairs and Bond angles
Lone pairs have a greater repulsion than bound electrons.
This repulsion has the effect of “squishing” the bond angle in the molecule.
CH4
NH3
H2O
# of lone pairs 1 2
Bond angle
109.5°
107°
104.5°

11. Repulsion of lone pair electrons

12. Question
What are the bond angles of CH3CO2CH3?

13. Answer How close were you? Is the angle between the C-O-C really 109°?
Why or why not?