3. Valence Shell Electron Pair Repulsion Theory (VSEPR Theory):
Looking at molecules in 3D!!!
Tuesday, December 6th, 2016

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

5. VSEPR Theory Based on Electron Dot (Lewis structures)
VSEPR 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

6. VSEPR overview tetrahedral trigonal pyramidal Bent Linear
Each shape has a name (you should know these 5)
Names of Shapes:
tetrahedral
trigonal pyramidal
Bent
Linear
trigonal planar

7. Triangular Planar
Bent
Tetrahedral
Linear
Trigonal pyramidal

8. Example 1: Methane (CH4) Example 2: Ammonia (NH3) Example 3: Water(H2O)

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

10. ..
ammonia
NH3 .. .. ..
Trigonal Pyramidal
Less repulsion between the bonding pairs of electrons

11. .. ..
water, H2O
109.5° (109.5°)
109.5° (107°)
109.5° (104.5°)

12. .. ..

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

14. Steps to Success
Draw the Lewis Structure for each atom
- Count up the valence electrons, determine central atom, bond terminal atoms, recount, add lone electron pairs, check & shift to share if necessary
Determine VSEPR Structure
-- # atoms bound
-- # unpaired electrons on central atom
Redraw as VSEPR Structure

15. Example 4: Carbon Dioxide (CO2)

16. Example 4: Carbon Dioxide (CO2)

17. Molecule Kit Lab Activity
With your partner, you will draw the lewis dot structures for several compounds.
You will construct the compounds from the kits provided and sketch your models.
Identify the molecular geometry (shape) of your compound from the central atom and label the bond angles.
Lastly, determine if you compound is polar or nonpolar (horse & cart)