1. Notes: 9.4 OBJECTIVES: Students will be able to (SWBAT)
Predict the shapes of molecules
QUESTIONS TO PONDER:
What is VSEPR?
Why is VSEPR important in determining molecular shape?
What is the difference between molecular and electron geometry?
Why is a water molecule shaped differently than expected from its Lewis structure?
What are some of the exceptions to the VSEPR model?

2. Molecular Shape (Chapter 9, Section 4)

3. 9.4 – Molecular Shape
VSEPR (Valence Shell Electron Pair Repulsion) Model or Theory
Model used to determine 3-D molecular shape
Based on arrangement that minimizes repulsion of shared and unshared electron pairs around central atom
Unshared pairs repel more than shared pairs
Unshared/lone pairs are not visible
The shapes of molecules determine whether or not molecules can get close enough to each other to react.

4. Electron Geometry vs. Molecular Geometry
First, draw the molecular Lewis Structures
Electron Geometry (what determines shape):
Lone and shared pairs determine the geometry
Use VSEPR chart to determine electron geometry
Molecular Geometry (what you see):
Only shared pairs determines the geometry
Use VSEPR chart to determine molecular geometry

5. ← shared pair → ← unshared pair ↓ DASH – going back
WEDGE – sticking out

6. Steps to Determine Molecular Shape
Draw Lewis Structure of molecule
Count # of bonded atoms around central atom
Count unshared (lone) pairs (2 dots)
Use VSEPR chart (like one in the book, pg 260) to determine shape
*Remember, electron pairs will position themselves around central atoms to avoid the most repulsion (move farthest away from each other)

7. 3D Model using VSEPR for NH3

8. Example
NH3

9. Example
NH3
Total # of bonded atoms = 3

10. Example
NH3
Total # of lone pairs = 1

12. Example ***remember, lone pairs are not visible! NH3 ↓ ↓
↓ ↓
According to VSEPR chart  NH3 will have trigonal pyramidal shape

14. Exceptions to the Octet Rule
Central atoms in some molecules will not have an octet
Boron  can have less than an octet; often has only 3 pairs of e- (trigonal planar)
Phosphorus  can have an expanded (more than) octet, up to 5 pairs
Sulfur  can have expanded octet up to 6 pairs

15. More than 1 central atom Many molecules form longer chains
Anything with multiple C or N’s often form longer chains of molecules with C and N as the “backbone”

16. Example Example: C6H14 (hexane)  each C is a “central atom”
Structural Formula (model)
3-D (projection) formula