1. TOPIC: Molecular Geometry (Shapes of Molecules) Essential Question: How do you determine the different shapes of molecules?

2. VSEPR Theory
Electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory
because electrons are negatively charged, they should be most stable when they are separated as much as possible
The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule

3. Electron-group repulsions and the five basic molecular shapes.
linear
trigonal planar
tetrahedral
trigonal bipyramidal
octahedral

4. Electrons vs. Molecular Geometry
The geometry of electron pairs around a central atom is called the electron geometry.
Lone pair electrons on a central atom will repel other pairs but will not be visible in the molecular geometry (no nuclei)
If there are lone pairs on the central atom the electron geometry and the molecular geometry will differ.

5. Two electron pairs on central atom
Examples:
CS2, HCN, BeF2

6. 3 electron pairs on central atom All are in bonds
Trigonal Planar Shape
Examples:
SO3, BF3, NO3-, CO32-

7. Four electron pairs on central atom All 4 in bonds
Examples:
CH4, SiCl4, SO42-, ClO4-

8. Trigonal Pyramidal Shape
4 Electron Pairs
3 bonded
1 un-bonded (lone pair)
Trigonal Pyramidal Shape
Examples: NH3, PF3, ClO3. H3O+
Bond angles are reduced from 109.5o to 107o due to extra repulsion by lone pair

9. 2 Un-bonded (lone pairs) BENT SHAPE
4 Electron Pairs
2 Bonded
2 Un-bonded (lone pairs)
BENT SHAPE
Examples: H2O, OF2, SCl2
Bond angles are reduced a little more due to repulsion
To 104.5o

10. The steps in determining a molecular shape
Molecular formula
Lewis structure
Electron-group arrangement (electron geometry)
Bond angles
Molecular geometry
Count all e- pairs around central atom
Note lone pairs and double bonds
Consider bonding e- pairs only
Step 1
Step 2
Step 3
Step 4