1. Go to the following site and see resonance structures and bond length

2. The VSEPR theory or model
The Valence Shell Electron Pair Repulsion (VSEPR) model:
is based on the number of regions of high electron density around a central atom (effective pairs).
can be used to predict structures of molecules or ions that contain only non-metals by minimizing the electrostatic repulsion between the regions of high electron density. Effective pairs must be as far as possible from each other.
can also be used to predict structures of molecules or ions that contain multiple bonds or unpaired electrons.
does fail in some cases, that is there are exceptions.

3. The VSEPR rules Draw the Lewis structure for the molecule or ion.
Count the effective pairs or total number of regions of high electron density (bonding and unshared electron pairs) around the central atom.
Double and triple bonds count as one effective pair (ONE REGION OF HIGH ELECTRON DENSITY).
An unpaired electron counts as one effective pair (ONE REGION OF HIGH ELECTRON DENSITY).
For molecules or ions that have resonance structures, you may use any one of the resonance structures.

4. # regions of high electron density
Molecule
Lewis Structure
# regions of high electron density
(EFFECTIVE PAIRS)
BeCl2
BF3
SHAPE
Linear 2
Trigonal Planar 3

5. # regions of high electron density
Molecule
Lewis Structure
# regions of high electron density
(EFFECTIVE PAIRS)
CH4
NH3
SHAPE 4
Tetrahedral
Trigonal Pyramidal 4
(3 bonded
1 lone pair)

6. # regions of high electron density
Molecule
Lewis Structure
# regions of high electron density
(EFFECTIVE PAIRS)
H2O
PF5
SHAPE
Bent or angular 4
(2 bonded
2 lone pairs) 5
Trigonal Bipyramidal

7. # regions of high electron density
Molecule
Lewis Structure
# regions of high electron density
(EFFECTIVE PAIRS)
SF6
CO2
SHAPE
Octahedral 6
Linear
180° 2

8. ABE NOTATION
Since the molecular geometry is determined by how many bonding and non-bonding electron groups surround the central atom, the first thing one needs to do is count how many of each there are. There is a notation that simplifies this bookkeeping:
ABxEy
The A represents the central atom, B represents the electron groups that form bonds to other atoms, and E represents the non-bonding electron groups. The subscripts, x and y, indicate how many of each kind are present.

9. An incidental benefit of using the ABE notation is that it provides a convenient way of remembering the hybridization at the central atom. The total number of substituents (bonding plus non-bonding groups) is equal to the number of atomic orbitals that participate in the hybrid orbital.
Molecule
ABE representation
# of substituents
Hybridization
CH4
AB4 4
sp3
NH3
AB3E
H2O
AB2E2
CO2
AB2 2 sp
SF6
AB6 6
sp3d2
I3- ion
AB2E3 5
sp3d

10. AB4 bonding groups: 4 non-bonding groups: 0
AB3E bonding groups: 3 non-bonding groups: 1
AB2E2 bonding groups: 2 non-bonding groups: 2
                               CH4 methane
              NH3 ammonia
              H2O water
AB2 bonding groups: 2 non-bonding groups: 0
                                      CO2 Carbon Dioxide
Note that bonding "electron groups" does not necessarily imply single bonds; it can mean double or triple bonds as well:

11. Displayed in the following table are the five most important electronic symmetries. Each row in the table is linked to a page that shows the different molecular symmetries possible for that electronic symmetry.
Class
Hybridization
Electronic Symmetry
AB2 sp
linear
AB3
sp2
trigonal planar
AB4
sp3
tetrahedral
AB5
sp3d
trigonal bipyramidal
AB6
sp3d2
octahedral

12. Go to the following site and see VSEPR to visualize
orbitals and shapes
Note: These sites have also been used in the pervious notes
together with the sources cited in specific slides

13. Polar and non polar molecules
A molecule like HF is said to be dipolar or
to have a dipole moment
 -
H F
Any diatomic molecule that has a
polar bond will show a dipole moment

14. Polyatomic molecules with polar bonds
Polar: if the dipoles
of the bonds don’t
cancel out.
Polyatomic molecules
with polar bonds cab be
Non Polar: if the dipoles of the bonds cancel out because
of the symmetric shape.

15. Example of Polar Polyatomic Molecules
NH3
H2O + - - +
The dipoles of the bonds don’t cancel out so the molecule has a dipole moment.
These molecules are dipoles.

16. Example of Non-Polar Polyatomic Molecules
BF3
CO2
CCl4 Cl Cl Cl Cl
The dipoles of the bonds cancel out because of the symmetric shape.
These molecules are non-polar molecules