1. Delocalized Electrons - Resonance Structures (14.3)
Some molecules are not well described by Lewis Structures.
Typically, structures with multiple bonds can have similar structures with the multiple bonds between different pairs of atoms

2. Resonance Structures
Example: experimentally, ozone has two identical bonds whereas the Lewis Structure requires one single (longer) and one double bond (shorter).

3. Examples – Draw Lewis Structures for each, and include all relevant resonance structures:
NO3-
CO32-
SO2
NO2

4. Resonance in Benzene
Benzene consists of 6 carbon atoms in a hexagon. Each C atom is attached to two other C atoms and one hydrogen atom.
There are alternating double and single bonds between the C atoms.
Experimentally, the C-C bonds in benzene are all the same length.
Experimentally, benzene is planar.

5. We write resonance structures for benzene in which there are single bonds between each pair of C atoms and the 6 additional electrons are delocalized over the entire ring:
Benzene belongs to a category of organic molecules called aromatic compounds (due to their odor).

6. Multiple Bonds
It is possible for more than one pair of electrons to be shared between two atoms (multiple bonds):
One shared pair of electrons = single bond (e.g. H2);
Two shared pairs of electrons = double bond (e.g. O2);
Three shared pairs of electrons = triple bond (e.g. N2).
Generally, bond distances decrease and bond strengths increase as we move from single through double to triple bonds. (4.2.4)

7. Bond Strength and Bond Length (4.2.4)
Multiple bonds are shorter than single bonds.
Multiple bonds are stronger than single bonds.
As the number of bonds between atoms increases, the atoms are held closer and more tightly together.
NOTE: A double bond between two atoms is not twice as strong as a single bond between the same two atoms.

10. Exceptions to the Octet Rule Odd Number of Electrons
There are three classes of exceptions to the octet rule:
Molecules with an odd number of electrons;
Molecules in which one atom has less than an octet;
Molecules in which one atom has more than an octet.
Odd Number of Electrons
Few examples. Generally molecules such as ClO2, NO, and NO2 have an odd number of electrons.

11. Less than an Octet
Relatively rare.
Molecules with less than an octet are typical for compounds of Groups 1, 2, and 13.
Most typical example is BF3.
Formal charges indicate that the Lewis structure with an incomplete octet is more important than the ones with double bonds.

12. More than an Octet
This is the largest class of exceptions.
Atoms from the 3rd period onwards can accommodate more than an octet.
Beyond the third period, the d-orbitals are low enough in energy to participate in bonding and accept the extra electron density.

13. Examples – Draw Lewis Structures for Each
PCl5
SF6
BeCl2
BF3
XeF4