1. Exceptions to Octet Rule
Lewis Structures &
Stability
Multiple Bonds
Bond Energies
Resonance
Exceptions to Octet Rule

2. Stability and Bond Energies
Bond energy is a measure of stability.
Bond energy – amount of energy required to break a bond holding two atoms together.
The values of bond energies decrease in order: triple > double > single.
Example: N2 > O2 > H2

3. Bond Length Bond length – distance of separation of the bonded nuclei
Bond length is related to the presence or absence of multiple bonding.
Bond length decreases as the number of bonds increase: single > double > triple

4. Resonance
In some cases, it is possible to write more than one Lewis structure that satisfies the octet rule.
Experimental evidence shows no double bond exists. The actual structure is a hybrid of these two.

5. Draw the resonance structures for:
Practice - Resonance
Draw the resonance structures for:
Ozone (O3)
Benzene (C6H6)

6. Ozone

7. Benzene

8. Exceptions to the Octet
Incomplete octet
Example: BeH2
Applies only to B and Be
Odd electron molecules
Example: NO
Expanded octet
Example: PF5
Applies to non-metals in periods 3-7
These elements have empty d-orbitals for excited electrons to jump into, creating more bonding sites

9. Homework
Pages 99 and 101
Problem #’s 4.17 – 4.20