1. 8.5 NOTES Resonance and Exceptions to the Octet Rule

2. Resonance: A blend of two or more Lewis Structures when one structure cannot indicate the true form of a molecule.
The “true” structure is actually the average of all of the resonance forms.
Resonance occurs when it is possible to achieve a correct structure by assigning electrons in different ways. Are you making a choice?
SO3   
SO2
CO3-2
Benzene (C6H6)

5. Octet Exceptions: There are several instances when atoms are unable to attain 8 electrons in their valence energy level.
Odd Number of Electrons
Assign pairs (and a single electron) to atoms to attain as close to 8 electrons as possible)
Period 2 elements CANNOT exceed 8 electrons.
These structures may have resonance.

6. Incomplete Octets
These occur when there are inadequate electrons to attain 8, but the elements also cannot double or triple bond to complete octets. (CONS!)

7. Expanded Octets
Larger atoms (Period 3 and higher) can sometimes hold more than 8 electrons. Phosphorus and Arsenic often achieve 10 electrons and Sulfur can have up to 12.