1. 2.5 Lewis Structures: Resonance and
One Possible Exception to the Octet Rule

2. Point Perhaps Missed - Triple bonds are stronger than Double which are stronger than Single. Triple Bonds are shorter than Double which are shorter than Single.

3. Resonance Multiple resonance structures are required when…
…one Lewis structure cannot determine a molecule’s bonds due to delocalized e–’s

4. Just as green is
a synthesis of blue and yellow…
…ozone is a synthesis of two resonance structures.
How many double bonds?
How many single bonds?
NONE!
NONE!

5. Draw a Lewis structure for ozone, O3.
Which?
Neither!

6. The observed structure of ozone:
…both O—O bonds are the same length.
…both outer O’s have a same e– density.

7. Bond order is the number of chemical bonds between a pair of atoms
Bond order is the number of chemical bonds between a pair of atoms. For resonance structures it is the number of bonds divided by number of bond sites.

8. Resonance Consider HCO2–
the electrons that form the double bond can move among the two O’s and the C.
They are delocalized.

9. Resonance
In benzene (C6H6), a hexagon with a circle signifies the delocalized electrons in the ring.

10. One Possible Exception to the Octet Rule
Ions or molecules with less than an octet.
BF3
boron trifluoride
Boron is OK with 6 electrons, instead of 8.