1. Molecular Structures Section 9.3
Chemistry

2. Objectives
Explain why resonance occurs, and Identify resonance structures.
Explain three exceptions to the octet rule, and Identify molecules in which these exceptions occur.

3. Resonance Structures
Resonance Structures: a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion.
Ex: CO3

4. Carbonate Resonance Structures

5. Resonance Structures
Resonance structures differ only in the position of the electron pairs, never the atom positions

6. 3 Exceptions to the Octet Rule
There are some exceptions to the octet rule.
What do you notice about the molecule to the right?

7. 1st Exception Central atom has more than 8 valence electrons
EXPANDED OCTET

8. 2nd Exception
What do you notice about the molecule to the right?

9. 2nd Exception
Some compounds form with fewer than 8 electrons around an atom
Coordinate covalent bond: One atom donates a pair of electrons to be shared with an atom or ion that needs two electrons to become stable

10. 3rd Exception
What do you notice about the molecule to the right?

11. 3rd Exception
Small group of molecules has an odd number of valence electrons and cannot form an octet

12. Homework
42-48(omit c in 48) page 258