1. Variations in Lewis Structures

2. Resonance Forms  Some substances are not adequately defined by a single Lewis Structure  For example, ozone ( O 3 ), can be drawn just as easily two different ways  The two equivalent structures are called resonance forms  Both are written side by side with a double headed arrow between

3.  In resonant forms, only the placement of electrons is different between the structures  The resonant compound behaves almost like an average of the two forms

4. Example  Draw two resonant forms of NO 3 -.

5. Bond Order  Single Bond – 1  Double Bond – 2  Triple Bond – 3  In resonance form the bond order is the mathematical average of the bond orders Bond Order = Sum of Bond Orders # of Bonded Atoms Example: What is the bond order of NO 3 - ?

6. Exceptions To Rule Of Eight  The octet rule is not always obeyed  If the number of electrons is odd, an octet cannot be achieved for every atom  ClO 2, NO, NO 2

7. Less Than Octet  Some atoms have less than an octet around them  In B and Be occurs often  BF 3  These compounds tend to be reactive due to the lack of the octet

8. More Than Octet  Some atoms form compounds with more than an octet  Observed only in elements in period 3 or higher  These elements have unfilled d orbitals which can be used to accommodate electrons  Larger atoms can often accommodate more electrons

9. Examples  Draw PCl 5  Draw ICl 4 -  Which never has more than an octet around it: S, C, P, Br