Untitled - Kelly Walsh High School

136 CHEMISTRY FOR THE UTTERLY CONFUSED
9-5 Resonance
Quick Tip
Don’t Forget!
Sometimes when writing the Lewis structure of a species, we may draw more
than one possible “correct” Lewis structure for a molecule. The nitrate ion,
NO 3 , is a good example. The structures that we write for this polyatomic anion
differ in which oxygen has a double bond to the nitrogen. None of these three
truly represents the actual structure of the nitrate ion—it is an average of all
three of these Lewis structures. We use resonance theory to describe this situation.
Resonance occurs when more than one Lewis structure (without moving
atoms) is possible for a molecule. The individual structures are called resonance
structures (or forms) and are written with a two-headed arrow (i) between
them. The three resonance forms of the nitrate ion are:
O
N
O
−
O O N
O
O
− −
O N O
When you draw resonance structures, you can only move electrons (bonds).
Never move the atoms.
Again, let us emphasize that the actual structure of the nitrate ion is not any of
the three shown. It is not flipping back and forth among the three. It is an average
of all three. All the bonds are the same, and are intermediate between single
bonds and double bonds in strength and length.
When writing the Lewis structures of polyatomic ions, don’t forget to show their
ionic charge.
O