What is resonance?

Resonance can be considered as the condition ocurring when more than one valid Lewis structure can be written for a particular molecule.
This can happen any time there are two or more adjacent p-tpye orbitals in the same plane.

Important!

Resonance structures are imaginary. They represent extremes of electron location. Resonance can be considered as a valence theory solution to a molecular orbital problem.

In general chemistry the concept of resonance was introduced through inorganic anions such as NO₃^-, NO₂^-, ClO₄^-, SO₄^-2, CO₃^-2.

Note the three Lewis structures below for the nitrate anion (ignore the curved arrows in structures 1 and 2 for the moment). The position of the colored atoms is the same in each structure. The only difference is the location of the electrons. Structure 1 has a double bond to the red oxygen, structure 2 to the blue oxygen, and structure 3 to the green oxygen. Structures 1-3 are not identical, but they are equivalent.


	Rule #1: Resonance structures must have the same number of electrons. (you should prove it to yourself by counting them).
	Rule #2: All resonance structures must be valid Lewis dot structures.
	Rule #3: The hybridization does not change between resonance structures.
	Rule #4: The positions of atoms are the same in all structures.
	Rule #5: Resonance structures do not have to be equivalent.

Important!


The actual structure of a molecule is a weighted average of all resonance contributors. The result is that the electrons are distributed over the entire molecule, and not isolated, as suggested by individual resonance structures. For the Resonance in Motion examples below you can think of is as if the strucures were changing infinitely fast, so that electrons and charges were everywhere at once.


See the resonance in motion

How does one show the interconversion between resonance structures?

The arrows in the figure tell the story. Each double headed arrow represents the movement of two electrons. To convert between structure 1 and structure 2, one of the lone pairs of electrons on the blue oxygen moves to form a new double bond. It would not be possible to stop at this point because it would result in an invalid Lewis dot structure, nitrogen would have 10 electrons around it. To maintain a valid Lewis dot structure one pair of electrons from the double bond between nitrogen and the red oxygen moves to become a lone pair on the red oxygen. The same process can be repeated to get from structure 2 to structure 3. Except in this case the electrons start moving from from the green oxygen and end at the blue oxygen.

Problems

1. Draw the arrows necessary to go from structure 3 to structure 1.
2. Draw the arrows necessary to go from structure 2 back to structure 1.
3. Instead of starting to move electrons from the blue oxygen in structure 1, could you start drawing new resonance structures by moving a lone pair on the green oxygen first? The red oxygen?