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Resonance

In certain cases, molecules can be represent by more than one reasonable Lewis structure that differ only in the location of π electrons.
Electrons in σ bonds have a fixed location and so they are said to be localised.
In contrast,  π electrons that can be drawn in different locations are said to be delocalised.
Collectively these Lewis diagrams are then known as resonance structures or resonance contributors or resonance canonicals.
The "real" structure has characteristics of each of the contributors, and is often represented as the resonance hybrid (think of a hybrid breed which is a mixed breed).  In a way, the resonance hybrid is a mixture of the contributors.

(note that a resonance hybrid cannot normally be written as an individual Lewis diagram !).

You should be able to draw all reasonable resonance structures for a given organic molecule. Examples.

The best way to "derive" resonance structures is by learning to "push" curly arrows and starting from a reasonable Lewis structure.

Rules to remember for recognising resonance structures:
  • Atoms never move.
  • You can only move electrons in π bonds or lone pairs (that are in p orbitals)
  • The overall charge of the system must remain the same.
  • The bonding framework of a molecule must remain intact.
 
Ranking resonance structures (for 1st row atoms, in order of importance):
  • Octet rule should be satisfied.
  • Charge separation requires energy (think electrostatics)
  • Charge separation that is against that predicted by electronegativity.

Here is an organic example. Note how the possible resonance structures are derived from the starting point by pushing curly arrows.

Resonance of Propanone
Now let's look at an inorganic example.  Again the possible resonance structures are derived from the starting point by pushing curly arrows.

Now would be a good time to check out some questions


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Dr. Ian Hunt, Department of Chemistry, University of Calgary