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Resonance Structures: How to get them !

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 p bonds or lone pairs (that are in p orbitals)
  • The overall charge of the system must remain the same.
Ranking resonance structures (for 1st row atoms, in order of importance):
  • Octet rule should be satisfied.
  • Charge separation requires energy (think electrostatics)
  • Charge separation against that predicted by electronegativity.
Here is an example. Note how the possible resonance structures are derived from the starting point by pushing curly arrows.

Resonance of Propanone


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

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