Chemical Bonding: Valence Electrons

Lewis structures have a major flaw. For many structures (O3, NO3-, etc) drawing a single structure won't show the actual distribution of electrons between atoms. For these structures experimental data tells us a single Lewis structure isn't an accurate picture of the molecule as found in the real world.

To get around this we draw resonance structures. For example, for O3 we draw:

Ozone Resonance Structure

The actual structure is an average of the two structures. Experimental data tells us that the bonds between the Oxygen atom are actually one and a half (1 1/2) and not double bonds.

What you need to know:

  • Resonance structures are necessary to show how electrons are distributed in chemical bonds in a molecule.
  • Remember, the molecule isn’t flipping back and forth between structures! It's an average of the resonance structures.
  • The <---> symbol drawn between resonance structures does not mean equilibrium or any sort of change. It only shows that there is more than one way to draw the structure. It's not a very good choice of symbols, really.