Resonance: When One Lewis Structure Isn't Enough

Sometimes, a single Lewis structure isn't enough to accurately represent the bonding and electron distribution in a molecule. This limitation leads us to the fundamental concept of resonance, a cornerstone of understanding molecular stability and reactivity in organic chemistry.

Consider benzene (C6H6):

Which of these two structures of benzene are correct?

 The two Kekulé resonance structures for benzene.

You will learn in these sections that neither of these individual structures perfectly describes benzene. Instead, both structures are equally valid, and the true electronic structure of benzene is actually an average or hybrid of these two forms.

What is Resonance?

Resonance occurs when a molecule or polyatomic ion has two or more possible valid Lewis structures that differ only in the placement of electrons (not atoms). These individual Lewis structures are called resonance structures (or resonance contributors). It is crucial to understand that resonance structures are hypothetical; they do not exist independently, but rather collectively describe the actual electronic structure of the molecule.

The true structure is a composite, or resonance hybrid, of all contributing resonance structures. This concept is vital for describing delocalized electrons – electrons that are not confined to a single bond or atom, but are spread out over several atoms. A single Lewis formula, which assigns an integer number of covalent bonds, cannot adequately express this delocalization.