When chemists can draw multiple valid Lewis dot structures for a single molecule, a special situation arises. The actual physical structure is a weighted average of all these theoretical contributing structures. Scientists refer to this true blended form as a resonance hybrid.
Resonance significantly stabilizes a molecule by spreading its available electron density over a larger spatial area. This electron spreading action directly reduces the overall potential energy of the entire chemical system. Chemists refer to this specific increase in molecular stability as the resonance energy.
A major misconception among chemistry students is that the molecule rapidly flips back and forth between different structures. In reality, the individual drawn resonance structures do not physically exist on their own at any time. The molecule exists constantly and permanently as a single, completely stable hybrid structure.
For instance, the six carbon-carbon bonds in a benzene ring are all chemically identical. They are intermediate in length and strength between a standard single bond and a standard double bond. Resonance helps chemists explain physical properties that simple dot structures simply fail to predict.