resonance
Resonance is the use of two or more Lewis structures to represent the covalent bonding in a molecule. One of the valid structures is referred to as a resonance structure. It is now understood that the true structure of a molecule which displays resonance is that of an average or a hybrid of all the resonance structures. In the case of the O 3 molecule, each of the covalent bonds between O atoms is best thought of as being one and a half bonds, as opposed to either a pure single bond or a pure double bond. This half-bond can be shown as a dotted line in both the Lewis structure and the molecular model see Figure below .