Resonance Structures

Sometimes more than one satisfactory Lewis structure can be written and there is no reason to select one over another. In such cases a single structural formula is inadequate for a correct representation, and we say that the true structure is a resonance hybrid of the several structures. Common examples of species requiring resonance structures are ozone, O₃, carbonate ion CO₃^-2, and benzene, C₆H₆. These structures are shown in Figure 1.1. The number of electron pairs shared by two atoms is sometimes called the bond order (BO). Thus a single bond has BO = 1, a double bond has BO = 2. In O₃ and C6H6, the apparent O-O and C-C bond orders are 1.5; in CO₃^-2 the apparent C-O bond order is 1.33; these numbers are averages over the various resonance structures.

Figure 1.1. Resonance structures.