Resonance Structures - April 8

Resonance is the phenomenon that occurs when two or more equally valid Lewis structure diagrams can be written for one molecule.

Ex: Ozone (O₃) This structure suggests that ozone consists of one coordinate covalent bond and one double covalent bond. Double bonds are usually shorter than single covalent bonds. However, for ozone, experimental evidence shows that both bond lengths are equal.

Therefore, the actual bond is an average of the two structures.

A double-headed arrow is used to denote the resonance forms of a molecule.

Theory of Resonance

Early chemists believed that the molecules themselves resonated back and forth between the two structures. Today, it is believed that they are hybrids, that have characteristics of all resonance forms, yet remain distinct. It is also important to note that the more resonance forms that can be written for a molecule, the more stable it is.

Resonance only occurs when a double or triple bond can be moved to a different terminal atom without changing the overall bonding structure or shape.

Ex: Carbon trioxide (CO₃^2-)