Resonance

For some molecules and polyatomic ions, no single Lewis structure — however carefully drawn — can correctly represent the true distribution of electrons and bonds. In such cases, the actual species is described as a resonance hybrid: a weighted blend of two or more valid Lewis structures, called resonance structures or canonical forms, which differ from one another only in the placement of electrons (never in the positions of the atomic nuclei).

Two clear, commonly cited examples illustrate the idea.

The carbonate ion, $\text{CO}_3^{2-}$. As shown in the Lewis-structure section of this chapter, one valid structure has carbon doubly bonded to one particular oxygen and singly bonded to the other two. But nothing distinguishes any one of the three oxygens as special — three equally valid resonance structures exist, each with the double bond on a different oxygen. Experimentally, however, all three carbon–oxygen bonds in $\text{CO}_3^{2-}$ are found to have exactly the same length, roughly $128\ \text{pm}$ — a value intermediate between a typical $\text{C}-\text{O}$ single bond ($\approx 143\ \text{pm}$) and a typical $\text{C}=\text{O}$ double bond ($\approx 121\ \text{pm}$). The real carbonate ion is best described not as any one of the three individual structures, but as their resonance hybrid, in which the double-bond character is evenly delocalised across all three carbon–oxygen bonds at once.

Benzene, $\text{C}_6\text{H}_6$. The classic Kekulé structure alternates single and double bonds around the six-membered ring, and a second, equally valid Kekulé structure simply reverses which bonds are single and which are double. Neither structure alone is correct: benzene's actual carbon–carbon bond length, measured experimentally at $139\ \text{pm}$, is uniform all the way around the ring and lies between a pure $\text{C}-\text{C}$ single bond ($154\ \text{pm}$) and a pure $\text{C}=\text{C}$ double bond ($134\ \text{pm}$) — exactly the signature of a resonance hybrid, arising here from the delocalisation of the six pi electrons over the whole ring rather than their being confined to three fixed double bonds. …