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Chemistry · Glossary

What is Resonance structures, resonance hybrid?

Also known as: resonance structure · resonance hybrid

Definition 3.10 University Chemistry — Year 1 · Chapter 3 — Lewis Structures, Resonance and VSEPR

When the electrons of a molecule can be placed in several Lewis structures that differ only by the position of π\pi bonds and lone pairs — the nuclei staying in place — each is a resonance structure (or mesomeric form), linked to the others by a double-headed arrow ↔\leftrightarrow. The real molecule is none of them: it is the resonance hybrid, a single structure whose electron distribution is a weighted average of theirs.

Resonance in ozone (one lone pair of the right-hand oxygen becomes a π bond while the π bond on the left becomes a lone pair) and in benzene, and one of the three equivalent structures of the nitrate ion, whose charge is shared by the three oxygens.
Resonance in ozone (one lone pair of the right-hand oxygen becomes a π\pi bond while the π\pi bond on the left becomes a lone pair) and in benzene, and one of the three equivalent structures of the nitrate ion, whose charge is shared by the three oxygens.

Examples

Example 3.13 (Ozone, nitrate, benzene)

Ozone has two equivalent structures: each O−O\ce{O-O} bond is double in one and single in the other, so both have a bond order 1.51.5 and the same length, 127.8 pm127.8\,\mathrm{pm}, between O=O\ce{O=O} and O−O\ce{O-O}. The nitrate ion NOX3X−\ce{NO3-} has three equivalent structures; each N−O\ce{N-O} bond has order 4/34/3. Benzene, CX6HX6\ce{C6H6}, has two Kekulé structures; its six C−C\ce{C-C} bonds are equal, 139.7 pm139.7\,\mathrm{pm}, between the single bond of ethane (153.6 pm153.6\,\mathrm{pm}) and the double bond of ethene (133.9 pm133.9\,\mathrm{pm}).

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