“Suppose that we ask: is it necessary that a molecule such as CO have a definite valence-bond structure? The answer, which is part of the new idea, is no; instead the CO molecule may have (and does have) a structure which is neither C=O or C≡O, but is somewhere between them, or which rather has some aspects of both. It is customary now to speak of the molecules as resonating between these two structures.”
-Linus Pauling, 1936.
As with the structure of carbon monoxide, the principle of resonance also explains what was once a chemical enigma – the true molecular structure of benzene.
Benzene, a double-bond conjugated six member hydrocarbon ring, can be represented by two structures that are equivalent in energy.
A simple model representing oscillation between the two primary structures is, however, insufficient as it does not explain one of the principle chemical properties of the molecule – its inability to saturate.
The application of the theory of resonance permitted the determination of a more accurate model. In the resonance model, the molecular configuration of benzene is such that all possible structures (including ones not shown above) contribute to the true structure – a combination of all structures at once, with each carbon-carbon bond energetically equivalent. As Pauling wrote in his 1946 Encyclopedia Britannica entry on resonance
It is sometimes found…that a choice cannot be made between two or more structures which are about equally stable, and of which no one accounts in a completely satisfactory way for the properties of the substance. The concept of quantum-mechanical resonance has provided the solution to this problem: namely, the actual normal state of such a molecule does not correspond to any one of the alternative reasonable structures, but rather to a combination of them, their individual contributions being determined by their nature and stability….Just as it is customary to speak of the electrons in an atom in its normal state as moving around the nucleus in roughly the way described by the old quantum theory…so is it customary, and for some purposes useful, to speak of the resonance of a molecule in its normal state between two or more structures.
Based on this theory, the benzene molecule is now often depicted as a hexagon with a circle in the middle representing the true resonating nature of the molecule.
This structure has since been verified by multiple experimental techniques such as electron diffraction, x-ray diffraction, and molecular spectroscopy.
Having recognized resonance as an important missing link in understanding molecular bond structure, Pauling applied his theory to a large collection of empirical results. (For one, he identified electronic resonance as the principle that allows for the formation of four equivalent bonds to be formed by carbon.) His analysis consistently explained gaps in the classical models of bond theory and aligned with the quantitative data available. Pauling’s theory of resonance has since contributed fundamentally to the scientific understanding of molecular shape and stability, and has permitted insight into the true nature of the chemical bond.