“Resonance structures are a way of showing delocalized electrons within a molecule. These structures cannot be represented by a single Lewis structure, so multiple resonance Lewis structures must be drawn in order to capture the actual electron density within the molecule.

The existence of these structures is caused by electron delocalization, which results in the shifting of lone pairs and bonds between species. However, within each group of resonance structures, the primary resonance structure will have the lowest potential energy compared to other available resonance structures. The difference between the potential energy of the actual structure and that of the contributing structure with the lowest potential energy is called the resonance energy. Properties of atoms, such as formal charge, contribute to the overall resonance energy of a molecule.

In order to investigate resonance, a student labels one of the two oxygen atoms in acetic acid with O¹⁸ (noted with a star). The student adds a combination of a weak base and CH₃Cl to the labeled acetic acid, and proceeds to analyze the products using NMR and mass spectrometry.

Figure 1. Lewis Structure of ¹⁸O-labeled Acetic Acid.”
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