Resonance and the Resonance Effect

For some molecules and ions, no single Lewis (electron-dot) structure can correctly represent the real distribution of electrons and bond lengths observed experimentally. When this happens, chemists draw two or more valid alternative structures -- called canonical forms or resonance structures -- that differ from one another only in how the electrons (never the atomic nuclei) are arranged, and describe the real molecule as a single resonance hybrid: a weighted blend of all the contributing structures, which is always more stable (lower in energy) than any one contributing structure would be on its own. This extra stability is called resonance or delocalisation stabilisation.

A few rules govern valid resonance structures: only electrons may move between structures, never atoms; every contributing structure must have the same total number of electrons and the same number of paired/unpaired electrons; and structures that are more stable in their own right -- more covalent bonds, less charge separation, and any negative charge sitting on the more electronegative atom -- contribute more heavily to the real hybrid than less stable ones. A classic example is the acetate ion, $\text{CH}_3\text{COO}^-$: it can be drawn with the double bond and the formal negative charge on either of its two oxygen atoms, giving two energetically identical contributing structures. Because both contribute equally, the real ion is a hybrid in which both carbon-oxygen bonds are exactly equivalent, each of intermediate length between a typical C-O single bond and a C=O double bond, and the full negative charge is shared equally between the two oxygen atoms rather than sitting fixed on one. …