Resonance

NOTE ON SOURCE NUMBERING: the source PDF heads this section '5.8', immediately after the section renumbered 5.8 above (see that section's note) — kept here as its own distinct section, renumbered 5.9, so the two different topics printed back-to-back under an apparently duplicated '5.8' label are not merged into one. Many polyatomic molecules and ions can be drawn as more than one genuinely valid Lewis structure — differing only in WHERE the bonding/lone-pair electrons sit, never in which atoms are actually bonded to which — and no single one of these individual structures alone correctly explains all of the real species' observed properties (for instance, its actual, single, intermediate bond length). $\text{CO}_3^{2-}$ is one such case: it can be drawn with the C=O double bond on any one of its three oxygens, giving three individually valid but non-identical Lewis structures. The real structure is understood as a BLEND of all such contributing structures at once, called the resonance hybrid; the different individual contributing structures are called canonical forms; and the whole phenomenon — a single real species being better represented by a blend of multiple electron-arrangement structures than by any one of them alone — is called resonance. Two summarising points follow. First, the resonance hybrid's actual energy is LOWER than the energy of any single one of its contributing canonical forms, so resonance genuinely STABILISES the polyatomic species it applies to (this is one of resonance's most important chemical consequences). Second, the resonance hybrid's overall bonding character — its real, single bond lengths and charge distribution — is the AVERAGE of all its contributing canonical forms taken together, not a simple 'switching' between them. Resonance energy is defined as the difference in energy between this (lower-energy, more stable) resonance hybrid and the single most stable individual canonical structure among its contributors; oz …