Resonance structures

Resonance theory applies whenever a molecule's Lewis structure contains a CONJUGATED system of pi bonds -- that is, two or more multiple bonds alternating with single bonds along a chain or ring -- or, more generally, wherever a p-orbital-bearing atom sits directly attached to a multiple bond. The theory rests on five connected ideas. First, the pi electrons of such a conjugated system are NOT confined ('localised') to any one specific pi bond; they are, in reality, spread out (delocalised) across the whole conjugated system. Second, this delocalisation is represented on paper by drawing two or more separate Lewis structures for the SAME single compound, each one showing a different possible arrangement of where the pi electrons could be (generated from one another using curved arrows to track the electron movement), and then joining these Lewis structures together with a double-headed arrow; these individual Lewis structures are called resonance structures (also 'contributing structures' or 'canonical structures') of the one real species -- benzene's two resonance structures, which differ only in WHICH set of three alternating ring bonds is drawn as double versus single, are the chapter's worked example. Third, across every resonance structure of a given species, the POSITIONS of the carbon atoms themselves never change at all -- only the positions of the pi ELECTRONS differ from one resonance structure to the next (so, in benzene's two resonance structures, the bond between C1 and C2 is drawn as a single bond in one structure and as a double bond in the other, purely because the pi electrons have shifted, while the carbon atoms themselves have not moved). Fourth, and most importantly, NO single resonance structure is itself a real, physically-existing molecule, and no single one, by itself, can explain every observed property of the real compound; the real, actual molecule is instead the RESONANCE HYBRID of every resonance structure that can validly be drawn for it, carrying some genuine character of each and every one of them at once -- benzene's real structure, for example, is often drawn as a plain hexagon with a dotted circle inscribed inside it, precisely because every one of its six C-C bonds genuinely has BOTH partial single-bond character AND partial double-bond character simultaneously, which is exactly why the real ring turns out to be a perfectly regular hexagon rather than having three long bonds and three short ones. Fifth, while each individual resonance structure does have its own calculable HYPOTHETICAL energy (worked out from standard bond-energy values), the real molecule's actual, measured energy is always LOWER than the calculated energy of any single one of its resonance structures taken alone -- meaning the resonance hybrid is always more stable than any single contributor to it, and the exact difference between the real molecule's actual energy and the LOWEST calculated energy among its resonance structures is called the resonance stabilisation energy (or simply resonance energy); resonance, in other words, ALWAYS acts to stabilise the real molecule relative to any single contributing structure. Two firm rules govern when a resonance structure may even be validly drawn at all: every atom that is part of the conjugated pi system must lie in the exact same plane as one another, and every resonance structure drawn for one given species must have the exact same total number of unpaired electrons as every other. When comparing two or more candidate resonance structures to judge which is more important (contributes more to the real hybrid), the MORE stable, more important structure is consistently the one that has (a) a greater number of ordinary covalent bonds, (b) a greater number of atoms with a fully complete octet (or, for hydrogen, a complete duplet), ( …