The Lewis model of mesomerism

Principle

Simple covalent bonds are longer than double covalent bonds (that means that the distance between the nuclei is longer). Nevertheless sulfur dioxide has astonishingly the same length of its two bonds! A clever trick permits to explain this fact by the Lewis model : $SO_2$ has the same length of its two bonds because the real molecule is somewhat an intermediary ("resonance hybrid") between two "resonance forms" ( canonical structures ) represented by the following Lewis structures: In fact there are four electrons ( red in the following resonance form) which are symetrically distributed between the atoms of the molecule: If we try to "keep" the Lewis model (2 electrons = one bond!) we must represent the real molecule by two Lewis structures (which don't exist indeed!) so that the real molecule shows to be an intermediary between these two structures: Beware however, the arrow of mesomerism is not an equilibrium arrow! A mesomerical molecule is like a rhinoceros: it exists indeed. A child who has never seen a rhinoceros can imagine how it is, if you explain that it is something between a dragon and an unicorn (which do not exist, but are well-known to this child !)

Occurrence

We limit ourselves to two cases where mesomerism is possible: A second canonical structure can be found if the lone electron pair is placed on the single bond. In this case, the atom B makes one bond too many and we have to place its "bonding electron pair" on atom C. Another canonical structure may be found placing a bonding pair (for instance the pair between A and B) on the single bond . But now atom C has one bond too many and the other bonding pair must be placed on atom D.

Examples

The nitrite ion: The buta-1,3-diene: or: This molecule has therefore 3 canonical structures.

Probability et stability

The canonical structures contribute with different probabilities to the real molecular structure. ( A rhinoceros is more like a dragon than like an unicorn ! )