When two or more valence-bond structures are possible, differing only in the placement of electrons, the molecule will usually show characteristics of both structures. The different structures are called resonance structures or resonance forms because they are not different compounds, just different ways of drawing the same compound. The actual molecule is said to be a resonance hybrid of its resonance forms.
as shown in the above structure
Spreading the positive charge over two atoms makes the ion more stable than it would be if the entire charge were localized only on one . We call this a resonance-stabilized ion. Resonance is most important when it allows a charge to be de localized over two or more atoms and increase its stability.
The more stable resonating structure form the major contributor, and the less stable form is the minor contributor. The structure of the actual compound resembles the major contributor more than it does the minor contributor.
- All the resonance structures must be valid Lewis structures for the compound.
- Only the placement of the electrons may be shifted from one structure to another. (Electrons in double bonds and lone pairs are the ones that are most commonly shifted.) Nuclei cannot be moved, and the bond angles must remain the same.
- The number of unpaired electrons (if any) must remain the same. Most stable compounds have no unpaired electrons, and all the electrons must remain paired in all the resonance structures.
- The major resonance contributor is the one with the lowest energy. Good contributors generally have all octets satisfied, as many bonds as possible, and as little charge separation as possible. Negative charges are more stable on more electronegative atoms, such as 0, N, and S.
- Resonance stabilization is most important when it serves to delocalize a charge over two or more atoms.