Resonance Effect

For certain molecules like carbonate ion (CO₃^2-), one single Lewis structure would not be enough to explain all of the properties. In that case, the molecule is said to have more than one structure.

Each of those structures can explain some of the properties but not all of the properties. The actual structure of the molecule is a hybrid of all the possible structures (canonical forms). This phenomenon is called resonance in organic chemistry. If resonance occurs, each bond would be both a single bond and a double bond at the same time, i.e. the bond order would lie between one and two.

Resonating structures should fulfil the following criteria:

1. All atoms should have the same positions in all the structures.
2. There should have the same number of paired and unpaired electrons.
3. The structures should have almost the same energies.

Note: Canonical forms do not have any existence in reality.

Resonance Energy

The energy difference between the most stable canonical form and the resonance hybrid is known as Resonance Energy. The more the resonance energy, the more the stability.

Rules for finding out the most stable canonical form:

• The canonical form with no charges is the most stable
• The canonical form with more number of covalent bonds is more stable
• The canonical form where unlike charges are in close proximity are more stable
• If they are to be charged, the negative charge should be on an electronegative atom. Then, this canonical form is said to be more stable.