Resonance is a phenomenon in which a vibrating system or external force drives another system to vibrate with greater amplitude at a specific preferential frequency. Frequencies at which the response amplitude is a relative maximum are known as the system's resonant frequencies or resonance frequencies. At resonant frequencies, small periodic driving forces have the ability to produce large amplitude oscillations, due to the storage of vibrational.
Resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure. A molecule or ion with such delocalized electrons is represented by several contributing structures (also called resonance structures or canonical structures).
The concept first appeared in 1899 in Johannes Thiele's "Partial Valence Hypothesis" to explain the unusual stability of benzene which would not be expected from August Kekulé's structure proposed in 1865 with alternating single and double bonds. Benzene undergoes substitution reactions, rather than addition reactions as typical for alkenes. He proposed that the carbon-carbon bond in benzene is intermediate of a single and double bond.
General characteristics of resonance
Molecules and ions with resonance (also called mesomerism) have the following basic characteristics:
Contributing structures of the carbonate ion
- They can be represented by several correct Lewis formulas, called "contributing structures", "resonance structures" or "canonical forms". The real structure is an intermediate of these structures represented by a resonance hybrid.
- The contributing structures are not isomers. They differ only in the position of electrons, not in the position of nuclei.
- Each Lewis formula must have the same number of valence electrons (and thus the same total charge), and the same number of unpaired electrons, if any.
- Bonds that have different bond orders in different contributing structures do not have typical bond lengths. Measurements reveal intermediate bond lengths.
- The real structure has a lower total potential energy than each of the contributing structures would have. This means that it is more stable than each separate contributing structure would be.
(Source: Wikipedia  )