Resonance

1.  Resonance energy

             Definition          

  The difference in potential energy between the actual species and the (computed) energy of the contributing structure with the lowest potential energy is called the resonance energy or delocalization energy.

Explanation

 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(
The Lewis structure of a compound can be generated by trial and error. We start by writing symbols that contain the correct number of valence electrons for the atoms in the molecule. We then combine electrons to form covalent bonds until we come up with a Lewis structure in which all of the elements (with the exception of the hydrogen atoms) have an octet of valence electrons.
Example: Let's apply the trial and error approach to generating the Lewis structure of carbon dioxide, CO₂. We start by determining the number of valence electrons on each atom from the electron configurations of the elements. Carbon has four valence electrons, and oxygen has six).

C: [He] 2s² 2p²

O: [He] 2s² 2p⁴⁾

A molecule or ion with such delocalized electrons is represented by several contributing structure also variously known as resonance structures , canonical structures,(When more than one Lewis structure can be drawn, the molecule or ion is said to have resonance. The individual Lewis structures are termed contributing resonance structures. Resonance is a common feature of many molecules and ions of interest in organic chemistry).

, which collectively constitute a resonance hybrid.