From a few experiments, it was observed that the bond parameters of some molecules were not the same as what was calculated on the basis of different bond theories. To explain these differences, the theory of resonance came into the picture which suggested that whenever a single Lewis structure is insufficient to describe a molecule correctly, then multiple Lewis structures can be superimposed over each other to describe the molecule leading to hybrid structures with similar energy, position of nuclei, bonding and non-bonding pairs of electrons.
Definition of the resonance structure
Resonance structures are the multiple Lewis structures of similar energy, the position of nuclei, bonding and the non-bonding pair of electrons that can accurately describe a molecule.
They are taken as canonical structures of the hybrid molecules formed by the superimposition of multiple Lewis structures. The hybrid molecule alone can accurately describe the molecule.
An indicative example – Resonance structure of SO3
The above figure shows a different canonical structure of SO3. They all are similar in energy, the position of nuclei, bonding and non-bonding pairs.
The three O-S bond are same in bond length which was not equivalent to the length of a double bond between O and S. So it requires resonance structures to describe it correctly.
Resonance in Chemistry
- Every structure is associated with a certain quantity of energy, which determines the stability of a molecule or ion. A resonance hybrid has a structure that is intermediate between the contributing structures. The total quantity of potential energy, however, is lower than the intermediate and the molecule is said to be a hybrid molecule.
- Resonance averages the bond characteristics as a whole.
- The canonical forms don’t exist in reality. They are discussed to make the study of molecules easy. The superimposition of canonical forms leads to the formation of hybrid molecules which accurately describes the molecule.
- The concept of resonating structures or canonical structures came into being so as to account for the different bond parameters found in the molecule than suggested by their Lewis structures.
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