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When a single structure is unable to explain all the properties of a compound, a number of other structures are suggested to that substance under certain roles. These various structures have no real existence and are called contributing, canonical or resonating structures". The structure, which is more stable than any one of the resonating structures and explains all the properties of the substance, is called Resonance Hybrid. This structure cannot be drawn on the paper.
As a result of resonance, the molecule acquires energy, which is lower than any of the contributing structures. As a result, the substance acquires minimum energy, and maximum stability. It may be noted that:
"Resonance gives stability and extra bond energy to the molecule of the substance. This extra bond energy is called Resonance Energy". It is also given by the following relation:
Resonance energy = Experimental bond Energy - Energy of the most stable canonical form Or (Resonance energy) = (Actual bond energy) - (Energy of pure covalent bond).

Conditions Necessary for Resonance
Following points must be kept in view to suggest the various canonical forms:
  1. The relative position of all the atoms in the canonical form must not change. For example, the two canonical forms X - Y - Z and X - Z - Y of a hypothetic molecule XYZ cannot be suggested. The arrangement of the electrons may change.
  2. The number of unpaired and paired electrons in the canonical forms must remain the same.
  3. The canonical structures should have approximately the same energy.
  4. The canonical structures should be so written that unlike charges they reside on neighbouring atoms. For example, in N2O, the structures, NN+ O+ cannot be canonical structures.
  5. In the canonical forms, the negative charge should reside on the most electronegative atom while positive charge should reside on the most electropositive atom. For example, in hydrofluoric acid, HF, the canonical form II  
    is more stable but real molecule is more like the structure I.
  6. Greater the number of covalent bonds, greater is the contributions of canonical forms. For example, in the following canonical forms of BF3, double bonded structures are more important.
Geometry of Covalent Molecules
The structures of CO32- ion

The single Lewis structure based on the presence of two single bonds and one double bond between carbon and oxygen atoms is inadequate to represent the molecule accurately as it represents unequal bond. According to experimental findings, all carbon to oxygen bonds in CO32- are equivalent. Therefore the carbonate ion is best described as a resonance hybrid of the canonical forms I, II and III as shown below:


The Structure of CO2 Molecule

The experimentally determined carbon to oxygen bond length in CO2 is 115 pm. The lengths of a normal carbon to oxygen double bond (C = 0) and carbon to oxygen triple bond (C 0) are not equal. Obviously, a single Lewis structure cannot depict this position. It becomes necessary to write more than one Lewis structure. The structure of CO2 is best described as a resonance hybrid of the canonical forms I, II and III.

The various structures of benzene are given below:


Possible Structures of Benzene
Structures I and II are due to Kekule. Structures III, IV and V are due to Dewar and structure VI is such that two carbon atoms have only seven electrons (i.e. one electron less than eight electrons required by octet rule). Structure VI is thus unimportant and hence has least contribution. Structures III to V are also less important. I and II are real resonating structures because these explain the equivalence of all C - C bonds. Thus each C - C bond length lies in between C - C and C = C bond.

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