Use of Contributing Structures
In Lewis formulas, covalent bonds are represented in accordance with the valence bond theory. Each single bond is made by two valence electrons, localized between the two bonded atoms. Each double bond has two additional localized π electrons, while each triple bond has four additional π electrons (two pairs) between the bonded atoms.
In molecules or ions that have a combination of one or more single and multiple bonds, often the exact position of the respective bonds in the Lewis formula cannot be indicated. The π electrons appear to be delocalized and the multiple bonds could be in different positions. In those cases the molecule cannot be represented by one single Lewis formula. To solve this problem, in valence bond theory the concept of resonance is used, and the molecule is represented by several contributing structures, each showing a possible distribution of single and multiple bonds. The molecular orbital theory already includes the concept of delocalized electrons and therefore has no need of the concept of resonance.
None of the contributing structures is considered to represent the actual structure, since bonds that have a different bond order in different contributing structures do not have, if measured, a bond length that is typical for a normal single or multiple bond. Moreover, the overall energy of the actual structure is lowered with the resonance energy.
Read more about this topic: Resonance (chemistry)
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