Rules For Writing Resonating Structures

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 Rules for Writing Resonating Structures

(i)    Each structure should be such as not to alter the position of atoms, i.e., the nuclei of the atoms in each structure must remain in fixed position. Only the arrangement of electrons may change. The bond angles and bond lengths should be closer to the real structure. Only then the contribution of that structure to the hybrid will be significant. For example, the Dewar structure of benzene are less important than the Kekule structures because in Dewar statutes there I one very long bond across the ring.

(ii)    Each structure must have the same number of unpaired electrons. We should not draw structures in a manner that one structure may not contain even a single unpaired electron and another contains two unpaired electrons. For example.

(iii)    The contributing structures should all have similar energies. This rule may be illustrated by taking the example of benzene. The Dewar structures have higher energy an are, therefore, less important than the more stable Kekule forms in the combination that represents the true structure of benzene.

(iv)    Those structures in which negative charge resides on the most electronegative atom and positive charge on the most electropositive atom will be closest to the true structure.
       for example , we may write the following resonating structures of HCI.

(v)    The contributing structures should be so written that unlike charges reside on neighboring atoms. This rule can be illustrated by writing various resonance structures for HN3 . The last structure is quite insignificant because of the+ ve charge on adjacent N atoms and double-ve charge on the third nitrogen atom.

(vi)    The greater the number of covalent bonds, greater is the contribution of that resonating structure. For example the double bonded structures of BF3 are important because of the formation of an additional covalent bond.

For example, in carbon dioxide molecule the structure (as given below) has very little importance because of its very high energy resulting form the loss of π-bonding stabilizations.

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