The concept of resonance was introduced to explain the properties os such molecules which cannot be represented by single electronic ( Lewis ) structure. In such a case it has been found that the molecule can be represented by two or more electronic structures each of which can explain most but not all the observed properties of the molecule. The real structure of the molecule lies somewhere between all these electronic structures, which however, cannot be represented on paper.

The structure of nitroalkanes , according to valence bond therory, may be written as

R-N(=O)--->O  or R-N+(=O)---O-

(classical)            (polar)

This polar structure is supported by their large dipole moments. Thus nitro alkanes may be represented as a resonance hybrid of the following two cannonical structures

R-N+(=O)---O-      <----->    R-N+( O-)=O (the singly bonded negative oxygen carries three pairs of electrons while the double bonded oxygen has two pairs of electrons).

Fron the M.O. point of view, the nitro-group is conjugated, and delocalisation of bonds increases its stability ( two oxygen atoms are equivalent as in carboxylate ion)

-1/2 O---N+---O-1/2 ( each oxygen sharing equal negative charge with nitrogen positively charged)