Resonance structures show that chemical species have equal bond lengths and bond angles when in resonance. The bond lengths of the N - O bond in HNO2 is 126pm.
Linus Pauling introduced the idea of resonance to explain the nature of bonding in compounds and ions where a single Lewis structure can not satisfactorily account for the observed properties of the chemical species.
If the two structures are resonance structures, they will have equal N - O bond lengths and bond angles. The bond length will be intermediate between that of a pure N - O single and N - O double bond. The actual N- O bond lengths in HNO2 are obtained as; 136 pm - 122 pm/2 = 126 pm.
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