Nitrate ion, NO3−, is a polyatomic ion that can be represented by multiple Lewis resonance structures. Each canonical form places a double bond between the nitrogen atom and one oxygen atom, while the other two N–O linkages are single bonds. All oxygen atoms possess sufficient lone-pair electrons to complete an octet, and the overall ion carries a –1 charge.
(a) Define bond order and describe how bond order is determined for a bond in a species that exhibits resonance.
(b) Calculate the average (effective) bond order for an N–O bond in NO3−.
(c) Using your answer to part (b), explain why all three N–O bonds in nitrate are experimentally observed to have the same bond length even though each resonance structure contains one N=O double bond and two N–O single bonds.
(d) Consider the series of species NO (nitric oxide), NO2−, and NO3−. Rank the N–O bond lengths from shortest to longest and justify your ranking in terms of the bond orders in the three species.