# Which has the greater N–O bond length, NO₂⁻ or NO₃⁻?

Which has the greater N–O bond length, $$\ce{NO_2^{-}}$$ or $$\ce{NO^_3^{-}}$$?

I don't understand why the N-O bonds should be of different lengths in these two molecules (don't the same bonds have the same length)? I didn't quite know how to determine this and so I've tried googling the answer, but none of the answers already provided helped clear things up for me... Why does $$\ce{NO_3^{-}}$$ have a greater N-O bond length?

There are two ways to think of it. First: draw out the (major) contributing Lewis structures for both ions. Each iin has one pi bond, but it's shared between two linkages in $$\ce{NO2^-}$$ ion versus three linkages in the $$\ce{NO3^-}$$ ion. So the $$\ce{NO3^-}$$ ion has less pi bonding in each linkage making those bonds weaker overall.
Second: In all of those Lewis structuresthere are nonbinding pairs of electrons on the oxygen atoms, plus a pair on the nitrogen in $$\ce{NO2^-}$$. There are more such pairs in $$\ce{NO3^-}$$ than in $$\ce{NO2^-}$$. These pairs tend to repel each other and the bonding pairs, and $$\ce{NO3^-}$$ requires more room, thus longer bonds, to accommodate the greater number of closely packed nonbonding electron pairs.