Azide ion $\left({\mathrm{N}}_3^{-}\right)$    


Bond order: $\left(\mathrm{I}\right)<2$ 

Bond Order: $\left(\mathrm{II}\right)>2$

Due to the delocalisation in $\mathrm{I}$ double character increase in single bond and $\mathrm{II}$ triple bond character decrease in triple bond (double bond character comes).