the N-O bond in NO2 - is shorter than the N-O bond NO3 -. State the reason.
in NO2- the lone pair is delocalised between the 2 O groups also. hence bond order equal to 1 + 1/2 = 3/2
in NO3- lone pair shared between three O atoms hence bond order od NO bond is 1 + 1/3 = 4/3
greater the bond order shorter the bond. hence bond length of NO2- less than that of NO3-
if you look at the structure of NO3- you'll notice that N makes 1 double bond and 2 single bonds. Thus because of hybridization, you can say each oxygen atom has 1/3 of a double bond on average.
Now look at NO2- (O=N-O-). There is one double bond and one single bond. Since each oxygen atom has about 1/2 of a double bond, the average bond length will be shorter than those in NO3-.