Question

How many types of $ N - O $ bond lengths are present in:
$A)HN{O_3} \\
  B)N{O_3}^ - $

Answer 1

Hint :we use the concept of resonance to explain the type of bonding present in $ HN{O_3} $ and $ N{O_3}^ - $ . In $ HN{O_3} $ the canonical forms show double-bond character in two bonds, causing them to be shorter than typical $ N - O $ bonds, that’s why the third $ N - O $ bond is elongated.

Complete Step By Step Answer:
The lewis structure of nitric acid has a resonating structure. It has a $ \pi $ bond, covalent bond and a dative bond between $ N $ and $ O $ but no ionic bond is present.
The structure clearly shows that it has four single bonds and one $ \pi $ bond. Two of the $ N - O $ bonds are equivalent and relatively short, and the third $ N - O $ bond is elongated because the $ O $ atom is also attached to a proton.

 $ B) $ The nitrate ion, according to its Lewis diagram, has two types of nitrogen-oxygen bonds, one double bond and two single bonds, suggesting that one nitrogen-oxygen bond in the nitrate ion is shorter and stronger than each of the other two.
It consists of one central nitrogen atom surrounded by three identically bonded oxygen atoms in a trigonal planar arrangement. There are four $ \sigma $ and one $ \pi $ bond between $ N $ and $ O $ , so, it seems there are two covalent bonds and one double covalent bond.

Note :
Remember the nitrate ion carries a formal charge of $ - 1 $ . This charge results from a combination formal charge in which each of the three oxygens carries a $ - \dfrac{2}{3} $ charge, whereas the nitrogen carries a $ + 1 $ charge, all these adding up to formal charge of the polyatomic nitrate ion.