The Lewis structure for the nitrate ion has a nitrogen atom in the center, with three oxygen atoms surrounding the nitrogen. There is one double nitrogen-oxygen bond and two single nitrogen-oxygen bonds. The nitrogen atom has no lone pairs of electrons.
The double-bonded oxygen atom has two lone pairs of electrons filling its octet. The remaining single-bonded oxygen atoms have three lone pairs of electrons surrounding them. The nitrate ion has three resonant structures. In each resonant structure, the nitrogen creates a double-bond with one of the three surrounding oxygen atoms. The oxygen atom that the nitrogen double-bonds with is different in each resonant structure.