A reasonable Lewis structure is H-O-stackrel(+)N(=O)O^-H−O−+N(=O)O−. One of the oxygen atoms has a formal negative charge, and the nitrogen atom is quaternized and bears a formal positive charge. Of course, the molecule is neutral, and the Lewis structure reflects this.
Note that when nitric acid dissociates:
HNO_3(aq) + H_2O(l) rarr H_3O^+ + NO_3^-HNO3(aq)+H2O(l)→H3O++NO−3
There are 3 formal charges on the nitrate ion: ""^(-)O-stackrel(+)N(=O)-O^-−O−+N(=O)−O−. The overall charge on the ion is still -1−1, but there is again formal charge separation in our representation of the anion.
