How to count formal charges in \[NO_2^ - \]?

Hint: In order to count the formal charge of a given compound, i.e., \[NO_2^ - \], we must have an idea about the number of the valence electron present in each element present in the compound. \[NO_2^ - \] has two chemical elements present in it, one is Nitrogen and the other is Oxygen.

Complete answer:
Let us first understand what a formal charge is. Formal charge is said to be a charge which is assigned to an atom that is present in a molecule, assuming that the electrons which are involved in all chemical bonding are shared equally between the atoms. The structure will be chosen in such a way that the formal charge present on each atom will be close to zero.
We can determine the formal charge of an atom present in a molecule by the following equation:
\[FC = V - N - \frac{B}{2}\]; Where V is the valence electron present in the neutral atom in a ground state.
N is the number of the non-bonding valence electron which is present in the atom of the molecule.
B is the number of the electrons that is shared between two atoms.
The formal charge in the Nitrogen atom is
\[FC = 5 - 2 - \frac{6}{2} = 5 - 2 - 3 = 0\].
The formal charge present in the double bonded oxygen atom is
\[FC = 6 - 4 - \frac{4}{2} = 6 - 4 - 2 = 0\].
The formal charge present in the single bonded oxygen atom is
\[FC = 6 - 6 - \frac{2}{2} = 6 - 6 - 1 = - 1\].

Note: We have to remember that in organic chemistry, formal charge is a very essential feature for writing a structure. The structure which will omit the formal charge is said to be incorrect or incomplete. But in inorganic chemistry this convention is not followed. We will draw the formal charge near to the atom which will be bearing that charge.