The nitrate ion (NO₃^–) consists of a nitrogen (N) atom with five valence electrons and three oxygen (O) atoms, each having six valence electrons. Nitrogen shares a double covalent bond with one oxygen atom (O=N⁺) and two single covalent bonds (O^–-N⁺) with the remaining two oxygen atoms. Its molecular structure is shown below.

To find the formal charge of NO₃^–, let us calculate the formal charges on the atoms.

Nitrogen

V = 5, N = 0, B = 8

Therefore,

q_f = 5 – 0 – 8/2 = 1

The formal charge on N in NO₃^– is 1.

Oxygen 1 and Oxygen 2

V = 6, N = 6, B = 2

Therefore,

q_f = 6 – 6 – 2/2 = -1

The formal charge on two of the oxygen atoms in NO₃^– is -1.

Oxygen 3

V = 6, N = 4, B = 4

q_f = 6 – 4 – 4/2 = 0

The net formal charge is: -1 + (2 x -1) + 0 = -1

The formal charge of NO₃^– is -1.