In the perchlorate ion (ClO₄^–), chlorine (Cl) has a double covalent bond with three of the four oxygen (O) atoms and a single covalent bond with the fourth oxygen atom as shown below.

Let us calculate the formal charge of ClO₄^– by calculating the formal charges on Cl and O in the molecule and then adding them up.

Chlorine

V = 7, N = 0, B = 14

Therefore,

q_f = 7 – 0 – 14/2 = 0

Oxygen 1, Oxygen 2, and Oxygen 3

V = 6, N = 4, B = 4

Therefore,

q_f = 6 – 4 – 4/2 = 0

Oxygen 4

V = 6, N = 6, B = 2

Therefore,

q_f = 6 – 6 – 2/2 = -1

The net formal charge is: 0 + 3 x 0 – 1 = -1

The formal charge of ClO₄^– ion is -1.