In the chlorate ion (ClO₃^–), chlorine (Cl) has a double bond with two of the three oxygen (O) atoms and a single bond with the third oxygen atom, as shown below.

Let us calculate the formal charge of ClO₃^– by calculating the formal charges on Cl and O in the molecule and adding them up.

Chlorine

V = 7, N = 2, B = 10

Therefore,

q_f = 7 – 2 – 10/2 = 0

Oxygen 1 and Oxygen 2

V = 6, N = 4, B = 4

Therefore,

q_f = 6 – 4 – 4/2 = 0

Oxygen 3

V = 6, N = 6, B = 2

Therefore,

q_f = 6 – 6 – 2/2 = -1

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

The formal charge of ClO₃^– is -1.