The resonance structure of carbonate (CO₃^2-) is discussed in our article on resonance structure. In this article, we will determine the formal charge on each atom and the formal charge of the molecule.

Carbon (C) has a double bond with two of the three oxygen (O) atoms and a single bond with the third, as shown below.

Let us now calculate the formal charges on C and O.

Carbon

V = 4, N = 0, B = 8

Therefore,

q_f = 6 – 0 – 12/2 = 0

The formal charge on C in CO₃^2- is 0.

Oxygen 1 and Oxygen 2

V = 6, N = 6, B = 2

Therefore,

q_f = 6 – 6 – 2/2 = -1

Oxygen 3

V = 6, N = 4, B = 4

Therefore,

q_f = 6 – 4 – 4/2 = 0

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

The formal charge of the carbonate ion is -2.