In sulfur trioxide (SO₃), sulfur (S) shares double covalent bonds with all oxygen (O) atoms. Its Lewis structure is shown below.

Let us calculate the formal charge of SO₃ by calculating the formal charges on S and O.

Sulfur

V = 6, N = 0, B = 12

Therefore,

q_f = 6 – 0 – 12/2 = 0

The formal charge on S in SO₃ is zero.

Oxygen 1, Oxygen 2, and Oxygen 3

V = 6, N = 4, B = 4

Therefore,

q_f = 6 – 4 – 4/2 = 0

The net formal charge is: 0 + 3 x 0 = 0

The formal charge of SO₃ is zero. The structure shown above is the most stable.