In sulfur dioxide (SO₂), sulfur (S) has a double covalent bond with one oxygen (O) atom and a single covalent bond with another. SO₂ undergoes the following resonance.

O^––S⁺=O ↔ O=S⁺–O^–

Let us calculate the formal charge of both these structures.

i. O^––S⁺=O

Sulfur

V = 6, N = 2, B = 6

Therefore,

q_f = 6 – 2 – 6/2 = 1

Oxygen 1

V = 6, N = 6, B = 2

Therefore,

q_f = 6 – 6 – 2/2 = -1

Oxygen 2

V = 6, N = 4, B = 4

Therefore,

q_f = 6 – 4 – 4/2 = 0

The net formal charge is: 1 – 1 + 0 = 0

ii. O=S⁺–O^–

Sulfur

V = 6, N = 2, B = 6

Therefore,

q_f = 6 – 2 – 6/2 = 1

Oxygen 1

V = 6, N = 4, B = 4

Therefore,

q_f = 6 – 4 – 4/2 = 0

Oxygen 2

V = 6, N = 6, B = 2

Therefore,

q_f = 6 – 6 – 2/2 = -1

The net formal charge is: 1 + 0 – 1 = 0

The formal charge of SO₂ is zero. It spends half its time in each of its two resonance structures. It flips back and forth swiftly between the two forms.