What is the oxidation number of sulfur in SO₂?

• The oxidation number of oxygen is -2 (Rule #6).
  There are two oxygen atoms.
• The overall charge of the compound is zero, so the sum of the oxidation numbers of all the atoms in the formula is zero (Rule #8).

Let the oxidation number of sulfur be represented as S.

S + 2 (-2) = 0

S = +4