Rules for Assigning Oxidation States

1. The oxidation number of a pure element is 0. (ex. O in O₂ is 0, Cu in copper metal is 0)
2. The sum of the oxidation numbers in a neutral molecule is zero. (ex. for H₂O, H = +1 and O = -2, so 2(+1) + 1(-2) = 0)
3. The sum of the oxidation numbers in an ion is the charge of the ion. (ex. Na⁺¹ ion has Na with oxidation number +1; SO₄^-2 ion has S = +6 and O = -2, so 1(+6) + 4(-2) = -2)
4. Oxygen is almost always -2 (unless it is bonded to an atom of higher electronegativity like fluorine - OF₂).
5. Hydrogen is almost always +1 (unless it is bonded to an atom with lower electronegativity like sodium - NaH).
6. Oxidation numbers of elements at the far left and right sides of the periodic table tend to have oxidation numbers that match the most common ionic charge formed by that element. (ex. Na = +1, Ca = +2, F = -1)