Rules for assigning oxidation number
1. In an uncombined element, the oxidation number is zero.
2. In a monoatomic ion, the oxidation number is equal to the charge of the ion.
3. In compounds, the oxidation number of hydrogen is +1 and of oxygen is -2, except in peroxides where it is -1.
4. In compounds where an element has more than one oxidation number, the sum of all oxidation numbers must equal the charge of the compound.
5. In a neutral compound, the sum of oxidation numbers of all elements must equal zero.
6. In polyatomic ions, the sum of oxidation numbers of all elements must equal the charge of the ion.
7. In a compound with a metal and a nonmetal, the oxidation number of the metal is positive and the oxidation number of the nonmetal is negative.
8. In a compound with two nonmetals, the more electronegative element has a negative oxidation number and the less electronegative element has a positive oxidation number.