Chemistry Form 5: Chapter 3 - Rules of Oxidation Number

There are several rules for assigning the oxidation number to an element. Learning these rules will simplify the task of determining the oxidation state of an element, and thus, whether it has undergone oxidation or reduction.

1. The oxidation number of an atom in the elemental state is zero.
   Example: Cl₂ and Al both are 0 
2. The oxidation number of a monatomic ion is equal to its charge.
   Example: In the compound NaCl, the sodium has an oxidation number of 1+ and the chlorine is 1-.
3. The algebraic sum of the oxidation numbers in the formula of a compound is zero.
   Example: the oxidation numbers in the NaCl above add up to 0
4. The oxidation number of hydrogen in a compound is 1+, except when hydrogen forms compounds called hydrides with active metals, and then it is 1-.
   Examples: H is 1+ in H₂O, but 1- in NaH (sodium hydride).
5. The oxidation number of oxygen in a compound is 2-, except in peroxides when it is 1-, and when combined with fluorine. Then it is 2+.
   Example: In H₂O the oxygen is 2-, in H₂O₂ it is 1-.
6. The algebraic sum of the oxidation numbers in the formula for a polyatomic ion is equal to the charge on that ion.
   Example: in the sulfate ion, SO₄^2-, the oxidation numbers of the sulfur and the oxygens add up to 2-. The oxygens are 2- each, and the sulfur is 6+.