Rules for Oxidation Numbers

1. The ON of an atom of an element in its free state is zero.
   (e.g.:  ON(Cl) in Cl₂ is 0; ON(Fe) in Fe is 0)

2. The ON of a monatomic ion is its charge.
   (e.g.:  ON(Fe) in Fe²⁺ is +2)

3. The more electronegative element has a negative ON.
   (e.g.:  ON(Cl) is negative and ON(K) is positive in KCl)

4. ON(O) is usually -2, and ON(H) is usually +1.
   (e.g.:  ON(H) is +1 and ON(O) is -2 in H₂SO₄)

5. The sum of the ON of all the atoms in a neutral compound is zero.
   (e.g.:  Consider H₂SO₄;  2 ON(H) + ON(S) + 4 ON(O) = 0)
   (Therefore:  2(+1) +ON(S) + 4(-2) = 0  gives:  ON(S) = +6)

6. The sum of the ON of all the atoms in a polyatomic ion is the charge on the ion.
   (e.g.:  Consider PO₄^3-;  ON(P) + 4 ON(P) = -3)
   (Therefore:  ON(P) + 4(-2) = -3  gives:  ON(P) = +5)

   This Power Point Show has more examples of Oxidation Numbers.