Predicting Oxidation Numbers

1. In free elements (that is, in uncombined state), each atom has an oxidation number of zero. Ex. In O2, the oxidation number of each oxygen atom is zero.
2. For ions composed of only one atom, the oxidation number is equal to the charge on the ion. Ex. The oxidation number of Ca2+ is +2.
3. All alkali metals (elements in column 1of the periodic table, with the exception of hydrogen) have an oxidation number of +1. Ex. The oxidation numbers of Li, K, and Na will always be +1.
4. All alkaline earth metals (elements in column 2 of the periodic table) have an oxidation number of +2. Ex. The oxidation number of Ba is +2.
5. The oxidation number of Aluminum (Al) is always +3.
6. The oxidation number of oxygen in most compounds (such as H2O and CO2) is -2. In hydrogen peroxide (H2O2) and peroxide (O22-) oxygen shows a -1 oxidation number.
7. The oxidation number of hydrogen is +1, except when in is bonded to a metal as a negative ion, in which case it is -1. Ex. H2O shows hydrogen as +1. NaH shows hydrogen as -1.
8. When halogens (elements in column 17 on the periodic table) form negative ions, they will have an oxidation number of -1. Ex. NaCl and CaCl2 both show chlorine with a -1 oxidation number.
9. In a neutral molecule, the sum of the oxidation numbers of all of the atoms must be zero. Ex. In H2O, each hydrogen is +1 and the oxygen is -2. So, (2 x +1) + (-2) = 0.
10. In a polyatomic ion, the sum of oxidation numbers of all the elements in the ion must be equal to the net charge of the ion. Ex. In the polyatomic ion known as hydroxide (OH-), the oxygen is -2 and the hydrogen is +1. So, (-2) + (+1) = -1, the same as the charge on the hydroxide ion (OH-)