Oxidation numbers

OXIDATION NUMBERS

It is often useful to follow chemical reactions by looking at changes in the oxidation numbers of the atoms in each compound during the reaction. Oxidation numbers also play an important role in the systematic nomenclature of chemical compounds. By definition, the oxidation number of an atom is the charge that atom would have if the compound was composed of ions.

1. The oxidation number of an atom is zero in a neutral substance that contains atoms of only one element. Thus, the atoms in O₂, O₃, P₄, S₈, and aluminum metal all have an oxidation number of 0.

2. The oxidation number of simple ions is equal to the charge on the ion. The oxidation number of sodium in the Na⁺ ion is +1, for example, and the oxidation number of chlorine in the Cl^– ion is -1.

3. The oxidation number of hydrogen is +1 when it is combined with a nonmetal as in CH₄, NH₃, H₂O, and HCl.

4. The oxidation number of hydrogen is -1 when it is combined with a metal as in. LiH, NaH, CaH₂, and LiAlH₄.

5. The metals in Group IA form compounds (such as Li₃N and Na₂S) in which the metal atom has an oxidation number of +1.

6. The elements in Group IIA form compounds (such as Mg₃N₂ and CaCO₃) in which the metal atom has a +2 oxidation number.

7. Oxygen usually has an oxidation number of -2. Exceptions include molecules and polyatomic ions that contain O-O bonds, such as O₂, O₃, H₂O₂, and the O₂^2- ion.

8. The elements in Group VIIA often form compounds (such as AlF₃, HCl, and ZnBr₂) in which the nonmetal has a -1 oxidation number.

9. The sum of the oxidation numbers in a neutral compound is zero.

H₂O: 2(+1) + (-2) = 0

10. The sum of the oxidation numbers in a polyatomic ion is equal to the charge on the ion. The oxidation number of the sulfur atom in the SO₄^2- ion must be +6, for example, because the sum of the oxidation numbers of the atoms in this ion must equal -2.

SO₄^2-: (+6) + 4(-2) = -2

11. Elements toward the bottom left corner of the periodic table are more likely to have positive oxidation numbers than those toward the upper right corner of the table. Sulfur has a positive oxidation number in SO₂, for example, because it is below oxygen in the periodic table.

SO₂: (+4) + 2(-2) = 0

EVALUATION

Calculate the oxidation number of the central elements in the following compounds.

• K₂Cr₂O₇
• KMnO₄
•  H₂SO₄
•  Cr_O7^-2