Oxidation Numbers for AP Chemistry

Practice problems for these concepts can be found at:

• Basics Multiple Choice Review Questions for AP Chemistry
• Basics Free-Response Questions for AP Chemistry

Oxidation numbers are bookkeeping numbers that allow chemists to do things like balance redox equations. Don't confuse oxidation numbers with the charge on an ion. Oxidation numbers are assigned to elements in their natural state or in compounds using the following rules:

• The oxidation number of an element in its elemental form (i.e., H₂, Au, Ag, N₂) is zero.
• The oxidation number of a monoatomic ion is equal to the charge on the ion. The oxidation number of Mg²⁺ is +2. Note that the charge is written with number first, then sign; for oxidation numbers it is sign, then number.
• The sum of all the oxidation numbers of all the elements in a neutral molecule is zero. The sum of all the oxidation numbers in a polyatomic ion is equal to the charge on the ion.
• The alkali metal ions have an oxidation number of +1 in all their compounds.
• The alkaline earth metals have an oxidation number of +2 in all their compounds.
• The oxidation number of hydrogen in compounds is +1, except it is –1 when combined with metals or boron in binary compounds.
• The oxidation number of halogens in their compounds is –1 except when combined with another halogen above them on the periodic table, or with oxygen.
• The oxidation number of oxygen is –2 in compounds, except for peroxides, in which it is –1.

Determine the oxidation number of sulfur in sulfuric acid, H2SO4. The sum of all the oxidation numbers must equal zero, since this is a neutral compound. The oxidation numbers of hydrogen (+1) and oxygen (–2) are known, so the oxidation number of sulfur can be determined:

2(+1) + ? + 4(–2) = 0

H₂SO₄

The oxidation number of sulfur in this compound must be +6.

Practice problems for these concepts can be found at:

• Basics Multiple Choice Review Questions for AP Chemistry
• Basics Free-Response Questions for AP Chemistry