22.2 Oxidation Numbers
22.2 Oxidation Numbers
Lesson Objectives
- Use the oxidation number rules to determine the oxidation number of an atom of any element in a pure
substance. - Define oxidation and reduction in terms of a change in oxidation number.
- Identify which atoms are being oxidized and which atoms are being reduced in a redox reaction.
- Know which types of reactions are usually redox reactions and which types are usually not. Use oxidation
number changes to decide whether a given reaction involves oxidation and reduction.
Lesson Vocabulary
- oxidation number
Check Your Understanding
Recalling Prior Knowledge
- What are the definitions of oxidation and reduction?
- What are the five principal types of chemical reactions?
Keeping track of what is being oxidized and what is being reduced in a redox reaction can sometimes be challenging
when ions are not directly involved. In this lesson, you will learn how to give every atom in a molecule an oxidation
number. These oxidation numbers will be helpful in identifying and analyzing redox reactions.
Assigning Oxidation Numbers
Anoxidation numberis a positive or negative number that is assigned to an atom to indicate its degree of oxidation
or reduction. The term oxidation state is often used interchangeably with oxidation number. Recall from the previous
lesson that the electron transfer that occurs in a redox reaction may be complete or it may be partial. A partial electron
transfer was described as a shift in the electron density near an atom as a result of a change in the other atoms to
which it is covalently bonded. That charge shift is based on the relative electronegativities of the atoms involved in
the bond.
Overall, the oxidation number of an atom in a molecule is the charge that the atom would have if all polar covalent
and ionic bonds resulted in a complete transfer of electrons from the less electronegative atom to the more elec-
tronegative one. Oxidation numbers can be assigned by looking at the Lewis structure for a given substance, but for
many simpler compounds, they can also be assigned using the set of rules outlined below.