Oxidation number

Definition

An oxidation number is a value assigned to an element in a chemical compound that represents its degree of oxidation (loss of electrons). It helps in determining how electrons are distributed among the atoms in the compound.

5 Must Know Facts For Your Next Test

1. Oxidation numbers are used to keep track of electron transfers in redox reactions.
2. The oxidation number of an atom in its elemental form is always zero.
3. Oxygen usually has an oxidation number of -2, except in peroxides where it is -1 and in compounds with fluorine where it is positive.
4. Hydrogen typically has an oxidation number of +1 when bonded with non-metals and -1 when bonded with metals.
5. The sum of the oxidation numbers in a neutral molecule must be zero, while in a polyatomic ion, it must equal the charge on the ion.

Review Questions

• What is the usual oxidation number for oxygen and what are its exceptions?
• How do you determine the overall charge using oxidation numbers for a polyatomic ion?
• Why is it important to assign correct oxidation numbers when balancing redox reactions?

Related terms

Redox Reaction:

A chemical reaction involving the transfer of electrons between two species. It consists of two half-reactions: one for reduction and one for oxidation.

Reduction:

The gain of electrons by a molecule, atom, or ion. It results in a decrease in the oxidation state.

Oxidation:

The loss of electrons by a molecule, atom, or ion. It results in an increase in the oxidation state.