Assigning oxidation numbers

Oxidation-reduction reactions (redox reactions) are reactions in which electrons are lost by an atom or ion in one reactant and gained by an atom or ion in another reactant. Although electrons are gained and lost in these reactions, the balanced equation for a redox reaction does not show the electrons that are being transferred. In order to tell whether a redox reaction has occurred or not, we need a way to keep track of electrons. The best way to do so is by assigning oxidation numbers to the atoms or ions involved in a chemical reaction.

Oxidation numbers are hypothetical numbers assigned to an individual atom or ion present in a substance using a set of rules. Oxidation numbers (or oxidation states as they are also called) can be positive, negative, or zero. It is VERY IMPORTANT to remember that oxidation numbers are always reported for one individual atom or ion and not for groups of atoms or ions.

The following rules are used to assign oxidation numbers. Chem 1115 students will have these rules available on exams. Chem 1215 students must memorize these rules.

Oxidation Number Rules

1. The oxidation number for an atom in its elemental form is always zero.
   • A substance is elemental if both of the following are true:
     • only one kind of atom is present
     • charge = 0
   • Examples:
     • S8: The oxidation number of S = 0
     • Fe: The oxidation number of Fe = 0
2. The oxidation number of a monoatomic ion = charge of the monatomic ion.
   • Examples:
     • Oxidation number of S2- is -2.
     • Oxidation number of Al3+ is +3.
3. The oxidation number of all Group 1A metals = +1 (unless elemental).
4. The oxidation number of all Group 2A metals = +2 (unless...