20.1 The Meaning of Oxidation and Reduction

• Oxidation and reduction always occur simultaneously. The substance gaining oxygen is oxidized, while the substance losing oxygen is reduced.
• Losing electrons is oxidation. Gaining electrons is reduction.
• Iron, often used in the form of the alloy steel, corrodes by being oxidized to ions of iron by oxygen.

20.2 Oxidation Numbers

• As a general rule, a bonded atom’s oxidation number is the charge that it would have if the electrons in the bond were assigned to the atom of the more electronegative element.
• An increase in the oxidation number of an atom or ion indicates oxidation. A decrease in the oxidation number of an atom or ion indicates reduction.

20.3 Balancing Redox Equations

• If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction. Therefore, the reaction as a whole must be a redox reaction.
• In a balanced redox equation, the total increase in oxidation number of the species oxidized must be balanced by the total decrease in the oxidation number of the species reduced.
• To balance a redox reaction using half-reactions, write separate half-reactions for the oxidation and the reduction. After you balance atoms in each half-reaction, balance electrons gained in the reduction with electrons lost in the oxidation.