SCH4U Rules for oxidation numbers: Date: Chemical reactions which involve electron transfer are called redox reactions (reduction-oxidation). Oxidation numbers are used by chemists to keep track of the positive or negative character of atoms or ions in redox reactions and to determine which species have been oxidized or reduced. An oxidation number is the real or apparent charge on an atom or ion has when all bonds are assumed to be lonic. Oxidation involves the loss of electrons, and an increase in oxidation number. I.e. Cu Cu + 2e (oxidation number increases by 2) Reduction involves the gain of electrons and a decrease in oxidation number. I.e. Zn+ 2e Zn (oxidation number decreases by 2) Oxidation Reduction Reactions 1. The oxidation number on each atom in any uncombined element is always zero. 2. The oxidation number of any monatomic ion is the net charge on the atom. 3. The oxidation number for oxygen is -2 in all its compounds (except peroxides where it is -1). A peroxide has an oxygen-oxygen