Rules for Finding Oxidation Numbers
The following are a list of rules to be used when finding the oxidation numbers of atoms in molecules, compounds, or polyatomic ions. A more in-depth discussion of the topic is found in the section on Oxidation Numbers.
1 The oxidation number of an atom in an uncombined elementA substance containing only one kind of atom and that therefore cannot be broken down into component substances by chemical means. is 0. Since atoms of the same element always form pure covalent bonds, they share electrons equally, neither losing nor gaining, e.g., Cl2.
2 The oxidation number of a monatomic ion equals the charge on that ion, e.g., Na+ and Cl–.
3 Some elements have the same oxidation number in nearly all their compounds.
...a...Elements in periodic groupThose elements that comprise a single column of the periodic table. Also called family. IA have oxidation numbers of +1, and elements in periodic group IIA have oxidation numbers of +2, e.g., Na+.
...b...The most electronegative element, fluorine, is always assigned both electrons from any bond in which it participates. This gives fluorine an oxidation number of –1 in all its compounds, e.g., OF2.
...c...Oxygen usually exhibits an oxidation number of –2, but exceptions occur in peroxides, superoxides, and when oxygen combines with fluorine.
...d...Hydrogen exhibits an oxidation number of +1 unless it is combined with an element more electropositive than itself, e.g., with lithium, in which case its oxidation number is –1.
4 The sum of the oxidation numbers of all atoms in a complete formula must be 0; that is, when an electron is lost by one atom (+1 contribution to oxidation number), the same electron must be gained by another atom (–1 contribution to oxidation number).
5 If a polyatomic ion is considered by itself, the sum of the oxidation numbers of its constituent atoms must equal the charge on the ion.