Assigning Oxidation Numbers (Oxidation States)
The oxidation number of an atom in a covalent molecule (or polyatomic ion) is defined
as the charge that would be associated with that atom if both electrons of each bond that
the atom forms were transferred completely to the atom of higher electronegativity.
Oxidation numbers are assigned by using the following set of rules.
1. Each atom in an element (in any of its allotropic forms) is assigned an oxidation
number of 0.
2. The oxidation number of a monatomic ion is equal to the charge of the ion: +1 for
Na+, +2 for Cu2+, –1 for Cl–, and so on.
3. Fluorine, the most electronegative element, has an oxidation number of –1 in all its
covalent compounds as well as in the F– ion.
4. Hydrogen in its compounds usually has an oxidation number of +1. The only
exceptions occur in metal hydrides, such as Na+H–, in which hydrogen has an
oxidation number of –1.
5. Oxygen in its compounds usually has the oxidation number –2. There are two
important exceptions: (a) When oxygen is combined with fluorine, as in F2O, its
oxidation number is +2. (b) In compounds containing oxygen-oxygen bonds, such as
hydrogen peroxide, the oxidation number of oxygen is –1.
6. The halogens other than fluorine have an oxidation number of –1 except when they
are combined with a more electronegative element, that is, oxygen or a more
electronegative halogen.
7. The sum of the oxidation numbers of all the atoms in a neutral compound is 0, and the
sum of oxidation numbers in an ion is equal to the charge on the ion.
Examples:
SF6
In this molecule, as in all its compounds, each fluorine atom has oxidation
number –1 (Rule 3). The molecule is neutral, so the sum of all the oxidation
numbers must equal 0 (Rule 7). So, because the sum of the oxidation numbers of
all the fluorine atoms is –6, sulfur must have oxidation number +6 (–6 + 6 = 0).
ClO4– In this anion, oxygen has oxidation number –2 (Rule 5). The sum of all the
oxidation numbers in this anion must equal the charge on the anion, or –1 (Rule
7). Therefore, since the sum of the oxidation numbers of all the oxygen atoms is
-8, chlorine must have oxidation number +7 (–8 + 7 = –1).
BeH2 This is a metal hydride, so the oxidation number of hydrogen is –1 (Rule 4). It is a
neutral molecule, so the sum of oxidation numbers must equal 0 (Rule 7). Since
the sum of the oxidation numbers of the two hydrogens is –2, beryllium must
have oxidation number +2 (–2 + 2 = 0).