The oxidation number, also known as oxidation state, represents the number of electrons that an element can either gain, lose or share to form a chemical bond with another element.
By definition, the oxidation number can be seen as the potential charge an atom can possess from either the sharing, gaining or loss of electrons in an oxidation-reduction reaction.
Calculating Oxidation Number
In order to calculate the oxidation of an element or compound we can utilize the following rules.
Rule 1: For an atom in its standard or elemental state its oxidation number is equal to zero. The following elements and molecules are good examples.
C (graphite) O2 (g) P4 (s) Cl2 (g) He (g) Zn (s)
Rule 2: For an ion, its charge is equal to its oxidation number.
Na+ Oxidation Number = +1 SO42– Oxidation Number = – 2
Rule 3: For metals from Group 1A, their oxidation number is equal to +1 when connected to other elements.
Rule 4: For metals from Group 2A, their oxidation number is equal to +2 when connected to other elements.
Rule 5: For hydrogen, the oxidation number is +1 when connected to non-metals and -1 when connected to boron or a metal.
Rule 6: For fluorine, the oxidation number is -1 when connected to other elements.
Rule 7: For oxygen, the oxidation number is -1 when it’s a peroxide, -1/2 when it’s a superoxide and -2 in most other cases.
A peroxide represents a compound containing two elements from Group 1A bonded to two oxygen atoms.
A superoxide represents a compound containing one element from Group 1A bonded to two oxygen atoms.
Rule 8: For the halogens (Cl, Br, I), their oxidation numbers are -1 unless they are connected to oxygen.
When the halogen (Cl, Br, I) is connected to oxygen then the oxidation number must be calculated.
Now let’s take a look at a few practice problems.
PRACTICE 1: Determine the oxidation number of the sulfur atom in the sulfate ion.
PRACTICE 2: Determine the oxidation number of the nitrogen atom.
PRACTICE 3: Determine the oxidation number of the carbon atom in carbon dioxide.
Beyond Oxidation Numbers
Oxidation Numbers can serve roles beyond redox reactions such as naming ionic compounds, determining the degree of oxidation within biological reactions, understanding electron configurations and other trends of the periodic table.