These rules are used to assign oxidation numbers to elements. They are listed by
Priority , i.e., in a case of apparent conflict, the rule that comes
first is observed.
| Rules for Assigning Oxidation Numbers |
| 1 |
The oxidation number of an uncombined element is 0. |
| 2 |
Alkali metals (Group I-A) have an oxidation number of +1. Alkaline earth elements (Group II-A)
have an oxidation number of +2. (Aluminum is +3). |
| 3 |
Hydrogen (H) in compounds is +1. |
| 4 |
Oxygen (O) in compounds is -2. |
| 5 |
Halogens (Group VIII-A) are -1 in binary compounds. |
| 6 |
Sulfur (S) is -2 in binary compounds. |
| 7 |
The algebraic sum of all the oxidation numbers must always equal the charge on the molecule
or ion.
Since molecules are electrically neutral, the sum of the positive and negative oxidation numbers
must be 0. |
| Practice Problems with Answers and Reasons. |
| Element |
Answer |
Reason |
| O2 |
0 |
O2 is an electrically neutral molecule and is assigned an oxidation number of 0. |
| Cl2 |
0 |
Cl 2 is an electrically neutral molecule and is assigned an oxidation number of 0. |
| O-2 |
-2 |
The oxidation number must match the charge on the ion. |
| Fe+3 |
+3 |
The oxidation number must match the charge on the ion. |
| S 8 |
0 |
S8 is an electrically neutral molecule and is assigned an oxidation number of 0. |
| F-1 |
-1 |
The oxidation number must match the charge on the ion. |
 |
-1 |
Since there is more than one atom of a given element in the compound, divide the total charge
(-2) by the number of atoms (2) to get the oxidation number per single atom. |
