In order to learn how to find the oxidation number of an atom in a given compound, it is important to learn what oxidation numbers are. The oxidation number of an atom is a number that represents the total number of electrons lost or gained by it.
Calculating Oxidation Numbers
An oxidation number can be assigned to a given element or compound by following the following rules.
- Any free element has an oxidation number equal to zero.
- For monoatomic ions, the oxidation number always has the same value as the net charge corresponding to the ion.
- The hydrogen atom (H) exhibits an oxidation state of +1. However, when bonded with an element with less electronegativity than it, it exhibits an oxidation number of -1.
- Oxygen has an oxidation of -2 in most of its compounds. However, in the case of peroxides, the oxidation number corresponding to oxygen is -1.
- All alkali metals (group 1 elements) have an oxidation state of +1 in their compounds.
- All alkaline earth metals (group 2 elements) exhibit an oxidation state of +2 in their compounds.
- In the compounds made up of two elements, a halogen (group 17 elements) have an oxidation number of -1 assigned to them.
- In the case of neutral compounds, the sum of all the oxidation numbers of the constituent atoms totals to zero.
- When polyatomic ions are considered, the sum of all the oxidation numbers of the atoms that constitute them equals the net charge of the polyatomic ion.
Thus, the oxidation number of an atom in a given compound can be calculated with the steps mentioned above.
In order to help students understand how to find oxidation number, the oxidation states of each individual atom in some example compounds are determined below.
An illustration explaining how to find oxidation number of the sulphur atom in a sodium sulfate molecule can be found above.
Hydrochloric Acid (HCl)
- As per the rules discussed above, the oxidation state of a group 17 element (halogen) in a diatomic molecule is -1. It is also discussed that hydrogen always exhibits an oxidation number of +1 unless it is paired with a less electronegative element.
- Since chlorine is more electronegative than hydrogen, an oxidation number of +1 can be assigned to the hydrogen atom in HCl.
- Therefore, the oxidation number of hydrogen is +1 and the oxidation of chlorine is -1 in HCl. These values can be verified by adding these oxidation numbers. Since the total is zero, which is the value of the oxidation number corresponding to a neutral molecule, the values are verified.
Carbon Dioxide (CO2)
- According to the rules to calculate oxidation number, which can be found in the previous subsection, the oxidation number of oxygen in its compounds (excluding peroxides) is -2.
- Since there are two oxygen atoms in carbon dioxide, the total of the oxidation numbers corresponding to each oxygen is -4.
- Since the CO2 molecule is neutral, the carbon atom must exhibit an oxidation state of +4 (the sum of all the oxidation numbers in a neutral molecule is zero).
- Therefore, the oxidation state of oxygen was found to be -2 and the oxidation number of carbon is +4 in a carbon dioxide molecule.
Students can understand how to find oxidation number with the help of the solved examples provided above. To learn more about oxidation states and other related concepts such as oxidizing agents, register with BYJU’S and download the mobile application on your smartphone.