Question

How would you find the oxidation numbers of ${\text{MgO}}$?

Answer 1

Hint: Oxidation number is also called oxidation state; it is defined as numbers of the electrons shared, lost, or gained by an atom when it is forming a chemical bond with different atoms. The oxidation number or oxidation state of an atom is assigned by two methods.
1) Sum all constant oxidation states of other atoms or ions or molecules bound to the atom whose oxidation state is to be determined.
2) Equate the oxidation state of a molecule, ion to the total charge on the molecule or ion.

Complete answer:
The rules used to determine the oxidation state are as follows:
1. Oxidation state of the alkali metals is always taken as +1.
2. Oxidation state of the alkaline earth metals is always taken as +2.
3. Oxidation state of the hydrogen is +1 but in the case of the hydrides, it is -1.
4. Oxidation state of the oxygen in oxides is -2, in peroxides is -1 while in superoxide it is -1/2.
5. Oxidation state of the halogen is -1.
6. Oxidation state of the boron family metal ions is +3.
Here, the compound given is ${\text{MgO}}$ it is magnesium hydroxide.
Now, the oxidation number of magnesium is determined as follows:
Here, the given compound is neutral, the total charge on the compound is zero.
\[\left( {{\text{oxidation}}\,{\text{number}}\,{\text{of}}\,{\text{Mg}}} \right){\text{ + }}\,\left( {{\text{oxidation number}}\,{\text{of}}\,{\text{O}}} \right){\text{ = Total}}\,{\text{charge}}\,{\text{on the molecule}}\]
Here, substitute oxidation number O as -2, and determine the oxidation state of the magnesium.
\[\left( {{\text{oxidation number of Mg}}} \right){\text{ + }}\,\left( {{\text{ - 2}}} \right){\text{ = 0}}\]
\[\left( {{\text{oxidation number of Mg}}} \right){\text{ = }}\, + 2\]
Thus, in the given compound ${\text{MgO}}$ the oxidation state of the oxygen is -2 and that of magnesium is +2.

Note: The total oxidation state of any species is nothing but equal to the charge present on that species. In the case of the cationic species, the total oxidation state is nothing but equal to the total positive charge present on the cationic species.In the case of the anionic species, the total oxidation state is nothing but equal to the total negative charge present on the anionic species.In the case of the neutral molecule total oxidation state is equal to zero.