Question

What is the oxidation number of Mg in $M{g}^{2+}$?

Answer 1

Hint: The oxidation number of an element which is also known as oxidation state describes the degree of oxidation of an atom within a chemical compound. In simple words, it is the charge acquired by the atom which represents loss or gain of electrons during the chemical change. It may be positive, negative or zero.

Complete answer:
Basic rules for determining the oxidation number of an atom in a chemical compound are as follows:
1. The oxidation number of a free element is always considered as zero.
2. The oxidation number of a mono-atomic ion is equal to the charge present on the atom.
3. The oxidation number of the hydrogen atom in the general case is $+1$. But in hydrides, the hydrogen atom is bonded with less electronegative elements, so in that case the oxidation number of hydrogen atoms is considered as $-1$.
4. The oxidation number of the oxygen atom in the general case is $-2$. But in peroxides, the oxidation number of oxygen atoms is considered as $-1$.
5. The oxidation number of the group I and group II elements is $+1$ and $+2$ respectively.
6. The oxidation number of halogens atoms in a compound is $-1$.
7. The summation of the oxidation number of all the atoms in a neutral molecule is zero.
8. For a polyatomic ion, the summation of the oxidation number of all the atoms is equal to the charge present on the ion.
Now, as per question we need to find the oxidation number of magnesium in $M{g}^{2+}$. As it is a mono-atomic ion i.e., it consists of only a single atom in its formula and we know that the oxidation number for a mono-atomic ion is equal to the charge present on the atom.
Therefore, we can conclude that the oxidation number of Mg in $M{g}^{2+}$ is $+2$.

Note:
It is important to note that the oxidation number of an atom is not the same as its valency. The valency of an atom is the maximum number of electrons it can share in order to become stabilized whereas the oxidation number is the actual number of electrons an atom can lose or gain in order to form a chemical bond.