Question

What is the oxidation number of ${H_2}O$ ?

Answer 1

The oxidation state or sometimes known as oxidation number, defines the degree of oxidation that an atom possesses in a chemical compound. In other words, oxidation number refers to the charge left on the central atom especially when all the bonding pairs of electrons get broken, with the charge allocated to the most electronegative atom.

Complete step by step answer:
There are few important rules to estimate the oxidation number of every atom in the compounds or ions which are as follows:
1. The oxidation number of hydrogen in a compound is generally $+ 1$ . The oxidation number of oxygen in a compound is generally $- 2$ , except to that in peroxides, where it is $- 1$ .
2. The summation of all the oxidation numbers in a polyatomic ion is generally equal to the charge exhibited by the ion.
The compound i.e. ${H_2}O$ as a whole does not possess any charge and is neutral which we can prove as follows:
According to Rule 1, the oxidation number of $H$ is $+ 1$ and the oxidation number of $O$ is $- 2$ .
So, there are two $H$ atoms and one $O$ atom.
Thus oxidation number of the whole compound i.e. ${H_2}O$ will become:
$[2 \times ( + 1)] + [1 \times ( - 2)] = 0$
In our case, ${H_2}O$ is neutral having a charge $0$ .
Hence, the oxidation number of ${H_2}O$ is 0.

Note: The more electronegative element in a substance is always allotted a negative oxidation state whereas, the less electronegative element is allotted a positive oxidation state. Always remember the fact that electronegativity is greatest at the top-right in the periodic table which declines toward the bottom-left.