Question

The oxidation number of oxygen is - 1 in:
A. ${\text{N}}{{\text{O}}_{\text{2}}}$
B. ${\text{Pb}}{{\text{O}}_{\text{2}}}$
C. ${\text{N}}{{\text{a}}_2}{{\text{O}}_{\text{2}}}$
D. ${\text{Mn}}{{\text{O}}_{\text{2}}}$

Answer 1

Hint: Oxidation number is the number of electrons gained or lost by the atoms. The charge represents the oxidation number of an atom.

Complete step by step answer:
An atom loss or gain electrons to form a bond with other atoms. So, the atoms get charged known as ions. The superscript of the ions represents the oxidation number of that atom.
In neutral molecules, the sum of the oxidation number is equal to zero.
In charged molecules, the sum of the oxidation number is equal to the charge of the molecule.
The atoms in elemental numbers have zero oxidation number.
The alkali metals have the oxidation number +1.
The transition metals show a variable oxidation number.
The oxidation number of oxygen in ${\text{N}}{{\text{O}}_{\text{2}}}$ is as follows:
The oxidation number of nitrogen is $ +4$.
$\left( { + 4 \times 1} \right)\, + \,\left( {x \times 2} \right)\, = \,0$
x = - 2
The oxidation number of oxygen is - 2 in ${\text{N}}{{\text{O}}_{\text{2}}}$.
So, option (A) is incorrect.
The oxidation number of oxygen in ${\text{Pb}}{{\text{O}}_{\text{2}}}$ is as follows:
The oxidation number of lead is +4.
$\left( { + 4 \times 1} \right)\, + \,\left( {x \times 2} \right)\, = \,0$
x = - 2
The oxidation number of oxygen is - 2 in ${\text{Pb}}{{\text{O}}_{\text{2}}}$.
So, option (B) is incorrect.
The oxidation number of oxygen in ${\text{N}}{{\text{a}}_2}{{\text{O}}_{\text{2}}}$ is as follows:
The oxidation number of sodium is +1.
$\left( { + 1 \times 2} \right)\, + \,\left( {x \times 2} \right)\, = \,0$
x = - 1
The oxidation number of oxygen is - 1 in ${\text{N}}{{\text{a}}_2}{{\text{O}}_{\text{2}}}$.
So, option (C) is correct.
The oxidation number of oxygen in ${\text{Mn}}{{\text{O}}_{\text{2}}}$ is as follows:
The oxidation number of manganese is +4.
$\left( { + 4 \times 1} \right)\, + \,\left( {x \times 2} \right)\, = \,0$
x = - 2
The oxidation number of oxygen is - 2 in ${\text{Mn}}{{\text{O}}_{\text{2}}}$.
So, option (D) is incorrect.

Therefore, option (C) ${\text{N}}{{\text{a}}_2}{{\text{O}}_{\text{2}}}$ is correct.

Note: The ${\text{N}}{{\text{a}}_2}{{\text{O}}_{\text{2}}}$ is known as sodium peroxide. The oxygen in peroxides always has - 1 oxidation number. In a molecule, the more electronegative atom has a negative oxidation number, and less electronegative has a positive oxidation number.