Question

The oxidation number of Sulphur in ${{\rm{S}}_{\rm{8}}}$, ${{\rm{S}}_{\rm{2}}}{{\rm{F}}_{\rm{2}}}$ and ${{\rm{H}}_2}{\rm{S}}$ respectively are __________

A. 0,+1 and -2
B. +2,+1 and -2
C. 0,+1 and +2
D. -2, +1 and -2

Answer 1

Oxidation number is the number of electrons that an atom loses or gains to form a chemical bond. If we know the type of compound, we will easily know the oxidation number of compounds.

Complete step-by-step answer:
To know the oxidation number of the compound, first we have to identify the type of compound in which the given compounds exist.
a) If a compound exists in elemental form (only one type of atom is present), the oxidation number of the element is always zero. So, in ${{\rm{S}}_{\rm{8}}}$, the oxidation number of S is zero.

b) For ions, the charge indicates the oxidation number. For example, the oxidation number of fluoride ions is –1. Also if the compound is neutral, the addition of oxidation numbers of all atoms or ions is always zero.
${{\rm{S}}_{\rm{2}}}{{\rm{F}}_{\rm{2}}}$ is a neutral compound. So, summation of the oxidation number of all atoms is zero. And the oxidation number of fluorine is –1.
Now, we calculate the oxidation number of ${{\rm{S}}_{\rm{2}}}{{\rm{F}}_{\rm{2}}}$. We take x as the oxidation number of S. Now, we will add the oxidation number of all atoms of the compound.
As the compound ${S_2F_2}$ is a neutral compound,
So, $\begin{array}{c}2x - 2 = 0\\\x = 1\end{array}$
Oxidation number of S in ${{\rm{S}}_{\rm{2}}}{{\rm{F}}_{\rm{2}}}$ is +1.

c) Similarly, we have to calculate the oxidation number of ${{\rm{H}}_2}{\rm{S}}$. ${{\rm{H}}_2}{\rm{S}}$ is a neutral compound and the oxidation number of Hydrogen is +1. We take x as the oxidation number of S.
So, for ${{\rm{H}}_2}{\rm{S}}$,
$\begin{array}{c}2 \times 1 + x = 0\\\2 + x = 0\\\x = - 2\end{array}$

So, the oxidation number of ${\rm{S}}$ in ${{\rm{H}}_2}{\rm{S}}$ is –2.

Hence, option A is correct.

Note: Students have to take care of taking the oxidation number of hydrogen in the calculation of an oxidation number of an element. When a hydrogen atom is bonded to a non-metal, its oxidation number is +1 and when it is bonded to a metal, its oxidation number is –1. In ${{\rm{H}}_2}{\rm{S}}$, hydrogen is bonded to a non-metal (sulphur), so its oxidation number is +1.