Question

Which of the following is not an inert gas?
A. Helium
B. Xenon
C. Hydrogen
D. Argon

Answer 1

Hint: The definition and understanding of the term “inert” when used for gases can be used to classify the given ones. They do have complete octets which make them the most stable atoms among all elements.

Step by step answer:
We have a modern periodic table that helps with classification of elements and understand, explain or deduce their properties and behavior which are similar to that of others, or different from others or unique for a given element. Here, we will talk about the last column of the periodic table that has been named as ${18^{{\rm{th}}}}$ group.

As we know that in a group, all the elements have the same valence shell electronic configuration so they exhibit similar properties to some extent.
We also know that moving towards right across a period, electrons get added to the valence shell. So starting from general valence shell electronic configuration for group $1$ as $n{s^1}$ we reach to group $18$ with $n{s^2}n{p^6}\left( {{\rm{only}}\;1{s^2}{\rm{ for He}}} \right)$. It means that the valence shell is completely filled in the ${18^{{\rm{th}}}}$ group elements.

We can understand the reactivity of an element as its tendency to lose or gain electrons to attain a completely filled valence shell makes it more stable. However, the ${18^{{\rm{th}}}}$ group elements already have a completely filled valence shell. So, they almost do not show any reactivity and are called inert gases
.
In the given options, ${\rm{He, Xe}}\;{\rm{and}}\;{\rm{Ar}}$ are positioned in the ${18^{{\rm{th}}}}$ group and therefore are inert gases whereas we know how reactive is hydrogen and due to its unique nature it has positioned rather separately from the periodic table.

Hence, option C is the correct one.

 Note: Inert gases are also called noble gases for the same reason as being the least or non-reactive elements. one should be careful that noble and inert gases are different from one another as noble gases do react but inerts do not.