Question

Which of the following gas is/are called rare gas.
A. Ne
B. He
C. Kr
D. All of these

Answer 1

Hint: Rare gases have very low concentrations in the atmosphere. Also, the chemical reactivity of these rare gas elements is very low. They show less chemical reactivity towards outer elements due to stable electronic configuration.

Complete Step by Step Solution:
On the extreme right of the periodic table, elements present in group 18 are called rare gases. All of the elements in group 18 have complete outer valence shell electronic configuration. The outer shell configuration for group 18 elements is represented as \[{\rm{n}}{{\rm{s}}^2}{\rm{n}}{{\rm{p}}^6}\]. The elements present in group 18 also called as noble gases are as follows:
1) Helium (He) (Z=2) which has 2 electrons in its valence shell and electronic configuration is \[{\rm{1}}{{\rm{s}}^2}\].
2) Neon (Ne) (Z=10), which has 8 electrons in its valence shell and electronic configuration is [He]\[{\rm{2}}{{\rm{s}}^2}{\rm{2}}{{\rm{p}}^6}\].
3) Argon (Ar) (Z=18) which has 8 electrons in its valence shell and electronic configuration is [Ne]\[{\rm{3}}{{\rm{s}}^2}{\rm{3}}{{\rm{p}}^6}\].
4) Krypton (Kr) (Z=36) which has 18 electrons in its valence shell and electronic configuration is [Ar] \[{\rm{3}}{{\rm{d}}^{10}}{\rm{4}}{{\rm{s}}^2}{\rm{4}}{{\rm{p}}^6}\].
5) Xenon (Xe) (Z=54) which has 18 electrons in its valence shell and electronic configuration is [Kr]\[{\rm{4}}{{\rm{d}}^{10}}{\rm{5}}{{\rm{s}}^2}{\rm{5}}{{\rm{p}}^6}\].
6) Radon (Rn) (Z=86) which has 32 electrons in its valence shell and electronic configuration is [Xe]\[{\rm{6}}{{\rm{s}}^2}{\rm{4}}{{\rm{f}}^{14}}{\rm{5}}{{\rm{d}}^{10}}{\rm{6}}{{\rm{p}}^6}\].

Due to the complete valence shell their ability to undergo chemical reactions is very-very less. The complete valence shell is stable and there is no need to gain or lose electrons. Therefore, there is a lack of chemical reactivity in these elements. These elements are mainly found as monoatomic. Also, their concentration is very low in the atmosphere (<1%) hence these are called rare gases.
So, option D is correct

Additional Information: Group 18 elements are also termed as zero group elements, noble gases, inert gases or rare gases.

Note: There are also rare earth elements in the periodic table. These are 17 elements such as Scandium, Yttrium and Lanthanide series from Lanthanum to Lutetium.