Which of the following gaseous molecules is monoatomic?
(A) Chlorine
(B) Helium
(C) Oxygen
(D) Nitrogen

Hint: Have a look at the periodic table. For any gas to exist in a monatomic state, it should have a stable configuration and should be less reactive. Write the electronic configuration of all the four elements given the options and accordingly determine which among them has the most stable configuration and is least reactive.


Complete step by step solution:
- Let’s begin by writing the electronic configurations of the elements given in the options.
- Chlorine has atomic number 17. The electronic configuration of chlorine is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{5}}$.
- Helium has the atomic number 2. The electronic configuration of helium is $1{{s}^{2}}$.
- Oxygen has the atomic number 8. The electronic configuration of oxygen is $1{{s}^{2}}2{{s}^{2}}2{{p}^{4}}$.
- Nitrogen has the atomic number 7. The electronic configuration of nitrogen is $1{{s}^{2}}2{{s}^{2}}2{{p}^{3}}$.
- We know that elements which have ground state stable configuration are least reactive. That is, they are inert. In the periodic table, the last group consists of the inert gas elements.
- In inert gas elements, all the orbitals are completely filled and they do not have valency. They are highly stable and therefore, they are inert.
- Among all the four elements given in the options, chlorine, oxygen and nitrogen exist in a diatomic state because their valence shell is incomplete and in order to be stable they form bonds with themselves.
- Helium has completely filled valence shell. It can’t form anymore covalent bonds due to lack of vacant orbitals. Therefore, it is highly stable and inert and thus, exists in monatomic state.
- Helium is the first element in the group of inert gases or noble gases.
- Therefore, Helium is monoatomic in gaseous state.

- Therefore, option (B) is the correct answer.


Note: Remember that the last group in the periodic table has most stable electronic configuration and thus, high stability. Therefore, they exist in a monatomic state in elemental form. That’s the reason why they are known as inert gas or noble gas elements.