Question

Which is the lightest gas?
(A) ${ N }_{ 2 }$
(B) $He$
(C) ${ O }_{ 2 }$
(D) ${ H }_{ 2 }$

Answer 1

Hint: Lighter gas is a gas that has a lower density than normal atmospheric gases and rises above them as a result. The lightest gas consists of only one proton in its nucleus. It is the most abundant chemical substance in the universe, especially in stars and gas giant planets.

Complete step by step answer:
> Well, Hydrogen is the simplest atom; it has a nucleus with one proton and an electron revolving it. Helium on the other hand has 2 protons and 2 neutrons in its nucleus, and 2 electrons revolving it. The mass of an electron is about 10000 times lesser than that of a proton or neutron which is approximately the same size, so we can leave the electrons as they don't contribute much mass.
> Hydrogen has 1 nucleon (proton) and helium has 4 (2 neutrons and 2 protons) so approximately, a helium atom is four times heavier than a hydrogen atom.
Hence, ${ H }_{ 2 }$ is the lightest gas. Therefore, Option D is the required answer.

Note: Well even though hydrogen can be separated by electrolysis of water, it can't be used because hydrogen is highly reactive and will easily combust in the presence of oxygen. We choose helium because it is an inert gas and so saves us the risk!