Question

The lightest gas on earth is:
a.) Hydrogen
b.) Oxygen
c.) Helium
d.) Chlorine

Answer 1

Hint: You should know that it is the most abundant chemical substance in the universe, which also constitutes 75% of all baryonic mass and it was also the first artificially produced in the early 16th century by the reaction of acids on metals.

Complete answer:
Hydrogen is the simplest atom, it has a nucleus with one proton and electron revolving it. The mass of an electron is about 10000 times lesser than that of a proton or neutron which are 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.

But this is when individual atoms are considered, but in natural form hydrogen is found as a diatomic molecule (i.e) two hydrogen atoms bonded together. Hydrogen has less mass because the atoms are smaller with one less proton, electron and neutron. Helium has 4 nucleons and a hydrogen molecule has only 2. So even in molecular form hydrogen is lighter than helium.

Additional Information:
This may create another question in your mind, 'if hydrogen is lighter why don't we use it in weather balloons?', the answer is simple, 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 from the risk.

Note: Hydrogen is the first element in the modern periodic table and has an atomic number of 1. It has a very big disadvantage, it is highly inflammable. Vehicles powered by hydrogen fuel cells emit water vapor and warm air- no polluting emissions.