The lightest gas on Earth is:
A.Hydrogen
B.Oxygen
C.Helium
D.Chlorine

Hint:The weight of the elements is dependent on the atomic mass of them, where the atomic mass is the mass of an atom of the element and can be defined as the total mass of the neutrons and the protons present in the nucleus of the atom as the electrons are considered to have negligible mass.

Complete answer:
Among the given elements, the lightest weight is that of hydrogen because its atomic mass is 1 and it has only one proton in its nucleus. In case of oxygen, the atomic mass is 16 and there are 8 protons and 8 neutrons in the nucleus. In case of helium, the atomic mass is 4 and there are 2 protons and 2 neutrons in the nucleus. In case of Chlorine, the atomic mass is 35 and there are 17 protons and 18 neutrons in the nucleus.

Hence, it is clear that the lightest element is hydrogen and the correct answer is option A.

Note:
The fact that hydrogen is the lightest gas on the Earth has been applied in many cases.
Hot air balloons are filled with hydrogen gases due to their light-weight nature and due to the fact that they are flammable. Hydrogen is also used for the hydrogenation of different compounds leading to the formation of different industrial products.
It is used as a coolant in generators due to a number of favourable properties that are a direct result of its lightweight nature.