Question

explain why helium is very rare in earth's atmosphere

Answer 1

Being lighter these gases face very low attraction due to the acceleration due to gravity. They have more tendency to escape out of earth's atmosphere.

Even the reason for Mars to have ample of CO2 instead of Oxygen is also the same 'escape of gas' from its atmosphere.

Gas molecules bounce around in our atmosphere. Some of them bounce up. If they are traveling at or greater than the escape velocity (11.3 km/s), they will leave the Earth and not return.

The kinetic energy of a molecule is: KE = 1/2(mv^2)

If we assume that energy is evenly distributed, we can see that a molecule with a lower mass will have a greater velocity and a molecule with a greater mass will have a lower velocity. So, the lighter molecules will have greater velocity and thus be more likely to attain escape velocity and leave.

Nitrogen is 14 times heavier than hydrogen and 3.5 times heavier than helium. Oxygen is 16 time heavier than hydrogen and 4 times heavier than helium. And, actually, for molecules the difference with helium is even greater because oxygen, nitrogen, and hydrogen are all diatomic.