Light gases like hydrogen and helium are very rare in the earth's atmosphere why?

Explanation

The escape velocity from the earth.s surface is `11.2 Km.s^-1`, approximately .At the upper temperature it is still lower. The atmospheric temperature was very high at the time of formation of the earth. At that temperature the rms speed of hydrogen gas molecules was `5 km.s^-1` or higher, As this is an average velocity , a large number of molcules were moving with velocities higher than the escape velocity . As a result, those molecules left the earth.s field of gravity forever. This incident occured over a long period of time. So light gases like hydrogen and helium are rare in the earth.s atmosphere.