why abundance of hydrogen in the earth's atmosphere is very small
Answers
Because hydrogen gas is so light, most of it escaped from the lower atmosphere
Hydrogen is the abundant in the Sun and in the large outer planets and hydrogen is the most abundant element in the universe as a whole, so at first sight it may seem strange that there is no significant gaseous hydrogen in earth’s atmosphere.
*The primary reason is the ‘lightness’ of hydrogen. hydrogen is the lightest of the elements.
*The lighter the molecules of a gas are, the faster the molecules can move. In the case of hydrogen in the atmospheric temperature range, some gaseous hydrogen molecules will move fast enough to exceed the earth’s escape velocity and can thus escape from the earth’s atmosphere into space. Over millions of years, earth’s gaseous hydrogen has escaped in this way.
Molecules of heavier gases (like oxygen and nitrogen) move more slowly, so very few if any reach the escape velocity, so the gases are retained in the atmosphere by earth’s gravity.
Bodies which are much more massive than the earth, such as the Sun and the Gas giant planets. Have a much larger escape velocity and so can manage to retain even the light, faster moving hydrogen.
*Just one other point to make is that there is of course a lot of hydrogen on earth in many chemical compounds. Water is one of these and as there is water vapour in the atmosphere, there is hydrogen in the atmosphere - but just not as free gaseous hydrogen.
Your question was about gaseous hydrogen and hopefully the points above answer your question as to why there is effectively none in earth’s atmosphere today.
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