Q. What is the reason that despite being third most abundant element on the Earth’s surface and most abundant chemical substance in the universe, Hydrogen is not found as a gas in Earth’s atmosphere?  
Answer: It is so light that readily escapes earth's gravity
Notes: Under ordinary conditions on Earth, elemental hydrogen exists as the diatomic gas, H2. However, hydrogen gas is very rare in the Earth’s atmosphere (1 ppm by volume) because of its light weight, which enables it to escape from Earth’s gravity more easily than heavier gases. However, hydrogen is the third most abundant element on the Earth’s surface, mostly in the form of chemical compounds such as hydrocarbons and water. Hydrogen gas is produced by some bacteria and algae and is a natural component of flatus, as is methane, itself a hydrogen source of increasing importance.