 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 . 
