Composition of Air

Average composition of earth's atmosphere ( up to an altitude of 25 km)

• Nitrogen                 78.09%
• Oxygen                   20.95%
• Argon                       0.93%
• Carbon dioxide        0.039%

and trace amount of other gases including Neon, Helium, Methane, Krypton, Hydrogen, 

Nitrous oxide, Carbon monoxide, Xenon, Ozone, Nitrogen dioxide, Iodine, Ammonia etc.  

Schematic diagram

Courtesy: bbc.co.uk