The Chemical Composition of Air

Atmosphere
99.998% of air consists of only four compounds. Tetra Images / Getty Images

Nearly all of the Earth's atmosphere is made up of only five gases: nitrogen, oxygen, water vapor, argon, and carbon dioxide. Several other compounds also are present. Although this CRC table does not list water vapor, air can contain as much as 5% water vapor, more commonly ranging from 1-3%. The 1-5% range places water vapor as the third most common gas (which alters the other percentages accordingly).

Below is the composition of air in percent by volume, at sea level at 15 C and 101325 Pa.

Nitrogen -- N2 -- 78.084%

Oxygen -- O2 -- 20.9476%

Argon -- Ar -- 0.934%

Carbon Dioxide -- CO2 -- 0.0314%

Neon -- Ne -- 0.001818%

Methane -- CH4 -- 0.0002%

Helium -- He -- 0.000524%

Krypton -- Kr -- 0.000114%

Hydrogen -- H2 -- 0.00005%

Xenon -- Xe -- 0.0000087%

Ozone -- O3 -- 0.000007%

Nitrogen Dioxide -- NO2 -- 0.000002%

Iodine -- I2 -- 0.000001%

Carbon Monoxide -- CO -- trace

Ammonia -- NH3 -- trace

Reference: CRC Handbook of Chemistry and Physics, edited by David R. Lide, 1997.