What Is the Chemical Composition of Air?

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).

This is 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.