Composition of air: the gases in Earth's atmosphere by percent
Dry air at sea level is a mixture of gases, mostly nitrogen and oxygen. The chart shows each gas with its formula and its percentage by volume.
| Name | Formula | % vol |
|---|---|---|
| Nitrogen | N₂ | 78.084 |
| Oxygen | O₂ | 20.946 |
| Argon | Ar | 0.934 |
| Carbon dioxide | CO₂ | 0.042 |
| Neon | Ne | 0.001818 |
| Helium | He | 0.000524 |
| Methane | CH₄ | 0.00019 |
| Krypton | Kr | 0.000114 |
| Hydrogen | H₂ | 0.00005 |
Dry air at sea level. Water vapor varies from about 0–4%. Carbon dioxide is rising by roughly 2–3 ppm per year.
What the chart shows
Nitrogen (N₂) makes up 78.084% of dry air and oxygen (O₂) 20.946%, so together they account for about 99%. Argon (Ar), at 0.934%, is nearly all of the rest. Carbon dioxide is about 0.042%, or 420 parts per million (ppm), and neon, helium, methane, krypton and hydrogen are present only in traces.
How to read the percentages
The values are percentages by volume. For gases, volume percent equals mole percent, because equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. The figures are for dry air; real air also contains from almost 0% to about 4% water vapor, which lowers the other percentages slightly.
Why the composition matters
Oxygen supports breathing and burning, and nitrogen dilutes it; a fire burns far more fiercely in pure oxygen. Argon is unreactive, so it shields metals during welding. It is common because potassium-40 in rocks slowly decays into argon-40.
Carbon dioxide and methane are only small fractions, but they are greenhouse gases that trap heat, and carbon dioxide is rising by about 2 to 3 ppm per year, mainly from burning fossil fuels. Industry separates nitrogen, oxygen and argon by cooling air until it liquefies and then distilling it.
Worked example
Dalton's law says each gas contributes to the total pressure in proportion to its share. At a sea-level pressure of 101.325 kPa, the partial pressure of oxygen is 0.20946 × 101.325 kPa = 21.22 kPa. In the same way, a 10.0 L sample of dry air contains about 7.81 L of nitrogen, 2.09 L of oxygen and 0.0934 L of argon.
FAQ
What percentage of air is oxygen?
Oxygen makes up 20.946% of dry air by volume, usually rounded to 21%. The share is almost the same at the top of Mount Everest; what falls with altitude is the total pressure, so each breath holds less oxygen.
What is the most abundant gas in the atmosphere?
Nitrogen, N₂, at 78.084% of dry air by volume. It is fairly unreactive because the triple bond joining its two atoms is very strong.
What is the molar mass of air?
The average molar mass of dry air is about 28.97 g/mol, found by weighting each gas's molar mass by its mole fraction. Gases with a lower molar mass, such as helium at 4.003 g/mol, rise in air.
Is air a compound or a mixture?
Air is a mixture. Its gases are not chemically bonded to each other, its composition varies, for example in water vapor, and it can be separated by physical methods such as fractional distillation of liquid air.