Every breath you take is a mix of gases. Almost all of it is two small molecules, and the rest is a handful of others in tiny amounts.
Chemists describe air by counting its molecules. Set water vapour aside for a moment, and in every 100 molecules of dry air, 78 are nitrogen, N₂, and 21 are oxygen, O₂. Nearly all of the last one is argon, Ar, a gas of single atoms that bond to nothing.
Nitrogen's two atoms are held by a triple bond, one of the strongest bonds there is, so nitrogen hardly reacts. Oxygen is the gas that burning and breathing use up, turning carbon into carbon dioxide.
Small amounts
The smallest amounts are counted in parts per million, ppm: molecules in every million molecules of air. In 2025 dry air held about 426 ppm of carbon dioxide, which is 0.0426%, and about 1.9 ppm of methane.
| Gas | Share |
|---|---|
| nitrogen, N₂ | 78.08% |
| oxygen, O₂ | 20.95% |
| argon, Ar | 0.93% |
| carbon dioxide, CO₂ | 0.0426%, or 426 ppm (2025) |
| neon, Ne | 18 ppm |
| methane, CH₄ | 1.9 ppm (2025) |
Water vapour comes and goes with the weather. It makes up about 1% of the air on average: more on a warm, humid day, and less on a cold, dry one. Water vapour, carbon dioxide and methane are a small part of the air, and they have a large effect on the temperature of the whole planet. The next section shows how.