Composition of the Earth's atmosphere

The Earth's atmosphere is a mixture of gases, not a single gas. Nitrogen and oxygen make up by far the largest parts of the mixture.

Approximately four-fifths (80%) of the atmosphere is nitrogen, and approximately one-fifth (20%) is oxygen. These are rounded proportions, so they do not show the small fraction made up of other gases. These include carbon dioxide, water vapour and noble gases.

The proportions of gases in the atmosphere are not exactly fixed, but its overall composition has remained broadly similar to today for around 200 million years.