Which gas makes up the largest share of Earth's atmosphere?

Nitrogen makes up the largest share of Earth’s atmosphere.

Dry air near Earth’s surface contains about 78 percent nitrogen by volume. Oxygen accounts for about 21 percent, while argon and carbon dioxide make up most of the remaining fraction. Water vapour varies considerably with location and weather, so it is usually listed separately when describing the composition of dry air.

Nitrogen is a relatively unreactive gas under ordinary conditions, but it is essential to life. Plants need biologically available nitrogen to build proteins and nucleic acids. Most organisms cannot use atmospheric nitrogen directly, so bacteria and industrial processes convert it into useful compounds.

A frequent mistake is to identify oxygen as the main atmospheric gas because animals need it for respiration. Oxygen is vital, but nitrogen is more abundant. Atmospheric composition also differs from the composition of the entire planet, whose interior contains many heavier elements and compounds.