Nitrogen makes up roughly 78% of Earth’s atmosphere by volume.
Most atmospheric nitrogen exists as diatomic nitrogen, written as N₂, meaning each molecule contains two nitrogen atoms joined by a strong triple bond. Oxygen accounts for about 21% of the atmosphere, while argon contributes just under 1%; carbon dioxide and other gases make up most of the remainder.
Nitrogen’s abundance does not mean it is immediately usable by most organisms. The strong N₂ bond makes atmospheric nitrogen relatively unreactive. Plants and animals generally depend on nitrogen compounds made available through biological nitrogen fixation, lightning or industrial processes. Certain bacteria convert N₂ into forms plants can use, beginning the nitrogen cycle.
A frequent mistake is to confuse atmospheric percentage by volume with the composition of dry air under every condition. Water vapor varies greatly with location and weather, so humid air contains a smaller percentage of nitrogen than dry air. The familiar 78% figure refers approximately to dry air near Earth’s surface.