Nitrogen makes up about 78 percent of dry air.

The percentage is a volume fraction, not a statement about the atmosphere's total mass. For gases compared under the same conditions, volume fractions also closely describe the proportions of molecules, so roughly 78 of every 100 molecules in dry air are nitrogen molecules.

Dry air is a useful reference mixture because humidity changes from place to place and hour to hour. Adding water vapor gives nitrogen a smaller share of the resulting moist mixture, even if the relative proportions among the dry gases remain essentially unchanged.

Atmospheric nitrogen is mostly N₂, with two nitrogen atoms held together by a strong triple bond. That structure makes the gas relatively unreactive under ordinary conditions, unlike many nitrogen compounds that participate readily in biological and environmental processes.

Earth's lower atmosphere is a mixture rather than a single substance. Nitrogen is the largest component, followed by oxygen, then argon at about 0.93 percent of dry air. Carbon dioxide and other trace gases account for much smaller fractions. A trace constituent can still have major effects: abundance alone does not determine a gas's importance for climate, chemistry, or living organisms.

The distinction between nitrogen gas and usable nitrogen is central to the nitrogen cycle. Organisms need nitrogen to build proteins and nucleic acids, but most cannot take atmospheric N₂ and incorporate it directly. Nitrogen fixation converts this relatively stable gas into forms that can enter biological chemistry. Certain microorganisms perform this conversion, and lightning and industrial processes provide additional routes for producing reactive nitrogen compounds.

Plants generally obtain nitrogen through compounds such as nitrate and ammonium rather than by extracting N₂ from surrounding air. Animals acquire nitrogen through food. Other microbial processes return nitrogen to the atmosphere, linking the large gaseous reservoir with soils, water, and living systems. The atmosphere's nitrogen abundance therefore coexists with nitrogen limitation in ecosystems: a large supply is not necessarily a chemically accessible supply.

The 78 percent figure is most useful for the well-mixed lower atmosphere. It should not be treated as an exact recipe for every altitude or every enclosed space. At very high altitudes, atmospheric composition changes, and in occupied or poorly ventilated environments local processes can alter the mixture people actually breathe.

Altitude also illustrates the difference between proportion and pressure. The nitrogen and oxygen fractions can stay broadly similar while total atmospheric pressure falls. Oxygen then has a lower partial pressure, which helps explain why breathing becomes difficult at high elevations without requiring the oxygen percentage to fall dramatically. A composition percentage describes the share of the mixture, not how many gas molecules occupy a particular volume.

Dry-air percentages are a reference composition, not a promise that every breath has precisely the same mixture. Humidity adds water vapor, while local sources and atmospheric mixing can alter trace constituents. The distinction between a bulk atmospheric description and a particular sample matters when interpreting measurements.