Which gas is most abundant in Earth's atmosphere?

• A. Oxygen
• B. Argon
• C. Nitrogen
• D. Carbon dioxide

Answer: (C)

Nitrogen is the most abundant gas in Earth's atmosphere, making up about 78% of the air by volume, with oxygen around 21% and the rest consisting of minor gases like argon and carbon dioxide. Nitrogen’s chemical inertness means it doesn’t readily react to form solids or dissolve into water, so it stays in the gaseous form for long timescales and accumulates as the atmosphere formed and evolved. In contrast, oxygen is constantly involved in biological and geological processes, being produced by photosynthesis but also consumed by respiration, combustion, and weathering, which keeps its atmospheric level near the 21% mark. The remaining gases are present only in trace amounts, which is why nitrogen dominates the composition.