What is the most abundant gas in Earth's atmosphere?

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

Answer: (A)

Understanding the composition of Earth's atmosphere by volume helps explain why nitrogen is the most abundant gas. In dry air, about 78 percent is nitrogen and roughly 21 percent is oxygen. The rest—argon, carbon dioxide, neon, helium, methane, and other trace gases—adds up to just a small fraction, with carbon dioxide around 0.04 percent and argon near 0.93 percent. Nitrogen’s dominance comes from its diatomic N2 molecule, which is held together by a very strong triple bond and is chemically quite inert, so it doesn’t react away quickly and remains plentiful. Oxygen is also abundant, but it participates in cycles of production and consumption that keep its level high yet not as dominant as nitrogen. Water vapor can be highly variable and, in some conditions, can exceed nitrogen locally, but it is not part of the standard dry-air composition. So, nitrogen is the most abundant gas in Earth's atmosphere.