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

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

Answer: (A)

Dominant gas in Earth's atmosphere is determined by relative abundance. In the air we breathe, nitrogen makes up about 78 percent, with oxygen around 21 percent and the rest consisting of argon, carbon dioxide, and other trace gases. Nitrogen stays the most abundant because it is chemically inert under many surface conditions, so it doesn’t readily form compounds that remove it from the atmosphere. This stability lets nitrogen persist and accumulate over geological time, even as oxygen is continually produced and consumed in life and weathering processes. That combination of high abundance and low reactivity is why nitrogen is the largest component of the atmosphere.