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

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

Answer: (B)

Nitrogen is the most abundant gas in Earth's atmosphere because it is chemically very stable in air and doesn’t readily react with other substances at atmospheric conditions. That stability means nitrogen remains in the gaseous form for long periods and isn’t easily removed or transformed, so it stays as the dominant component. In dry air, nitrogen makes up about 78 percent, while oxygen is around 21 percent, and the remaining fraction consists of trace gases like argon and carbon dioxide. Oxygen is more reactive and continually cycled through living systems and combustion, which is why its share is much smaller than nitrogen. Argon and carbon dioxide occur only in much smaller amounts.