Which gas makes up the majority of Earth's atmosphere?

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

Answer: (C)

The main idea here is knowing which gas dominates Earth's atmosphere. Nitrogen makes up the majority, about 78% of the air, while oxygen is the next most abundant at roughly 21%. The remaining portion consists of trace gases such as argon and carbon dioxide. Nitrogen stays so plentiful because it is chemically inert under typical Earth-surface conditions, so it doesn’t react away quickly and remains in the atmosphere. Oxygen, while essential for most life and combustion, is constantly being used up and produced by biological and photochemical processes, so it never reaches the same overwhelming share as nitrogen. That combination of high abundance and low reactivity is why nitrogen is the dominant gas.