Which gas makes up nearly 21% of Earth's atmosphere?

Which gas makes up nearly 21% of Earth's atmosphere?

• A. Helium
• B. Nitrogen
• C. Argon
• D. Oxygen

Answer: (D)

Understanding which gas constitutes about 21% of Earth's atmosphere helps you see how air is composed. The atmosphere is mostly nitrogen, making up roughly three-quarters of the air, while the remaining portion is mainly oxygen at about one-fifth. The rest consists of trace amounts of other gases like argon, carbon dioxide, neon, and a tiny amount of helium. So the gas that sits around the 21% mark is oxygen. This matters because oxygen supports most life and is involved in combustion, highlighting why it’s such a substantial component of dry air.

Understanding which gas constitutes about 21% of Earth's atmosphere helps you see how air is composed. The atmosphere is mostly nitrogen, making up roughly three-quarters of the air, while the remaining portion is mainly oxygen at about one-fifth. The rest consists of trace amounts of other gases like argon, carbon dioxide, neon, and a tiny amount of helium. So the gas that sits around the 21% mark is oxygen. This matters because oxygen supports most life and is involved in combustion, highlighting why it’s such a substantial component of dry air.