What is the approximate composition of Earth's atmosphere?

What is the approximate composition of Earth's atmosphere?

• A. 78% N, 21% O2, 1% Argon
• B. 21% N, 78% O2, 1% Argon
• C. 78% Argon, 21% N2, 1% O2
• D. 60% N, 40% O2, 0% Argon

Answer: (A)

Earth’s atmosphere is dominated by two gases: nitrogen and oxygen. In dry air, nitrogen makes up about 78 percent, oxygen about 21 percent, and a small amount of argon — around 1 percent. That’s why the statement listing roughly 78% nitrogen, 21% oxygen, and 1% argon fits the known composition best. The other options misstate the relative amounts (for example, oxygen far exceeds nitrogen in one, or nitrogen and oxygen percentages are swapped) and don’t reflect argon’s minor presence. Remember that trace gases and water vapor vary with conditions, but the dry-air mix is well approximated by ~78% N2, ~21% O2, ~1% Ar.

Earth’s atmosphere is dominated by two gases: nitrogen and oxygen. In dry air, nitrogen makes up about 78 percent, oxygen about 21 percent, and a small amount of argon — around 1 percent. That’s why the statement listing roughly 78% nitrogen, 21% oxygen, and 1% argon fits the known composition best. The other options misstate the relative amounts (for example, oxygen far exceeds nitrogen in one, or nitrogen and oxygen percentages are swapped) and don’t reflect argon’s minor presence. Remember that trace gases and water vapor vary with conditions, but the dry-air mix is well approximated by ~78% N2, ~21% O2, ~1% Ar.