What is the approximate composition of Earth's atmosphere by percent?

• A. 84% nitrogen; 12% oxygen; 4% argon
• B. 78% nitrogen; 21% oxygen; 1% other
• C. 65% nitrogen; 25% oxygen; 10% other
• D. 72% nitrogen; 26% oxygen; 2% other

Answer: (B)

The main idea is that dry air at sea level is dominated by two gases, nitrogen and oxygen, with only a small amount of other gases. The standard approximate composition is about 78% nitrogen, 21% oxygen, and around 1% other gases (argon, carbon dioxide, neon, helium, methane, etc.). Water vapor varies with humidity and altitude, so the exact percentages can shift, but the 78/21/1 split for dry air is the common reference. The other options misstate these proportions, making the nitrogen, oxygen, or the “other” portion inaccurate. So the best match is roughly 78% nitrogen, 21% oxygen, 1% other.