The normal concentration of oxygen in Earth's atmosphere is approximately what percent?

The normal concentration of oxygen in Earth's atmosphere is approximately what percent?

• A. 16
• B. 21
• C. 25
• D. 79

Answer: (B)

Oxygen makes up about 21 percent of Earth's atmosphere by volume at sea level. Dry air is roughly 78–79 percent nitrogen, with about 20.95–21 percent oxygen and small amounts of other gases. That’s why the normal, rounded figure used is 21 percent. The other values don’t fit the actual composition: 16 percent would be too low for a healthy, typical atmosphere, 25 percent would be unusually high, and 79 percent represents nitrogen, not oxygen.

Oxygen makes up about 21 percent of Earth's atmosphere by volume at sea level. Dry air is roughly 78–79 percent nitrogen, with about 20.95–21 percent oxygen and small amounts of other gases. That’s why the normal, rounded figure used is 21 percent. The other values don’t fit the actual composition: 16 percent would be too low for a healthy, typical atmosphere, 25 percent would be unusually high, and 79 percent represents nitrogen, not oxygen.