Question

What percentage of the atmosphere is oxygen?

Answer 1

21%

Explanation:

Dry atmosphere is 78% Nitrogen, 21% oxygen and 0.9% argon. The remaining 0.1% is mostly Carbon Dioxide, the rest is trace gases.

The amount of water vapor in the atmosphere is extremely variable, from near 0 to as high as 4%. This variable is why we normally just talk about dry air.