What atmospheric gas comprises approximately 21% of the air we breathe?

Oxygen is the atmospheric gas that makes up approximately 21% of the air we breathe. It is essential for the survival of most living organisms on Earth, as it is utilized in cellular respiration to produce energy. The presence of oxygen in the atmosphere is crucial for various biological and ecological processes, including combustion and respiration.

While nitrogen constitutes about 78% of the atmosphere and is the most abundant gas, it does not play a direct role in the respiratory processes of most life forms. Argon, although present in small amounts (around 0.93%), is an inert gas and does not participate in biological processes. Carbon dioxide, which is a trace gas making up about 0.04% of the atmosphere, is important for plant photosynthesis but is not utilized in the same way as oxygen in respiration. Therefore, it is oxygen that holds a distinct and critical role in sustaining life, leading to the conclusion that it comprises around 21% of the air we breathe.