Which gas composes about 21% of the atmosphere?

Oxygen is a vital component of Earth's atmosphere, making up approximately 21% of the total volume of gases present. This gas is essential for the survival of most living organisms, as it is used for respiration in animals and is also a product of photosynthesis in plants. The presence of oxygen in the atmosphere supports a wide range of biological processes, including combustion and cellular respiration, making it crucial to life on Earth.

While nitrogen constitutes the bulk of the atmosphere at about 78%, and carbon dioxide and argon make up much smaller percentages (around 0.04% and 0.93%, respectively), oxygen's proportion is significant in enabling aerobic life and various ecological processes.