What is the most common constituent of our air?

Nitrogen is the most abundant component of Earth's atmosphere, comprising about 78% of the air we breathe. It plays a critical role in supporting life, yet it is often overlooked due to its inert nature and lack of reactivity compared to oxygen. Nitrogen contributes to the stability of the atmosphere and is essential in the nitrogen cycle, impacting various biological processes.

In contrast, while oxygen is vital for respiration and makes up about 21% of the atmosphere, it is not the predominant component. Carbon dioxide is present in much smaller amounts (about 0.04%) and is crucial for photosynthesis but does not compare to the relative abundance of nitrogen. Helium is a trace gas in the atmosphere and constitutes less than 0.001% of the air. Thus, nitrogen stands out as the primary constituent of our atmosphere, making it the correct choice.