What gas makes up 78% of the air?

The gas that makes up 78% of the air is nitrogen. Nitrogen is a colorless, odorless gas that is essential for the growth of plants and is a critical component of amino acids, which are the building blocks of proteins. In the Earth's atmosphere, nitrogen exists primarily in a diatomic form (N2), which is relatively inert and does not readily react with other substances under normal conditions.

In contrast, oxygen, carbon dioxide, and hydrogen make up much smaller percentages of the atmosphere. Oxygen accounts for about 21% of the air and is necessary for respiration in most living organisms. Carbon dioxide is present in trace amounts, approximately 0.04%, and plays a key role in photosynthesis and the greenhouse effect. Hydrogen, while abundant in the universe, is found only in negligible amounts in the Earth's atmosphere as a result of its low molecular weight, which allows it to escape Earth's gravitational pull. Thus, nitrogen's significant presence in the atmosphere is vital for ecological balance and nutrient cycles.