Which gas is most abundant in Earth’s atmosphere?

Nitrogen is the most abundant gas in Earth's atmosphere, comprising about 78% of the atmosphere by volume. This gas plays a crucial role in several biological and chemical processes, including being a key component of amino acids and proteins necessary for life. Although oxygen is essential for respiration in many living organisms and constitutes approximately 21% of the atmosphere, it is not nearly as abundant as nitrogen.

Carbon dioxide, while important for processes such as photosynthesis and as a greenhouse gas, represents only a small fraction of the atmosphere—around 0.04%. Argon, although more abundant than carbon dioxide, makes up less than 1% of the atmosphere. Therefore, nitrogen's dominance in the atmospheric composition is clear, making it the correct answer.