The dominant gas in Earth's atmosphere is nitrogen, which comprises approximately 78% of the atmosphere by volume. Nitrogen plays a crucial role in supporting life, yet it is inert and does not directly contribute to biological processes. The second most abundant gas is oxygen, accounting for roughly 21%. The balance of these gases is essential for maintaining ecosystems and atmospheric stability, allowing for processes such as respiration and photosynthesis to occur, which are vital for sustaining life on Earth.