The principal gas in the Earth's atmosphere is nitrogen, which makes up about 78% of the atmosphere by volume. The second most abundant gas is oxygen, contributing around 21%. Nitrogen plays a critical role in various biological and chemical processes on Earth. It is relatively inert, meaning it does not readily react with other substances under normal conditions, which helps maintain the stability of the atmosphere. The balance of nitrogen and oxygen is essential for sustaining life, as oxygen is vital for respiration in most living organisms, while nitrogen is crucial for plant nutrition through biological fixation.