The primary gas in Earth's atmosphere is nitrogen, comprising about 78% of the atmosphere by volume. This inert gas is essential for life, although it is not in a form that organisms can readily use. Nitrogen plays a crucial role in the nitrogen cycle, where it is converted into a usable form via processes like nitrogen fixation through bacteria and lightning. Oxygen follows nitrogen, making up about 21% of the atmosphere. The balance of gases in Earth’s atmosphere is vital for sustaining life, regulating climate, and supporting various biological processes.