The main gas present in Earth's atmosphere is nitrogen, which makes up about 78% of the atmosphere by volume. This inert gas is crucial for life on Earth because it is a key component of amino acids, proteins, and nucleic acids, which are essential for all living organisms. However, nitrogen itself is not directly usable by most organisms in its gaseous form, and it must be converted into reactive forms, such as ammonium and nitrates, through biological processes like nitrogen fixation. The atmosphere also contains oxygen (about 21%), argon, carbon dioxide, and trace gases, all of which play vital roles in supporting life and regulating Earth's climate.