Which gas makes up the largest share of Earth's atmosphere?

Nitrogen is the dominant gas in Earth’s atmosphere. It makes up about 78% of the air by volume, while oxygen accounts for roughly 21%. The remaining small fraction includes argon, carbon dioxide, water vapour, and trace gases. Nitrogen is relatively unreactive, which helps keep the atmosphere stable over long periods. Living things use nitrogen in essential molecules, but most organisms cannot use atmospheric nitrogen directly without biological or industrial conversion.