Which gas is most common in the air?

Although we breathe air mainly for its oxygen, the most abundant gas in Earth atmosphere is actually nitrogen, making up about 78% of the air by volume. Oxygen comes second at roughly 21%, with argon, carbon dioxide, and trace gases making up the remaining 1%. Nitrogen is relatively inert, so it passes in and out of our lungs largely unused, while our bodies extract the oxygen we need. Plants and certain bacteria play a vital role in converting nitrogen into forms life can use.