Which gas is most abundant in air?

Nitrogen is by far the most abundant gas in Earths atmosphere, making up about 78 percent of the air by volume. Oxygen, the gas we depend on for breathing, is second at about 21 percent, and the remaining 1 percent is mostly argon with trace amounts of carbon dioxide, neon and other gases. Nitrogen is relatively unreactive, which is one reason it has accumulated and remained in the atmosphere over billions of years. Although plants and animals need nitrogen to build proteins and DNA, most cannot use atmospheric nitrogen directly; it must first be converted into usable forms by bacteria or by industrial processes.