Definition of composition of air: Earth’s atmosphere  consists of a mixture of gases called the air. The gases are usually odourless and colourless. As a result, we cannot touch or see the air. We can only feel the air. The earth’s atmosphere consists of  78% of Nitrogen, 21% of oxygen, 0.93%  of Argon, 0.04% of Carbon dioxide, and traces of helium, methane, neon, krypton and hydrogen, as well as water vapour. This is also known as Air composition of Earth.