Earth's atmosphere is made up of a layer of gas. It is held in place by gravity, preventing it from flying away into space. The Earth's atmosphere protects life by absorbing UV radiation, holding in heat to warm earth's surface and reducing temperature extremes that can occur between day and night. The gases that comprise the atmosphere are commonly referred to as air, which is what all living things on earth breathe.
Many people mistakenly believe that oxygen is the most abundant gas in the air breathed on Earth, but that honour actually goes to nitrogen, which makes up 78 per cent of the air. Nitrogen occurs as N2 --- two nitrogen atoms bonded together. This bond is so strong that nitrogen is chemically inert. The nitrogen animals breathe is exhaled right back out without being absorbed. Since nitrogen is essential for life --- it is found in RNA, DNA and proteins --- it must be converted to compounds with less stable bonds to be used by animals. One way this happens is through nitrogen fixation in plants.
Making up almost 21 per cent of the air all living things breathe, oxygen is absorbed by the lungs, or lung-like structures in lower animals, and transported to all the cells in the body by the blood. Oxygen is the most unstable, and therefore the most chemically active, gas found in air. While oxygen is a requirement for all animals, some may wonder why the atmosphere doesn't contain a higher concentration. There are two reasons for this: Breathing pure oxygen for extended periods of time leads to oxygen toxicity; and oxygen is highly flammable, so increasing the concentration in the air would increase air's flammability.
The third-most abundant gas in the air on Earth is argon, although it makes up less than 1 per cent of air. Argon is classified as a noble gas in chemistry, meaning it is fairly stable and does not react easily with other compounds. The argon in the air comes mainly from the decay of a certain potassium isotope in the Earth's crust. The bulk of argon used in science is acquired by fractional distillation of air in its liquid form.
There are several additional gases present in the atmosphere in minute amounts. These gases are referred to as trace gases and include water vapour, carbon dioxide, methane, helium, hydrogen and ozone. These gases each have their own purpose and forms of production. Methane, for example, is a strong greenhouse gas, meaning it helps trap heat in the earth's atmosphere. Ozone is found in two distinct layers of the atmosphere: the stratosphere, where it forms a protective layer, and the lower atmosphere, where it is actually one of the byproducts of smog.