The Earth’s atmosphere is a layer of gas held in place by gravity, which prevents it from escaping into space. It protects life by absorbing UV radiation, by holding in heat to warm the Earth’s surface and by reducing temperature extremes 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.

It’s a common misconception that oxygen is the most abundant gas in the air breathed on Earth; that honor goes to nitrogen, which makes up 78 percent of the air. Nitrogen occurs as N2 — two nitrogen atoms bonded together. The bond is very strong, making the gas chemically inert. Although inhaled nitrogen passes into the bloodstream, it is not used by the cells in the body. However, 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 percent of the air all living things breathe, oxygen is absorbed by the lungs, or lung-like structures in lower animals, and transported to all cells in the body by the blood. Oxygen is the most unstable, and therefore the most chemically active, gas found in air. Although all animals need oxygen, it can be deadly in higher-than-normal concentrations: Breathing pure oxygen for extended periods leads to oxygen toxicity. In addition to its role in biology, oxygen is essential for combustion, the chemical process responsible for fire.

The third-most abundant gas in the air on Earth is argon, although it makes up less than 1 percent of air. Argon is classified as a noble gas in chemistry, meaning it is very stable and seldom reacts with other compounds. The argon in the air comes mainly from the decay of potassium-40, a radioactive 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 vapor
  
• carbon dioxide
  
• methane
  
• helium
  
• hydrogen
  
• ozone

These gases each have their own purpose and forms of production. Methane, for example, is a powerful greenhouse gas, trapping heat in the earth’s atmosphere. Ozone is found in two distinct layers of the atmosphere: high in the stratosphere, where it blocks harmful ultraviolet light from the sun, and the lower atmosphere, where it is one of the components of smog.