Oxygen is the most common chemical element found on or in Earth. It is one of the main elements that make up air, and it is necessary for the survival of all plants and animals. Scientists use symbols to stand for the chemical elements. The symbol for oxygen is O.
Oxygen makes up about one fifth of the atmosphere.
In its free state, oxygen is a gas without color, odor, or taste. It turns into a pale blue liquid at temperatures below –297° F (–183° C) and becomes solid at about –360° F (–218° C). Oxygen combines easily with many other elements to form substances called compounds. The most common compound is water, which is two parts hydrogen to one part oxygen.
All animals need oxygen to stay alive. When they breathe in air, the oxygen in the air combines with the elements carbon and hydrogen in the body. This releases energy that the animals use to survive. The animals then breathe out a compound of carbon and oxygen called carbon dioxide. Plants take in that carbon dioxide as part of a process called photosynthesis.