Schematic diagram of oxygen atom.

A chemical element with atomic number 8. Oxygen is a colorless, odorless gas essential for almost all life forms on Earth. It is the most abundant element on Earth and the third most abundant element in the universe. As a highly reactive element, oxygen forms oxides with most elements as well as with other compounds. It is, for example, part of the water molecule.

Animals take in oxygen when they breathe, use it to extract energy from food, and release carbon dioxide. In contrast, plants take in carbon dioxide and release oxygen during photosynthesis. In the Earth’s atmosphere, 21 per cent of it is oxygen.

Combustion involves a chemical reaction between oxygen and carbon in a fuel such as coal or wood. A large amount of heat is released, and carbon dioxide is produced.