The gas oxygen (O2), composed of molecules of two oxygen atoms, occupies 21% of the Earth's atmosphere by volume. It is colorless, odorless, and tasteless. Oxygen also comprises 86% of the oceans and 60% of the human body, and is the third most abundant element found in the Sun. Almost all plants and animals require oxygen for respiration to maintain life.
Oxygen is very reactive and oxides of most elements are known. A chemical reaction in which an oxide is formed is known as oxidation. The rate at which oxidation occurs varies with the element with which oxygen is reacting. Rust, or iron oxide, for example forms relatively slowly, over days or weeks. Burning or combustion, however, involves a very rapid oxidation. Carbon in fossil fuels, for example, can be quickly oxidised to carbon monoxide and carbon dioxide, with a considerable amount of heat being given off. We can convert this heat into useful energy for heating, electricity and locomotion.
Within the stratosphere, oxygen molecules combine with free oxygen atoms to form ozone (O3). Ozone absorbs ultraviolet (UV) radiation from the Sun, and protects life on Earth from its damaging effect. Although abundant between 19 and 30 km altitude, the air at these levels in the atmosphere is thin. If all the ozone in the stratosphere was compressed to ordinary atmosphere pressure at ground level, it would occupy a layer only 3 mm thick.