oxygen

Colourless, odourless, tasteless, non-metallic, gaseous element, atomic number 8, relative atomic mass 15.9994. It is the most abundant element in the Earth's crust (almost 50% by mass), forms about 21% by volume of the atmosphere, and is present in combined form in water and many other substances. Oxygen is a by-product of photosynthesis and the basis for respiration in plants and animals.

Oxygen is very reactive and combines with all other elements except the noble gases (rare gases) and fluorine. Combustion (burning) and rusting are two examples of reactions involving oxygen. It is present in carbon dioxide, silicon dioxide (quartz), iron ore, calcium carbonate (limestone). In nature it exists as a molecule composed of two atoms (O₂); single atoms of oxygen are very short-lived owing to their reactivity. They can be produced in electric sparks and by the Sun's ultraviolet radiation in the upper atmosphere, where they rapidly combine with molecular oxygen to form ozone (O₃), an allotrope of oxygen.

Oxygen is obtained for industrial use by the fractional distillation of liquid air, by the electrolysis of water, or by heating manganese(IV) oxide with potassium chlorate. In the laboratory it is prepared by the action of the catalyst manganese(IV) oxide on hydrogen peroxide. The simple laboratory test for oxygen is that it relights a glowing spill. Oxygen is essential for combustion, and is used with ethyne (acetylene) in high-temperature oxyacetylene welding and cutting torches.

The element was first identified by English chemist Joseph Priestley in 1774 and independently in the same year by Swedish chemist Karl Scheele. It was named by French chemist Antoine Lavoisier in 1777.