Oxygen is a chemical element wi symbolO an atomic nummer 8. It is a member o the chalcogen group on the periodic cairt an is a heichly reactive nonmetal an oxidisin agent that readily forms oxides wi maist elements as well as ither compoonds.^[2] Bi mass, oxygen is the third-maist abundant element in the universe, efter hydrogen an helium.^[3] At staundart temperatur an pressur, twa atoms o the element bind tae form dioxygen, a colourless an odourless diatomic gas wi the formula O
₂. This is an important pairt o the atmosphere an diatomic oxygen gas constitutes 20.8% o the Yird's atmosphere.^[4] Addeetionally, as oxides the element maks up awmaist hauf o the Yird's crust.^[5]

Dioxygen is uised in cellular respiration an mony major classes o organic molecules in livin organisms conteen oxygen, sic as proteins, nucleic acids, carbohydrates, an fats, as dae the major consteetuent inorganic compoonds o ainimal shells, teeth, an bane. Maist o the mass o livin organisms is oxygen as a component o watter, the major constituent o lifeforms. Conversely, oxygen is continuously replenished bi photosynthesis, which uises the energy o sunlicht tae produce oxygen frae watter an carbon dioxide. Oxygen is ower chemically reactive tae remeen a free element in air withoot bein continuously replenished bi the photosynthetic action o livin organisms. Anither form (allotrope) o oxygen, ozone (O
₃), strangly absorbs ultraviolet UVB radiation an the heich-altitude ozone layer helps pertect the biosphere frae ultraviolet radiation. But ozone is a pollutant near the surface whaur it is a bi-product o smog. At law yird orbit altitudes, sufficient atomic oxygen is present tae cause corrosion o spacecraft.^[6]

Oxygen wis discovered independently bi Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, an Joseph Priestley in Wiltshire, in 1774, but Priestley is eften gien priority acause his wark wis published first. The name oxygen wis coined in 1777 bi Antoine Lavoisier,^[7] whase experiments wi oxygen helped tae discredit the then-popular phlogiston theory o combustion an corrosion. Its name derives frae the Greek roots ὀξύς oxys, "acid", leeterally "sharp", referrin tae the soor taste o acids an -γενής -genes, "producer", leeterally "begetter", acause at the time o namin, it wis mistakenly thocht that aw acids required oxygen in thair composeetion.

Common uise o oxygen includes residential heatin, internal combustion ingines, production o steel, plastics an textiles, brazin, waldin an cuttin o steels an ither metals, rocket propellant, oxygen therapy, an life support seestems in aircraft, submarines, spaceflicht an divin.