<unk>   is the element with atomic number 8 and represented by the symbol O. It is a member of the chalcogen group on the periodic table, and is a highly reactive nonmetallic period 2 element that readily forms compounds (notably oxides) with almost all other elements. At standard temperature and pressure two atoms of the element bind to form dioxygen, a colorless, odorless, tasteless diatomic gas with the formula<unk> .<unk>  is the third most abundant element in the universe by mass after hydrogen and helium and the most abundant element by mass in the Earth's crust.<unk>  constitutes 88.8% of the mass of water and 20.9% of the volume of air.
All major classes of structural molecules in living organisms, such as proteins, carbohydrates, and fats, contain<unk> , as do the major inorganic compounds that comprise animal shells, teeth, and bone.<unk>  in the form of<unk>  is produced from water by cyanobacteria, algae and plants during photosynthesis and is used in cellular respiration for all complex life.<unk>  is toxic to anaerobic organisms, which were the dominant form of early life on Earth until<unk>  began to accumulate in the atmosphere 2.5 billion years ago. Another form (allotrope) of<unk> , ozone (O3), helps protect the biosphere from ultraviolet radiation with the high-altitude ozone layer, but is a pollutant near the surface where it is a by-product of smog.<unk>  was independently discovered by Joseph Priestley and Carl Wilhelm Scheele in the 1770s, but Priestley is usually given priority because he published his findings first. The name<unk>  was coined in 1777 by Antoine Lavoisier, whose experiments with<unk>  helped to discredit the then-popular phlogiston theory of combustion and corrosion.<unk>  is produced industrially by fractional distillation of liquefied air, use of zeolites to remove carbon dioxide and nitrogen from air, electrolysis of water and other means. Uses of<unk>  include the production of steel, plastics and textiles; rocket propellant;<unk>  therapy; and life support in aircraft, submarines, spaceflight and diving.
Oxygen
Calfactant
Anethole
Nitrogen oxide
Alanycarb
Combivent
Methoxyflurane
Enflurane