Oxygen (from the Greek roots (oxys) (acid, literally "sharp," from the taste of acids) and (-genēs) (producer, literally
begetter) 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 O2. Oxygen 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.
Diatomic oxygen gas constitutes 20.9% of the volume of air.
All major classes of structural molecules in living organisms, such as proteins, carbohydrates, and fats, contain oxygen,
as do the major inorganic compounds that comprise animal shells, teeth, and bone. Oxygen in the form of O2 is
produced from water by cyanobacteria, algae and plants during photosynthesis and is used in cellular respiration for all
complex life. Oxygen is toxic to obligately anaerobic organisms, which were the dominant form of early life on Earth until
O2 began to accumulate in the atmosphere 2.5 billion years ago. Another form (allotrope) of oxygen, 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.
Oxygen was independently discovered by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in
Wiltshire, in 1774, but Priestley is often given priority because his publication came out of print first. The name oxygen
was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston
theory of combustion and corrosion. Oxygen 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 oxygen include
the production of steel, plastics and textiles; rocket propellant; oxygen therapy; and life support in aircraft, submarines,
spaceflight and diving.
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