Name, symbol: oxygen, O
Element category: nonmetal, chalcogen


Discovery    C. Scheele (1772)
Named by    A. Lavoisier (1777)

Physical properties

Phase    gas
Density    (0 °C, 101.325 kPa) 1.429 g/L
Liquid density at b.p.    1.141 g•cm−3
Melting point    54.36 K, -218.79 °C, -361.82 °F
Boiling point    90.20 K, -182.95 °C, -297.31 °F
Critical point    154.59 K, 5.043 MPa
Heat of fusion    (O2) 0.444 kJ•mol−1
Heat of vaporization    (O2) 6.82 kJ•mol−1
Molar heat capacity    (O2) 29.378 J•mol−1•K−1


Having the symbol O and the atomic number 8, oxygen is the most known chemical element, whose name has Greek roots. It derives from ὀξύς (oxys) (“acid”, literally “sharp”, referring to the sour taste of acids) and -γόνος (-gοnos) (“producer”, literally “begetter”), because at the time it was named,  it was mistakenly thought that all acids required oxygen in their composition. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a very pale blue, odorless, tasteless diatomic gas with the formula O2.  Oxygen is a very important part of the atmosphere and is necessary to sustain terrestrial life.

Oxygen is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element that readily forms compounds (notably oxides) with almost all other elements. It  is a strong oxidizing agent and has the second-highest electronegativity of all reactive elements, second only to fluorine. If judging by its mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium and the most abundant element by mass in the Earth’s crust, making up almost half of the crust’s mass.Free oxygen is too chemically reactive to appear on Earth without the photosynthetic action of living organisms, which use the energy of sunlight to produce elemental oxygen from water. Elemental O2 only began to accumulate in the atmosphere after the evolutionary appearance of these organisms, roughly 2.5 billion years ago.Diatomic oxygen gas constitutes 20.8% of the volume of air.

Because most of its mass is in the water, oxygen has most of the mass of living organisms (for example, about two-thirds of the human body’s mass). Elemental oxygen is produced by cyanobacteria, algae and plants and is used in cellular respiration for all complex life. Another form (allotrope) of oxygen, named 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. At even higher earth orbit altitudes atomic oxygen is a significant presence and a cause of erosion for spacecraft.

Although oxygen was discovered by Carl Wilhelm Scheele, in Upsala in 1773 or earlier and then by Joseph Priestley Wiltshire in 1774, the latter one gets the privilege of being better known since his work was published first. The name of the chemical element was actually 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. 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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