Oxygen

Oxygen, ₈O
	Liquid oxygen boiling
General properties
Allotropes	O2, O3 (Ozone)
Appearance	gas: colorless; liquid: pale blue
Standard atomic weight (Ar, standard)	[15.99903, 15.99977] conventional: 15.999
Oxygen in the periodic table
	nitrogen ← oxygen → fluorine
Atomic number (Z)	8
Group	group 16 (chalcogens)
Period	period 2
Element category	diatomic nonmetal
Block	p-block
Electron configuration	[He] 2s2 2p4
Electrons per shell	2, 6
Physical properties
Phase at STP	gas
Melting point	54.36 K (−218.79 °C, −361.82 °F)
Boiling point	90.188 K (−182.962 °C, −297.332 °F)
Density (at STP)	1.429 g/L
when liquid (at b.p.)	1.141 g/cm3
Triple point	54.361 K, 0.1463 kPa
Critical point	154.581 K, 5.043 MPa
Heat of fusion	(O2) 0.444 kJ/mol
Heat of vaporization	(O2) 6.82 kJ/mol
Molar heat capacity	(O2) 29.378 J/(mol·K)
Atomic properties
Oxidation states	2, 1, −1, −2
Electronegativity	Pauling scale: 3.44
Ionization energies	1st: 1313.9 kJ/mol; 2nd: 3388.3 kJ/mol; 3rd: 5300.5 kJ/mol; (more);
Covalent radius	66±2 pm
Van der Waals radius	152 pm
	Spectral lines
	Spectral lines
Miscellanea
Crystal structure	cubic
Speed of sound	330 m/s (gas, at 27 °C)
Thermal conductivity	26.58×10−3 W/(m·K)
Magnetic ordering	paramagnetic
Magnetic susceptibility	+3449.0·10−6 cm3/mol (293 K)
CAS Number	7782-44-7
History
Discovery	Carl Wilhelm Scheele (1771)
Named by	Antoine Lavoisier (1777)
Main isotopes of oxygen
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Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O
₂. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up almost half of the Earth's crust.

Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids, carbohydrates, and fats, as do the major constituent inorganic compounds of animal shells, teeth, and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form (allotrope) of oxygen, ozone (O
₃), strongly absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of smog and thus a pollutant.

Oxygen was discovered independently 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 work was published first. Priestley, however, called oxygen "dephlogisticated air", and did not recognize it as a chemical element. The name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and correctly characterized the role it plays in combustion.

Common uses of oxygen include production of steel, plastics and textiles, brazing, welding and cutting of steels and other metals, rocket propellant, oxygen therapy, and life support systems in aircraft, submarines, spaceflight and diving.