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Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. Oxygen is Earth's most abundant element, and after
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
and
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...
, it is the third-most abundant element in the universe. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula . Diatomic oxygen gas currently constitutes 20.95% of the Earth's atmosphere, though this has changed considerably over long periods of time. Oxygen makes up almost half of the Earth's crust in the form of oxides.Atkins, P.; Jones, L.; Laverman, L. (2016).''Chemical Principles'', 7th edition. Freeman. Many major classes of organic molecules in living organisms contain oxygen atoms, such as
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s, 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 Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
, 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 (), 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 isolated by
Michael Sendivogius Michael Sendivogius (; pl, Michał Sędziwój; 2 February 1566 – 1636) was a Polish alchemist, philosopher, and medical doctor. A pioneer of chemistry, he developed ways of purification and creation of various acids, metals and other chemi ...
before 1604, but it is commonly believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774. Priority is often given for Priestley 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,
plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adapta ...
s and textiles, brazing, welding and cutting of steels and other
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
s, rocket propellant, oxygen therapy, and life support systems in aircraft, submarines, spaceflight and diving.


History of study


Early experiments

One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work ''Pneumatica'', Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element
fire Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. At a certain point in the combustion reaction, called the ignition point, flames ...
and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. Cook & Lauer 1968, p. 499. In the late 17th century,
Robert Boyle Robert Boyle (; 25 January 1627 – 31 December 1691) was an Anglo-Irish natural philosopher, chemist, physicist, alchemist and inventor. Boyle is largely regarded today as the first modern chemist, and therefore one of the founders ...
proved that air is necessary for combustion. English chemist John Mayow (1641–1679) refined this work by showing that fire requires only a part of air that he called ''spiritus nitroaereus''. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects. From this, he surmised that nitroaereus is consumed in both respiration and combustion. Mayow observed that
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient ti ...
increased in weight when heated, and inferred that the nitroaereus must have combined with it. He also thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work ''Tractatus duo'' in the tract "De respiratione".


Phlogiston theory

Robert Hooke,
Ole Borch Ole Borch (7 April 1626 – 13 October 1690) (latinized to ''Olaus Borrichius'' or ''Olaus Borrichus'') was a Danish scientist, physician, grammarian, and poet. He was royal physician to both Kings Frederick III of Denmark and Christian V of Den ...
, Mikhail Lomonosov, and
Pierre Bayen Pierre Bayen (7 February 1725–14 February 1798) was a French chemist. He analyzed water drunk by the Kingdom of France, and he wrongly suggested that using pewter glasses rendered the water toxic. He became a member of the French Academy of ...
all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element. Emsley 2001, p. 299 This may have been in part due to the prevalence of the philosophy of combustion and
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
called the ''phlogiston theory'', which was then the favored explanation of those processes. Established in 1667 by the German alchemist
J. J. Becher Johann Joachim Becher (; 6 May 1635 – October 1682) was a German physician, alchemist, precursor of chemistry, scholar and adventurer, best known for his development of the phlogiston theory of combustion, and his advancement of Austrian camera ...
, and modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts. One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Highly combustible materials that leave little residue, such as wood or coal, were thought to be made mostly of phlogiston; non-combustible substances that corrode, such as iron, contained very little. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea; instead, it was based on observations of what happens when something burns, that most common objects appear to become lighter and seem to lose something in the process.


Discovery

Polish alchemist, philosopher, and physician
Michael Sendivogius Michael Sendivogius (; pl, Michał Sędziwój; 2 February 1566 – 1636) was a Polish alchemist, philosopher, and medical doctor. A pioneer of chemistry, he developed ways of purification and creation of various acids, metals and other chemi ...
(Michał Sędziwój) in his work ''De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti'' (1604) described a substance contained in air, referring to it as 'cibus vitae' (food of life,) and according to Polish historian Roman Bugaj, this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of
potassium nitrate Potassium nitrate is a chemical compound with the chemical formula . This alkali metal nitrate salt is also known as Indian saltpetre (large deposits of which were historically mined in India). It is an ionic salt of potassium ions K+ and ...
. In Bugaj's view, the isolation of oxygen and the proper association of the substance to that part of air which is required for life, provides sufficient evidence for the discovery of oxygen by Sendivogius. This discovery of Sendivogius was however frequently denied by the generations of scientists and chemists which succeeded him. It is also commonly claimed that oxygen was first discovered by Swedish pharmacist Carl Wilhelm Scheele. He had produced oxygen gas by heating mercuric oxide (HgO) and various nitrates in 1771–72. Scheele called the gas "fire air" because it was then the only known
agent Agent may refer to: Espionage, investigation, and law *, spies or intelligence officers * Law of agency, laws involving a person authorized to act on behalf of another ** Agent of record, a person with a contractual agreement with an insuranc ...
to support combustion. He wrote an account of this discovery in a manuscript titled ''Treatise on Air and Fire'', which he sent to his publisher in 1775. That document was published in 1777. Emsley 2001, p. 300 In the meantime, on August 1, 1774, an experiment conducted by the British clergyman Joseph Priestley focused sunlight on mercuric oxide contained in a glass tube, which liberated a gas he named "dephlogisticated air". Cook & Lauer 1968, p. 500 He noted that candles burned brighter in the gas and that a mouse was more active and lived longer while breathing it. After breathing the gas himself, Priestley wrote: "The feeling of it to my lungs was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards." Priestley published his findings in 1775 in a paper titled "An Account of Further Discoveries in Air", which was included in the second volume of his book titled ''
Experiments and Observations on Different Kinds of Air ''Experiments and Observations on Different Kinds of Air'' (1774–86) is a six-volume work published by 18th-century British polymath Joseph Priestley which reports a series of his experiments on "airs" or gases, most notably his discovery of ...
''. Because he published his findings first, Priestley is usually given priority in the discovery. The French chemist
Antoine Laurent Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS (


Lavoisier's contribution

Lavoisier conducted the first adequate quantitative experiments on oxidation and gave the first correct explanation of how combustion works. He used these and similar experiments, all started in 1774, to discredit the phlogiston theory and to prove that the substance discovered by Priestley and Scheele was a chemical element. In one experiment, Lavoisier observed that there was no overall increase in weight when tin and air were heated in a closed container. He noted that air rushed in when he opened the container, which indicated that part of the trapped air had been consumed. He also noted that the tin had increased in weight and that increase was the same as the weight of the air that rushed back in. This and other experiments on combustion were documented in his book ''Sur la combustion en général'', which was published in 1777. In that work, he proved that air is a mixture of two gases; 'vital air', which is essential to combustion and respiration, and ''azote'' (Gk. ' "lifeless"), which did not support either. ''Azote'' later became '' nitrogen'' in English, although it has kept the earlier name in French and several other European languages.


Etymology

Lavoisier renamed 'vital air' to ''oxygène'' in 1777 from the Greek roots '' (oxys)'' ( acid, literally "sharp", from the taste of acids) and ''-γενής (-genēs)'' (producer, literally begetter), because he mistakenly believed that oxygen was a constituent of all acids. Chemists (such as Sir
Humphry Davy Sir Humphry Davy, 1st Baronet, (17 December 177829 May 1829) was a British chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, several elements for ...
in 1812) eventually determined that Lavoisier was wrong in this regard, but by then the name was too well established. ''Oxygen'' entered the English language despite opposition by English scientists and the fact that the Englishman Priestley had first isolated the gas and written about it. This is partly due to a poem praising the gas titled "Oxygen" in the popular book ''
The Botanic Garden ''The Botanic Garden'' (1791) is a set of two poems, ''The Economy of Vegetation'' and ''The Loves of the Plants'', by the British poet and naturalist Erasmus Darwin. ''The Economy of Vegetation'' celebrates technological innovation and scien ...
'' (1791) by Erasmus Darwin, grandfather of Charles Darwin.


Later history

John Dalton's original atomic hypothesis presumed that all elements were monatomic and that the atoms in compounds would normally have the simplest atomic ratios with respect to one another. For example, Dalton assumed that water's formula was HO, leading to the conclusion that the atomic mass of oxygen was 8 times that of hydrogen, instead of the modern value of about 16. In 1805, Joseph Louis Gay-Lussac and Alexander von Humboldt showed that water is formed of two volumes of hydrogen and one volume of oxygen; and by 1811 Amedeo Avogadro had arrived at the correct interpretation of water's composition, based on what is now called
Avogadro's law Avogadro's law (sometimes referred to as Avogadro's hypothesis or Avogadro's principle) or Avogadro-Ampère's hypothesis is an experimental gas law relating the volume of a gas to the amount of substance of gas present. The law is a specific ca ...
and the diatomic elemental molecules in those gases.These results were mostly ignored until 1860. Part of this rejection was due to the belief that atoms of one element would have no chemical affinity towards atoms of the same element, and part was due to apparent exceptions to Avogadro's law that were not explained until later in terms of dissociating molecules. The first commercial method of producing oxygen was chemical, the so-called
Brin process Brin process is a now-obsolete industrial scale production process for oxygen. In this process barium oxide reacts at 500–600 °C with air to form barium peroxide which decomposes at above 800 °C by releasing oxygen. :2 BaO + O2 ⇌ 2 ...
involving a reversible reaction of barium oxide. It was invented in 1852 and commercialized in 1884, but was displaced by newer methods in early 20th century. By the late 19th century scientists realized that air could be liquefied and its components isolated by compressing and cooling it. Using a
cascade Cascade, Cascades or Cascading may refer to: Science and technology Science *Cascade waterfalls, or series of waterfalls * Cascade, the CRISPR-associated complex for antiviral defense (a protein complex) * Cascade (grape), a type of fruit * Bioc ...
method, Swiss chemist and physicist Raoul Pierre Pictet evaporated liquid sulfur dioxide in order to liquefy carbon dioxide, which in turn was evaporated to cool oxygen gas enough to liquefy it. He sent a telegram on December 22, 1877, to the French Academy of Sciences in Paris announcing his discovery of liquid oxygen. Just two days later, French physicist
Louis Paul Cailletet Louis-Paul Cailletet (21 September 1832 – 5 January 1913) was a French physicist and inventor. Life and work Cailletet was born in Châtillon-sur-Seine, Côte-d'Or. Educated in Paris, Cailletet returned to Châtillon to manage his fathe ...
announced his own method of liquefying molecular oxygen. Only a few drops of the liquid were produced in each case and no meaningful analysis could be conducted. Oxygen was liquefied in a stable state for the first time on March 29, 1883, by Polish scientists from Jagiellonian University, Zygmunt Wróblewski and Karol Olszewski. In 1891 Scottish chemist James Dewar was able to produce enough liquid oxygen for study. Emsley 2001, p. 303 The first commercially viable process for producing liquid oxygen was independently developed in 1895 by German engineer Carl von Linde and British engineer William Hampson. Both men lowered the temperature of air until it liquefied and then distilled the component gases by boiling them off one at a time and capturing them separately. Later, in 1901, oxyacetylene welding was demonstrated for the first time by burning a mixture of acetylene and compressed . This method of welding and cutting metal later became common. In 1923, the American scientist Robert H. Goddard became the first person to develop a rocket engine that burned liquid fuel; the engine used gasoline for fuel and liquid oxygen as the oxidizer. Goddard successfully flew a small liquid-fueled rocket 56 m at 97 km/h on March 16, 1926, in Auburn, Massachusetts, US. In academic laboratories, oxygen can be prepared by heating together potassium chlorate mixed with a small proportion of manganese dioxide. Oxygen levels in the atmosphere are trending slightly downward globally, possibly because of fossil-fuel burning.


Characteristics


Properties and molecular structure

At standard temperature and pressure, oxygen is a colorless, odorless, and tasteless gas with the molecular formula , referred to as dioxygen. As ''dioxygen'', two oxygen atoms are chemically bound to each other. The bond can be variously described based on level of theory, but is reasonably and simply described as a covalent double bond that results from the filling of molecular orbitals formed from the
atomic orbital In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any ...
s of the individual oxygen atoms, the filling of which results in a bond order of two. More specifically, the double bond is the result of sequential, low-to-high energy, or
Aufbau ''Aufbau'' is a term which was used in publications from 1919 to 1947 in the German language. The term can be translated as "structure", "construction" or as "rebuilding", "reconstruction". Peter Galison advocated its use as a "keyword", in the ...
, filling of orbitals, and the resulting cancellation of contributions from the 2s electrons, after sequential filling of the low σ and σ* orbitals; σ overlap of the two atomic 2p orbitals that lie along the O–O molecular axis and π overlap of two pairs of atomic 2p orbitals perpendicular to the O–O molecular axis, and then cancellation of contributions from the remaining two 2p electrons after their partial filling of the π* orbitals.Jack Barrett, 2002, "Atomic Structure and Periodicity", (Basic concepts in chemistry, Vol. 9 of Tutorial chemistry texts), Cambridge, UK: Royal Society of Chemistry, p. 153, . Se
Google Books
accessed January 31, 2015.
This combination of cancellations and σ and π overlaps results in dioxygen's double-bond character and reactivity, and a triplet electronic ground state. An electron configuration with two unpaired electrons, as is found in dioxygen orbitals (see the filled π* orbitals in the diagram) that are of equal energy—i.e., degenerate—is a configuration termed a
spin triplet In quantum mechanics, a triplet is a quantum state of a system with a spin of quantum number =1, such that there are three allowed values of the spin component, = −1, 0, and +1. Spin, in the context of quantum mechanics, is not a mechanical r ...
state. Hence, the ground state of the molecule is referred to as triplet oxygen.An orbital is a concept from quantum mechanics that models an electron as a wave-like particle that has a spatial distribution about an atom or molecule. The highest-energy, partially filled orbitals are
antibonding In chemical bonding theory, an antibonding orbital is a type of molecular orbital that weakens the chemical bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms. Such an orbital has one or more no ...
, and so their filling weakens the bond order from three to two. Because of its unpaired electrons, triplet oxygen reacts only slowly with most organic molecules, which have paired electron spins; this prevents spontaneous combustion. In the triplet form, molecules are paramagnetic. That is, they impart magnetic character to oxygen when it is in the presence of a magnetic field, because of the spin
magnetic moment In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electroma ...
s of the unpaired electrons in the molecule, and the negative exchange energy between neighboring molecules. Liquid oxygen is so magnetic that, in laboratory demonstrations, a bridge of liquid oxygen may be supported against its own weight between the poles of a powerful magnet. Singlet oxygen is a name given to several higher-energy species of molecular in which all the electron spins are paired. It is much more reactive with common organic molecules than is normal (triplet) molecular oxygen. In nature, singlet oxygen is commonly formed from water during photosynthesis, using the energy of sunlight. It is also produced in the troposphere by the photolysis of ozone by light of short wavelength and by the immune system as a source of active oxygen. Carotenoids in photosynthetic organisms (and possibly animals) play a major role in absorbing energy from singlet oxygen and converting it to the unexcited ground state before it can cause harm to tissues.


Allotropes

The common allotrope of elemental oxygen on Earth is called dioxygen, , the major part of the Earth's atmospheric oxygen (see Occurrence). O2 has a bond length of 121  pm and a bond energy of 498  kJ/mol. O2 is used by complex forms of life, such as animals, in cellular respiration. Other aspects of are covered in the remainder of this article. Trioxygen () is usually known as ozone and is a very reactive allotrope of oxygen that is damaging to lung tissue. Ozone is produced in the upper atmosphere when combines with atomic oxygen made by the splitting of by ultraviolet (UV) radiation. Since ozone absorbs strongly in the UV region of the spectrum, the ozone layer of the upper atmosphere functions as a protective radiation shield for the planet. Near the Earth's surface, it is a pollutant formed as a by-product of
automobile exhaust Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...
. At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft. The metastable molecule
tetraoxygen The tetraoxygen molecule (O4), also called oxozone, is an allotrope of oxygen consisting of four oxygen atoms. History Tetraoxygen was first predicted in 1924 by Gilbert N. Lewis, who proposed it as an explanation for the failure of liquid oxyg ...
() was discovered in 2001, and was assumed to exist in one of the six phases of solid oxygen. It was proven in 2006 that this phase, created by pressurizing to 20  GPa, is in fact a rhombohedral
cluster may refer to: Science and technology Astronomy * Cluster (spacecraft), constellation of four European Space Agency spacecraft * Asteroid cluster, a small asteroid family * Cluster II (spacecraft), a European Space Agency mission to study th ...
. This cluster has the potential to be a much more powerful oxidizer than either or and may therefore be used in rocket fuel. A metallic phase was discovered in 1990 when solid oxygen is subjected to a pressure of above 96 GPa and it was shown in 1998 that at very low temperatures, this phase becomes superconducting.


Physical properties

Oxygen dissolves more readily in water than in nitrogen, and in freshwater more readily than in seawater. Water in equilibrium with air contains approximately 1 molecule of dissolved for every 2 molecules of (1:2), compared with an atmospheric ratio of approximately 1:4. The solubility of oxygen in water is temperature-dependent, and about twice as much () dissolves at 0 °C than at 20 °C (). At 25 °C and of air, freshwater can dissolve about 6.04 
milliliters The litre (international spelling) or liter (American English spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3). ...
 (mL) of oxygen per liter, and seawater contains about 4.95 mL per liter. At 5 °C the solubility increases to 9.0 mL (50% more than at 25 °C) per liter for freshwater and 7.2 mL (45% more) per liter for sea water. Oxygen condenses at 90.20  K (−182.95 °C, −297.31 °F) and freezes at 54.36 K (−218.79 °C, −361.82 °F). Both liquid and
solid Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
are clear substances with a light
sky-blue Sky blue is a shade of light blue comparable to that of a clear daytime sky. The term (as "sky blew") is attested from 1681. A 1585 translation of Nicolas de Nicolay's 1576 ''Les navigations, peregrinations et voyages faicts en la Turquie'' i ...
color caused by absorption in the red (in contrast with the blue color of the sky, which is due to Rayleigh scattering of blue light). High-purity liquid is usually obtained by the fractional distillation of liquefied air. Liquid oxygen may also be condensed from air using liquid nitrogen as a coolant. Liquid oxygen is a highly reactive substance and must be segregated from combustible materials. The spectroscopy of molecular oxygen is associated with the atmospheric processes of aurora and airglow. The absorption in the
Herzberg continuum Herzberg is German for "heart mountain" and may refer to: Places in Germany *Herzberg am Harz, a town in the Osterode district, Lower Saxony *Herzberg, Brandenburg, a town in the Elbe-Elster district, Brandenburg *Herzberg, Ostprignitz-Ruppin, a to ...
and Schumann–Runge bands in the ultraviolet produces atomic oxygen that is important in the chemistry of the middle atmosphere. Excited-state singlet molecular oxygen is responsible for red chemiluminescence in solution. Table of thermal and physical properties of oxygen (O2) at atmospheric pressure:


Isotopes and stellar origin

Naturally occurring oxygen is composed of three stable
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
s, 16O, 17O, and 18O, with 16O being the most abundant (99.762% natural abundance). Most 16O is synthesized at the end of the helium fusion process in massive
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s but some is made in the neon burning process. 17O is primarily made by the burning of hydrogen into
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...
during the CNO cycle, making it a common isotope in the hydrogen burning zones of stars. Most 18O is produced when 14N (made abundant from CNO burning) captures a 4He nucleus, making 18O common in the helium-rich zones of evolved, massive stars. Fourteen radioisotopes have been characterized. The most stable are 15O with a
half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ...
of 122.24 seconds and 14O with a half-life of 70.606 seconds. All of the remaining radioactive isotopes have half-lives that are less than 27 s and the majority of these have half-lives that are less than 83 milliseconds. The most common decay mode of the isotopes lighter than 16O is β+ decay to yield nitrogen, and the most common mode for the isotopes heavier than 18O is beta decay to yield fluorine.


Occurrence

Oxygen is the most abundant chemical element by mass in the Earth's biosphere, air, sea and land. Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium. Emsley 2001, p. 297 About 0.9% of the Sun's mass is oxygen. Oxygen constitutes 49.2% of the Earth's crust by mass as part of oxide compounds such as silicon dioxide and is the most abundant element by mass in the Earth's crust. It is also the major component of the world's oceans (88.8% by mass). Oxygen gas is the second most common component of the Earth's atmosphere, taking up 20.8% of its volume and 23.1% of its mass (some 1015 tonnes). Emsley 2001, p. 298Figures given are for values up to above the surface Earth is unusual among the planets of the Solar System in having such a high concentration of oxygen gas in its atmosphere: Mars (with 0.1% by volume) and Venus have much less. The surrounding those planets is produced solely by the action of ultraviolet radiation on oxygen-containing molecules such as carbon dioxide. The unusually high concentration of oxygen gas on Earth is the result of the oxygen cycle. This biogeochemical cycle describes the movement of oxygen within and between its three main reservoirs on Earth: the atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for modern Earth's atmosphere. Photosynthesis releases oxygen into the atmosphere, while respiration, decay, and combustion remove it from the atmosphere. In the present equilibrium, production and consumption occur at the same rate. Free oxygen also occurs in solution in the world's water bodies. The increased solubility of at lower temperatures (see Physical properties) has important implications for ocean life, as polar oceans support a much higher density of life due to their higher oxygen content. Water polluted with plant nutrients such as nitrates or phosphates may stimulate growth of algae by a process called
eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyt ...
and the decay of these organisms and other biomaterials may reduce the content in eutrophic water bodies. Scientists assess this aspect of water quality by measuring the water's biochemical oxygen demand, or the amount of needed to restore it to a normal concentration. Emsley 2001, p. 301


Analysis

Paleoclimatologists Paleoclimatology ( British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to ...
measure the ratio of oxygen-18 and oxygen-16 in the shells and skeletons of marine organisms to determine the climate millions of years ago (see oxygen isotope ratio cycle). Seawater molecules that contain the lighter
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
, oxygen-16, evaporate at a slightly faster rate than water molecules containing the 12% heavier oxygen-18, and this disparity increases at lower temperatures. Emsley 2001, p. 304 During periods of lower global temperatures, snow and rain from that evaporated water tends to be higher in oxygen-16, and the seawater left behind tends to be higher in oxygen-18. Marine organisms then incorporate more oxygen-18 into their skeletons and shells than they would in a warmer climate. Paleoclimatologists also directly measure this ratio in the water molecules of ice core samples as old as hundreds of thousands of years. Planetary geologists have measured the relative quantities of oxygen isotopes in samples from the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
, the Moon, Mars, and meteorites, but were long unable to obtain reference values for the isotope ratios in the Sun, believed to be the same as those of the primordial solar nebula. Analysis of a silicon wafer exposed to the solar wind in space and returned by the crashed Genesis spacecraft has shown that the Sun has a higher proportion of oxygen-16 than does the Earth. The measurement implies that an unknown process depleted oxygen-16 from the Sun's disk of protoplanetary material prior to the coalescence of dust grains that formed the Earth. Oxygen presents two spectrophotometric
absorption band According to quantum mechanics, atoms and molecules can only hold certain defined quantities of energy, or exist in specific states. When such quanta of electromagnetic radiation are emitted or absorbed by an atom or molecule, energy of th ...
s peaking at the wavelengths 687 and 760  nm. Some remote sensing scientists have proposed using the measurement of the radiance coming from vegetation canopies in those bands to characterize plant health status from a
satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...
platform. This approach exploits the fact that in those bands it is possible to discriminate the vegetation's reflectance from its
fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
, which is much weaker. The measurement is technically difficult owing to the low signal-to-noise ratio and the physical structure of vegetation; but it has been proposed as a possible method of monitoring the carbon cycle from satellites on a global scale.


Biological production and role of O2


Photosynthesis and respiration

In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis. According to some estimates, green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on Earth, and the rest is produced by terrestrial plants. Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year. A simplified overall formula for photosynthesis is : 6 + 6 + photons → + 6 or simply :
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
+ water + sunlight → glucose + dioxygen Photolytic
oxygen evolution Oxygen evolution is the process of generating molecular oxygen (O2) by a chemical reaction, usually from water. Oxygen evolution from water is effected by oxygenic photosynthesis, electrolysis of water, and thermal decomposition of various oxides ...
occurs in the thylakoid membranes of photosynthetic organisms and requires the energy of four photons.Thylakoid membranes are part of chloroplasts in algae and plants while they simply are one of many membrane structures in cyanobacteria. In fact, chloroplasts are thought to have evolved from cyanobacteria that were once symbiotic partners with the progenitors of plants and algae. Many steps are involved, but the result is the formation of a proton gradient across the thylakoid membrane, which is used to synthesize adenosine triphosphate (ATP) via photophosphorylation. Raven 2005, 115–27 The remaining (after production of the water molecule) is released into the atmosphere.Water oxidation is catalyzed by a manganese-containing
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
complex known as the oxygen evolving complex (OEC) or water-splitting complex found associated with the lumenal side of thylakoid membranes. Manganese is an important cofactor, and
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
and chloride are also required for the reaction to occur. (Raven 2005)
Oxygen is used in mitochondria in the generation of ATP during oxidative phosphorylation. The reaction for aerobic respiration is essentially the reverse of photosynthesis and is simplified as : + 6 → 6 + 6 + 2880 kJ/mol In vertebrates, diffuses through membranes in the lungs and into red blood cells.
Hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythroc ...
binds , changing color from bluish red to bright red ( is released from another part of hemoglobin through the
Bohr effect The Bohr effect is a phenomenon first described in 1904 by the Danish physiologist Christian Bohr. Hemoglobin's oxygen binding affinity (see oxygen–haemoglobin dissociation curve) is inversely related both to acidity and to the concentration o ...
). Other animals use hemocyanin ( molluscs and some arthropods) or
hemerythrin Hemerythrin (also spelled haemerythrin; grc, αἷμα, haîma, blood, grc, ἐρυθρός, erythrós, red) is an oligomeric protein responsible for oxygen (O2) transport in the marine invertebrate phyla of sipunculids, priapulids, brachiop ...
( spiders and lobsters). A liter of blood can dissolve 200 cm3 of . Until the discovery of anaerobic
metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
, oxygen was thought to be a requirement for all complex life. Reactive oxygen species, such as superoxide ion () and hydrogen peroxide (), are reactive by-products of oxygen use in organisms. Parts of the immune system of higher organisms create peroxide, superoxide, and singlet oxygen to destroy invading microbes. Reactive oxygen species also play an important role in the
hypersensitive response Hypersensitive response (HR) is a mechanism used by plants to prevent the spread of infection by microbial pathogens. HR is characterized by the rapid death of cells in the local region surrounding an infection and it serves to restrict the g ...
of plants against pathogen attack. Oxygen is damaging to obligately anaerobic organisms, which were the dominant form of early life on Earth until began to accumulate in the atmosphere about 2.5 billion years ago during the Great Oxygenation Event, about a billion years after the first appearance of these organisms. An adult human at rest inhales 1.8 to 2.4 grams of oxygen per minute. This amounts to more than 6 billion tonnes of oxygen inhaled by humanity per year.(1.8 grams/min/person)×(60 min/h)×(24 h/day)×(365 days/year)×(6.6 billion people)/1,000,000 g/t=6.24 billion tonnes


Living organisms

The free oxygen partial pressure in the body of a living vertebrate organism is highest in the respiratory system, and decreases along any
arterial system An artery (plural arteries) () is a blood vessel in humans and most animals that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; the two exceptions are the pul ...
, peripheral tissues, and
venous system Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated ...
, respectively. Partial pressure is the pressure that oxygen would have if it alone occupied the volume.


Build-up in the atmosphere

Free oxygen gas was almost nonexistent in Earth's atmosphere before photosynthetic archaea and bacteria evolved, probably about 3.5 billion years ago. Free oxygen first appeared in significant quantities during the Paleoproterozoic eon (between 3.0 and 2.3 billion years ago). Even if there was much dissolved
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
in the oceans when oxygenic photosynthesis was getting more common, it appears the banded iron formations were created by anoxyenic or micro-aerophilic iron-oxidizing bacteria which dominated the deeper areas of the photic zone, while oxygen-producing cyanobacteria covered the shallows. Free oxygen began to
outgas Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (which ...
from the oceans 3–2.7 billion years ago, reaching 10% of its present level around 1.7 billion years ago. The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the extant anaerobic organisms to
extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the Endling, last individual of the species, although the Functional ext ...
during the Great Oxygenation Event (''oxygen catastrophe'') about 2.4 billion years ago. Cellular respiration using enables aerobic organisms to produce much more ATP than anaerobic organisms. Cellular respiration of occurs in all eukaryotes, including all complex multicellular organisms such as plants and animals. Since the beginning of the Cambrian period 540 million years ago, atmospheric levels have fluctuated between 15% and 30% by volume. Towards the end of the Carboniferous period (about 300 million years ago) atmospheric levels reached a maximum of 35% by volume, which may have contributed to the large size of insects and amphibians at this time. Variations in atmospheric oxygen concentration have shaped past climates. When oxygen declined, atmospheric density dropped, which in turn increased surface evaporation, causing precipitation increases and warmer temperatures. At the current rate of photosynthesis it would take about 2,000 years to regenerate the entire in the present atmosphere.


Extraterrestrial free oxygen

In the field of astrobiology and in the search for extraterrestrial life oxygen is a strong biosignature. That said it might not be a definite biosignature, being possibly produced abiotically on celestial bodies with processes and conditions (such as a peculiar
hydrosphere The hydrosphere () is the combined mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to change in shape. This ...
) which allow free oxygen, like with Europa's and Ganymede's thin oxygen atmospheres.


Industrial production

One hundred million tonnes of are extracted from air for industrial uses annually by two primary methods. The most common method is fractional distillation of liquefied air, with distilling as a vapor while is left as a liquid. The other primary method of producing is passing a stream of clean, dry air through one bed of a pair of identical zeolite molecular sieves, which absorbs the nitrogen and delivers a gas stream that is 90% to 93% . Simultaneously, nitrogen gas is released from the other nitrogen-saturated zeolite bed, by reducing the chamber operating pressure and diverting part of the oxygen gas from the producer bed through it, in the reverse direction of flow. After a set cycle time the operation of the two beds is interchanged, thereby allowing for a continuous supply of gaseous oxygen to be pumped through a pipeline. This is known as pressure swing adsorption. Oxygen gas is increasingly obtained by these non- cryogenic technologies (see also the related
vacuum swing adsorption Pressure swing adsorption (PSA) is a technique used to separate some gas species from a mixture of gases (typically air) under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at nea ...
). Oxygen gas can also be produced through electrolysis of water into molecular oxygen and hydrogen. DC electricity must be used: if AC is used, the gases in each limb consist of hydrogen and oxygen in the explosive ratio 2:1. A similar method is the electrocatalytic evolution from oxides and oxoacids. Chemical catalysts can be used as well, such as in
chemical oxygen generator A chemical oxygen generator is a device that releases oxygen via a chemical reaction. The oxygen source is usually an inorganic superoxide, chlorate, or perchlorate; ozonides are a promising group of oxygen sources. The generators are usually ig ...
s or oxygen candles that are used as part of the life-support equipment on submarines, and are still part of standard equipment on commercial airliners in case of depressurization emergencies. Another air separation method is forcing air to dissolve through ceramic membranes based on zirconium dioxide by either high pressure or an electric current, to produce nearly pure gas.


Storage

Oxygen storage Methods of oxygen storage for subsequent use span many approaches, including high pressures in oxygen tanks, cryogenics, oxygen-rich compounds and reaction mixtures, and chemical compounds that reversibly release oxygen upon heating or pressure chan ...
methods include high-pressure oxygen tanks, cryogenics and chemical compounds. For reasons of economy, oxygen is often transported in bulk as a liquid in specially insulated tankers, since one liter of liquefied oxygen is equivalent to 840 liters of gaseous oxygen at atmospheric pressure and . Such tankers are used to refill bulk liquid-oxygen storage containers, which stand outside hospitals and other institutions that need large volumes of pure oxygen gas. Liquid oxygen is passed through heat exchangers, which convert the cryogenic liquid into gas before it enters the building. Oxygen is also stored and shipped in smaller cylinders containing the compressed gas; a form that is useful in certain portable medical applications and oxy-fuel welding and cutting.


Applications


Medical

Uptake of from the air is the essential purpose of respiration, so oxygen supplementation is used in medicine. Treatment not only increases oxygen levels in the patient's blood, but has the secondary effect of decreasing resistance to blood flow in many types of diseased lungs, easing work load on the heart. Oxygen therapy is used to treat emphysema,
pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severit ...
, some heart disorders ( congestive heart failure), some disorders that cause increased
pulmonary artery pressure A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and t ...
, and any disease that impairs the body's ability to take up and use gaseous oxygen. Cook & Lauer 1968, p. 510 Treatments are flexible enough to be used in hospitals, the patient's home, or increasingly by portable devices. Oxygen tents were once commonly used in oxygen supplementation, but have since been replaced mostly by the use of oxygen masks or nasal cannulas.
Hyperbaric Hyperbaric medicine is medical treatment in which an ambient pressure greater than sea level atmospheric pressure is a necessary component. The treatment comprises hyperbaric oxygen therapy (HBOT), the medical use of oxygen at an ambient pressure ...
(high-pressure) medicine uses special oxygen chambers to increase the partial pressure of around the patient and, when needed, the medical staff. Carbon monoxide poisoning, gas gangrene, and decompression sickness (the 'bends') are sometimes addressed with this therapy. Increased concentration in the lungs helps to displace carbon monoxide from the heme group of
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythroc ...
. Oxygen gas is poisonous to the
anaerobic bacteria An anaerobic organism or anaerobe is any organism that does not require molecular oxygen for growth. It may react negatively or even die if free oxygen is present. In contrast, an aerobic organism (aerobe) is an organism that requires an oxygenat ...
that cause gas gangrene, so increasing its partial pressure helps kill them. Decompression sickness occurs in divers who decompress too quickly after a dive, resulting in bubbles of inert gas, mostly nitrogen and helium, forming in the blood. Increasing the pressure of as soon as possible helps to redissolve the bubbles back into the blood so that these excess gasses can be exhaled naturally through the lungs. Normobaric oxygen administration at the highest available concentration is frequently used as first aid for any diving injury that may involve inert gas bubble formation in the tissues. There is epidemiological support for its use from a statistical study of cases recorded in a long term database.


Life support and recreational use

An application of as a low-pressure breathing gas is in modern space suits, which surround their occupant's body with the breathing gas. These devices use nearly pure oxygen at about one-third normal pressure, resulting in a normal blood partial pressure of . This trade-off of higher oxygen concentration for lower pressure is needed to maintain suit flexibility.
Scuba Scuba may refer to: * Scuba diving ** Scuba set, the equipment used for scuba (Self-Contained Underwater Breathing Apparatus) diving * Scuba, an in-memory database developed by Facebook * Submillimetre Common-User Bolometer Array, either of two in ...
and surface-supplied underwater divers and submariners also rely on artificially delivered . Submarines, submersibles and atmospheric diving suits usually operate at normal atmospheric pressure. Breathing air is scrubbed of carbon dioxide by chemical extraction and oxygen is replaced to maintain a constant partial pressure.
Ambient pressure Ambient or Ambiance or Ambience may refer to: Music and sound * Ambience (sound recording), also known as atmospheres or backgrounds * Ambient music, a genre of music that puts an emphasis on tone and atmosphere * ''Ambient'' (album), by Moby * ...
divers breathe air or gas mixtures with an oxygen fraction suited to the operating depth. Pure or nearly pure use in diving at pressures higher than atmospheric is usually limited to
rebreathers A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is ...
, or decompression at relatively shallow depths (~6 meters depth, or less), or medical treatment in recompression chambers at pressures up to 2.8 bar, where acute oxygen toxicity can be managed without the risk of drowning. Deeper diving requires significant dilution of with other gases, such as nitrogen or helium, to prevent
oxygen toxicity Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen () at increased partial pressures. Severe cases can result in cell damage and death, with effects most often seen in the central nervous system, l ...
. People who climb mountains or fly in non-pressurized fixed-wing aircraft sometimes have supplemental supplies.The reason is that increasing the proportion of oxygen in the breathing gas at low pressure acts to augment the inspired partial pressure nearer to that found at sea-level. Pressurized commercial airplanes have an emergency supply of automatically supplied to the passengers in case of cabin depressurization. Sudden cabin pressure loss activates
chemical oxygen generator A chemical oxygen generator is a device that releases oxygen via a chemical reaction. The oxygen source is usually an inorganic superoxide, chlorate, or perchlorate; ozonides are a promising group of oxygen sources. The generators are usually ig ...
s above each seat, causing oxygen masks to drop. Pulling on the masks "to start the flow of oxygen" as cabin safety instructions dictate, forces iron filings into the sodium chlorate inside the canister. A steady stream of oxygen gas is then produced by the exothermic reaction. Oxygen, as a mild euphoric, has a history of recreational use in
oxygen bar An oxygen bar is an establishment, or part of one, that sells oxygen for recreational use. Individual scents may be added to enhance the experience. The flavors in an oxygen bar come from bubbling oxygen through bottles containing aromatic solutio ...
s and in sports. Oxygen bars are establishments found in the United States since the late 1990s that offer higher than normal exposure for a minimal fee. Professional athletes, especially in
American football American football (referred to simply as football in the United States and Canada), also known as gridiron, is a team sport played by two teams of eleven players on a rectangular field with goalposts at each end. The offense, the team wi ...
, sometimes go off-field between plays to don oxygen masks to boost performance. The pharmacological effect is doubted; a
placebo A placebo ( ) is a substance or treatment which is designed to have no therapeutic value. Common placebos include inert tablets (like sugar pills), inert injections (like Saline (medicine), saline), sham surgery, and other procedures. In general ...
effect is a more likely explanation. Available studies support a performance boost from oxygen enriched mixtures only if it is inhaled ''during'' aerobic exercise. Other recreational uses that do not involve breathing include pyrotechnic applications, such as George Goble's five-second ignition of barbecue grills.


Industrial

Smelting of iron ore into steel consumes 55% of commercially produced oxygen. In this process, is injected through a high-pressure lance into molten iron, which removes sulfur impurities and excess carbon as the respective oxides, and . The reactions are exothermic, so the temperature increases to 1,700 ° C. Another 25% of commercially produced oxygen is used by the chemical industry. Ethylene is reacted with to create ethylene oxide, which, in turn, is converted into ethylene glycol; the primary feeder material used to manufacture a host of products, including antifreeze and polyester polymers (the precursors of many
plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adapta ...
s and fabrics). Large quantities of oxygen or air is used in oxy-cracking process and for the production of acrylic acid, diformyl-furane, and benzylic acid. On the other hand, the electrochemical synthesis of hydrogen peroxide from oxygen is a promising technology to replace the currently used hydroquinone-process. Last but not least, catalytic oxidation is used in afterburners to get rid of hazardous gases. Most of the remaining 20% of commercially produced oxygen is used in medical applications, metal cutting and welding, as an oxidizer in rocket fuel, and in water treatment. Oxygen is used in oxyacetylene welding, burning acetylene with to produce a very hot flame. In this process, metal up to thick is first heated with a small oxy-acetylene flame and then quickly cut by a large stream of . Cook & Lauer 1968, p. 508


Compounds

The oxidation state of oxygen is −2 in almost all known compounds of oxygen. The oxidation state −1 is found in a few compounds such as peroxides. Compounds containing oxygen in other oxidation states are very uncommon: −1/2 ( superoxides), −1/3 ( ozonides), 0 ( elemental, hypofluorous acid), +1/2 ( dioxygenyl), +1 ( dioxygen difluoride), and +2 ( oxygen difluoride).


Oxides and other inorganic compounds

Water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
() is an oxide of
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
and the most familiar oxygen compound. Hydrogen atoms are covalently bonded to oxygen in a water molecule but also have an additional attraction (about 23.3 kJ/mol per hydrogen atom) to an adjacent oxygen atom in a separate molecule. These hydrogen bonds between water molecules hold them approximately 15% closer than what would be expected in a simple liquid with just
van der Waals force In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and ...
s.Also, since oxygen has a higher electronegativity than hydrogen, the charge difference makes it a polar molecule. The interactions between the different dipoles of each molecule cause a net attraction force. Due to its electronegativity, oxygen forms chemical bonds with almost all other elements to give corresponding oxides. The surface of most metals, such as
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
and titanium, are oxidized in the presence of air and become coated with a thin film of oxide that passivates the metal and slows further
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
. Many oxides of the
transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
s are non-stoichiometric compounds, with slightly less metal than the chemical formula would show. For example, the mineral
FeO Iron(II) oxide or ferrous oxide is the inorganic compound with the formula FeO. Its mineral form is known as wüstite. One of several iron oxides, it is a black-colored powder that is sometimes confused with rust, the latter of which consists of ...
( wüstite) is written as \ce_\ce, where ''x'' is usually around 0.05. Oxygen is present in the atmosphere in trace quantities in the form of
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
(). The Earth's crustal rock is composed in large part of oxides of silicon (
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
, as found in
granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies un ...
and quartz), aluminium ( aluminium oxide , in bauxite and corundum), iron ( iron(III) oxide , in hematite and rust), and calcium carbonate (in
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
). The rest of the Earth's crust is also made of oxygen compounds, in particular various complex silicates (in silicate minerals). The Earth's mantle, of much larger mass than the crust, is largely composed of silicates of magnesium and iron. Water- soluble silicates in the form of , , and are used as detergents and adhesives. Cook & Lauer 1968, p. 507 Oxygen also acts as a ligand for transition metals, forming
transition metal dioxygen complexes Transition or transitional may refer to: Mathematics, science, and technology Biology * Transition (genetics), a point mutation that changes a purine nucleotide to another purine (A ↔ G) or a pyrimidine nucleotide to another pyrimidine (C ↔ ...
, which feature metal–. This class of compounds includes the heme proteins
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythroc ...
and myoglobin. An exotic and unusual reaction occurs with , which oxidizes oxygen to give O2+PtF6, dioxygenyl hexafluoroplatinate. Cook & Lauer 1968, p.505


Organic compounds

Among the most important classes of organic compounds that contain oxygen are (where "R" is an organic group): alcohols (R-OH); ethers (R-O-R); ketones (R-CO-R); aldehydes (R-CO-H); carboxylic acids (R-COOH); esters (R-COO-R); acid anhydrides (R-CO-O-CO-R); and amides (). There are many important organic solvents that contain oxygen, including: acetone, methanol,
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
, isopropanol, furan,
THF Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...
, diethyl ether, dioxane, ethyl acetate, DMF, DMSO, acetic acid, and formic acid. Acetone () and phenol () are used as feeder materials in the synthesis of many different substances. Other important organic compounds that contain oxygen are: glycerol, formaldehyde, glutaraldehyde, citric acid, acetic anhydride, and acetamide. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms. The element is similarly found in almost all biomolecules that are important to (or generated by) life. Oxygen reacts spontaneously with many
organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
compounds at or below room temperature in a process called
autoxidation Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organic ...
. Cook & Lauer 1968, p. 506 Most of the
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s that contain oxygen are not made by direct action of . Organic compounds important in industry and commerce that are made by direct oxidation of a precursor include ethylene oxide and
peracetic acid Peracetic acid (also known as peroxyacetic acid, or PAA) is an organic compound with the formula CH3CO3H. This peroxy acid is a colorless liquid with a characteristic acrid odor reminiscent of acetic acid. It can be highly corrosive. Peracetic ...
.


Safety and precautions

The
NFPA 704 "NFPA 704: Standard System for the Identification of the Hazards of Materials for Emergency Response" is a standard maintained by the U.S.-based National Fire Protection Association. First "tentatively adopted as a guide" in 1960, and revised sev ...
standard rates compressed oxygen gas as nonhazardous to health, nonflammable and nonreactive, but an oxidizer. Refrigerated liquid oxygen (LOX) is given a health hazard rating of 3 (for increased risk of hyperoxia from condensed vapors, and for hazards common to cryogenic liquids such as frostbite), and all other ratings are the same as the compressed gas form.


Toxicity

Oxygen gas () can be
toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a sub ...
at elevated partial pressures, leading to convulsions and other health problems.Since 's partial pressure is the fraction of times the total pressure, elevated partial pressures can occur either from high fraction in breathing gas or from high breathing gas pressure, or a combination of both. Cook & Lauer 1968, p. 511 Oxygen toxicity usually begins to occur at partial pressures more than 50 kilo
pascals The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI), and is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is defin ...
 (kPa), equal to about 50% oxygen composition at standard pressure or 2.5 times the normal sea-level partial pressure of about 21 kPa. This is not a problem except for patients on mechanical ventilators, since gas supplied through oxygen masks in medical applications is typically composed of only 30–50% by volume (about 30 kPa at standard pressure). At one time, premature babies were placed in incubators containing -rich air, but this practice was discontinued after some babies were blinded by the oxygen content being too high. Breathing pure in space applications, such as in some modern space suits, or in early spacecraft such as Apollo, causes no damage due to the low total pressures used. In the case of spacesuits, the partial pressure in the breathing gas is, in general, about 30 kPa (1.4 times normal), and the resulting partial pressure in the astronaut's arterial blood is only marginally more than normal sea-level partial pressure. Oxygen toxicity to the lungs and central nervous system can also occur in deep
scuba diving Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface air supply. The name "scuba", an acronym for " Self-Contained Underwater Breathing Apparatus", was coined by Chr ...
and
surface supplied diving Surface-supplied diving is diving using equipment supplied with breathing gas using a diver's umbilical from the surface, either from the shore or from a diving support vessel, sometimes indirectly via a diving bell. This is different from scub ...
. Prolonged breathing of an air mixture with an partial pressure more than 60 kPa can eventually lead to permanent pulmonary fibrosis. Exposure to an partial pressures greater than 160 kPa (about 1.6 atm) may lead to convulsions (normally fatal for divers). Acute oxygen toxicity (causing seizures, its most feared effect for divers) can occur by breathing an air mixture with 21% at or more of depth; the same thing can occur by breathing 100% at only .


Combustion and other hazards

Highly concentrated sources of oxygen promote rapid combustion.
Fire Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. At a certain point in the combustion reaction, called the ignition point, flames ...
and explosion hazards exist when concentrated oxidants and fuels are brought into close proximity; an ignition event, such as heat or a spark, is needed to trigger combustion. Oxygen is the oxidant, not the fuel. Concentrated will allow combustion to proceed rapidly and energetically. Steel pipes and storage vessels used to store and transmit both gaseous and liquid oxygen will act as a fuel; and therefore the design and manufacture of systems requires special training to ensure that ignition sources are minimized. The fire that killed the Apollo 1 crew in a launch pad test spread so rapidly because the capsule was pressurized with pure but at slightly more than atmospheric pressure, instead of the normal pressure that would be used in a mission. Liquid oxygen spills, if allowed to soak into organic matter, such as wood, petrochemicals, and asphalt can cause these materials to detonate unpredictably on subsequent mechanical impact.


See also

* Geological history of oxygen * Hypoxia (environmental) for depletion in aquatic ecology * Ocean deoxygenation * Hypoxia (medical), a lack of oxygen *
Limiting oxygen concentration The limiting oxygen concentration (LOC), also known as the minimum oxygen concentration (MOC), is defined as the limiting concentration of oxygen below which combustion is not possible, independent of the concentration of fuel. It is expressed i ...
* Oxygen compounds *
Oxygen plant Oxygen plants are industrial systems designed to generate oxygen. They typically use air as a feedstock and separate it from other components of air using pressure swing absorption or membrane separation techniques. Such plants are distinct from ...
* Oxygen sensor


Notes


References


General references

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External links


Oxygen
at '' The Periodic Table of Videos'' (University of Nottingham)
Oxidizing Agents > Oxygen



Roald Hoffmann article on "The Story of O"


*
Scripps Institute: Atmospheric Oxygen has been dropping for 20 years
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