Difference between revisions of "Oxygen" - New World Encyclopedia

From New World Encyclopedia
(→‎Isotopes: editing)
Line 54: Line 54:
 
[[Antoine Lavoisier|Antoine Laurent Lavoisier]], who helped disprove the phlogiston theory, named the gas "oxygen". He coined the term from two [[Ancient Greek|Greek]] words—''οξυς (oxys)'' (acid, sharp) and ''γεινομαι (geinomai)'' (engender)—based on the belief that all acids contain oxygen. Since then, the definition of an acid has been revised so that oxygen is not always part of the molecular structure of every acid.
 
[[Antoine Lavoisier|Antoine Laurent Lavoisier]], who helped disprove the phlogiston theory, named the gas "oxygen". He coined the term from two [[Ancient Greek|Greek]] words—''οξυς (oxys)'' (acid, sharp) and ''γεινομαι (geinomai)'' (engender)—based on the belief that all acids contain oxygen. Since then, the definition of an acid has been revised so that oxygen is not always part of the molecular structure of every acid.
  
== Characteristics ==
+
== Notable characteristics ==
Oxygen is a chemical element in the [[Periodic table]]. Most molecules of free oxygen have the molecular formula O<sub>2</sub>, in which two oxygen [[atom]]s are doubly bonded to each other.
+
In the periodic table, oxygen is located at the top of group 16 (old group VIA), which is a family of nonmetals called the ''[[chalcogen]]s'' or the ''oxygen family''. It also lies between [[nitrogen]] and [[fluorine]] in period 2. In its ordinary, gaseous form, oxygen occurs as diatomic molecules with the formula O<sub>2</sub>, in which two oxygen [[atom]]s are doubly bonded to each other.
  
  

Revision as of 17:06, 2 May 2006

For other uses, see Oxygen (disambiguation).
8 nitrogenoxygenfluorine
-

O

S
O-TableImage.png
periodic table
General
Name, Symbol, Number oxygen, O, 8
Chemical series Nonmetals, chalcogens
Group, Period, Block 16, 2, p
Appearance colorless
O,8.jpg
Atomic mass 15.9994(3) g/mol
Electron configuration 1s2 2s2 2p4
Electrons per shell 2, 6
Physical properties
Phase gas
Density (0 °C, 101.325 kPa)
1.429 g/L
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
Heat of vaporization (O2) 6.82 kJ/mol
Heat capacity (25 °C) (O2)
29.378 J/(mol·K)
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K       61 73 90
Atomic properties
Crystal structure cubic
Oxidation states 2, −1
(neutral oxide)
Electronegativity 3.44 (Pauling scale)
Ionization energies
(more)
1st: 1313.9 kJ/mol
2nd: 3388.3 kJ/mol
3rd: 5300.5 kJ/mol
Atomic radius 60 pm
Atomic radius (calc.) 48 pm
Covalent radius 73 pm
Van der Waals radius 152 pm
Miscellaneous
Magnetic ordering paramagnetic
Thermal conductivity (300 K) 26.58 mW/(m·K)
Speed of sound (gas, 27 °C) 330 m/s
CAS registry number 7782-44-7
Notable isotopes
Main article: Isotopes of oxygen
iso NA half-life DM DE (MeV) DP
16O 99.76% O is stable with 8 neutrons
17O 0.038% O is stable with 9 neutrons
18O 0.21% O is stable with 10 neutrons

Oxygen (chemical symbol O, atomic number 8) is the second most common element on Earth and the third most common element in the universe. At ordinary temperatures and pressures, free oxygen (unbound to any other element) is a colorless, odorless, tasteless gas that makes up about 21% (by volume) of air. In combination with other elements, oxygen forms a variety of compounds, the most important of which is water. According to recent estimates, it constitutes more than 46% of the mass of the Earth's crust and about 28% of the mass of Earth as a whole.

Oxygen is essential for the respiratory function of humans, animals, plants, and some bacteria.


On Earth, it is usually covalently or ionically bonded to other elements. Unbound oxygen (usually called molecular dioxygen, O2, a diatomic molecule) first appeared in significant quantities on Earth during the Paleoproterozoic era (between 2500 million years ago and 1600 million years ago) as a product of the metabolic action of early anaerobes (archaea and bacteria). According to most experts, this new presence of large amounts of free oxygen drove most of the organisms then living to extinction. The atmospheric abundance of free oxygen in later geological epochs and up to the present has been largely driven by photosynthetic organisms, roughly three quarters by phytoplankton and algae in the oceans and one quarter from terrestrial plants.

Discovery of oxygen

Oxygen was first discovered by Michał Sędziwój, a Polish alchemist and philosopher, in the late sixteenth century. Sędziwój realized that air is a mixture of substances, one of which (later called oxygen) is a life-giving substance. He correctly equated this "elixir of life" with the gas given off by heating niter (or saltpeter, the mineral form of potassium nitrate).

Oxygen was rediscovered by the Swedish pharmacist Carl Wilhelm Scheele sometime before 1773, but his finding was not published until after the independent discovery by Joseph Priestley on August 1, 1774. Priestley published his discovery in 1775, and Scheele, in 1777; consequently, Priestley is usually given the credit.

Priestley subscribed to the "phlogiston theory", according to which a burning material releases an invisible, weightless substance called phlogiston, and the surrounding air (or gas) needs to have the capacity to absorb this phlogiston. Priestley found that the gas he discovered could support combustion for longer than ordinary air. He surmised that this gas contained no phlogiston and could absorb more of it than could ordinary air. He therefore called the gas dephlogisticated air.

Antoine Laurent Lavoisier, who helped disprove the phlogiston theory, named the gas "oxygen". He coined the term from two Greek words—οξυς (oxys) (acid, sharp) and γεινομαι (geinomai) (engender)—based on the belief that all acids contain oxygen. Since then, the definition of an acid has been revised so that oxygen is not always part of the molecular structure of every acid.

Notable characteristics

In the periodic table, oxygen is located at the top of group 16 (old group VIA), which is a family of nonmetals called the chalcogens or the oxygen family. It also lies between nitrogen and fluorine in period 2. In its ordinary, gaseous form, oxygen occurs as diatomic molecules with the formula O2, in which two oxygen atoms are doubly bonded to each other.


In its most stable form, oxygen exists as a diradical (triplet oxygen). Though radicals are commonly associated with highly reactive compounds, triplet oxygen is surprisingly (and fortunately) unreactive towards most compounds. Singlet oxygen, a name given to several higher energy species in which all the electron spins are paired, is much more reactive towards common organic molecules. Carotenoids effectively absorb energy from singlet oxygen and convert it back into the unexcited ground state.

Oxygen is a major component of air, produced by plants during photosynthesis, and is necessary for aerobic respiration in animals.

Liquid O2 and solid O2 have a light blue color and both are highly paramagnetic. Liquid O2 is usually obtained by the fractional distillation of liquid air. Liquid and solid O3 (ozone) have a deeper color of blue.

Allotropes

The presence of atmospheric oxygen has led to the formation of ozone (O3) and the ozone layer within the stratosphere. The ozone layer is extremely important for sustaining life, as it absorbs harmful ultraviolet (UV) radiation:

O2 + UV energy → 2O
O + O2 + UV energy → O3

The absorbed solar energy also raises the temperature of the atmosphere within the ozone layer, creating a thermal barrier that helps trap the atmosphere below (as opposed to bleeding out into space).

A recently discovered allotrope of oxygen, tetraoxygen (O4), is a deep red solid that is created by pressurizing O2 to the order of 20 gigapascals (GPa). Its properties are being studied for use in rocket fuels and similar applications, as it is a much more powerful oxidizer than either O2 or O3.

Isotopes

Oxygen has seventeen known isotopes, with atomic masses ranging from 12.03 u to 28.06 u (where u = unified atomic mass unit). Three of these isotopes—16O, 17O, and 18O—are stable, and 16O is the most abundant (over 99.7%). The remaining isotopes are radioactive, with half-lives shorter than three minutes.

An atomic weight of 16 was assigned to oxygen before the definition of the unified atomic mass unit was based upon 12C. Physicists used the atomic mass of 16 when referring to the isotope 16O only, while chemists equated it with the mean atomic mass of the naturally abundant mixture of isotopes. This discrepancy led to slightly different atomic weight scales.

Applications

Liquid oxygen finds use as an oxidizer in rocket propulsion. Oxygen is essential to respiration, so oxygen supplementation has found use in medicine (as oxygen therapy). People who climb mountains or fly in airplanes sometimes have supplemental oxygen supplies (to increase the inspired Oxygen partial pressure nearer to that found at sea-level requires increasing the proportion as a percentage of air). Oxygen is used in welding (such as the oxyacetylene torch), and in the making of steel and methanol.

Oxygen presents two absorption bands centered in the wavelengths 687 and 760 nanometers. Some scientists have proposed to use the measurement of the radiance coming from vegetation canopies in those oxygen bands to characterize plant health status from a satellite platform. This is because in those bands, it is possible to discriminate the vegetation's reflectance from the vegetation's fluorescence, which is much weaker. The measurement presents several technical difficulties due to the low signal to noise ratio and due to the vegetation's architecture, but it has been proposed as a possibility to monitor the carbon cycle from satellites on a global scale.

Oxygen, as a mild euphoric, has a history of recreational use that extends into modern times. Oxygen bars can be seen at parties to this day. In the 19th century, oxygen was often mixed with nitrous oxide to promote an analgesic effect; a stable 50% gaseous mixture (Entonox) is commonly used in medicine today as an analgesic, and 30% oxygen with 70% Nitrous Oxide is the common basic anaesthetic mixture.


Oxygen cycle

The oxygen cycle.

The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within and between its three main reservoirs: the atmosphere, biosphere, and lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for the modern Earth's atmosphere and life as we know it. If all photosynthesis were to cease, the Earth's atmosphere would be devoid of all but trace amounts of oxygen within 5000 years. The oxygen cycle would no longer exist.

The vast majority of molecular oxygen is contained in rocks and minerals within the Earth (99.5%). Only a small fraction has been released as free oxygen to the biosphere (0.01%) and atmosphere (0.49%). The main source of oxygen within the biosphere and atmosphere is photosynthesis which breaks down carbon dioxide and water to create sugars and oxygen:

CO2 + H2O + energy → CH2O + O2

An additional source of atmospheric oxygen comes from photolysis, whereby high energy ultraviolet radiation breaks down atmospheric water and nitrite into component molecules. The free H and N atoms escape into space leaving O2 in the atmosphere:

2H2O + energy → 4H + O2
2N2O + energy → 4N + O2

The main way oxygen is lost from the atmosphere is via respiration and decay mechanisms in which animal life consumes oxygen and releases carbon dioxide. Because lithospheric minerals are reduced in oxygen, surface weathering of exposed rocks also consumes oxygen. An example of surface weathering chemistry is formation of iron-oxides (rust) such as that found in the red sands of Australia:

4FeO + 3O2 → 2Fe2O3

Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create carbonate shell material (CaCO3) that is rich in molecular oxygen. When the organism dies its shell is deposited on the shallow sea floor and buried over time to create limestone rock in the lithosphere. Weathering processes initiated by organisms can also free oxygen from the lithosphere. Plants and animals extract nutrient minerals from rocks and release oxygen in the process.


The following tables offer estimates of oxygen cycle reservoir capacities and fluxes. These numbers are based primarily on estimates from Walker, J.C.G.


Table 1: Major reservoirs involved in the oxygen cycle

Reservoir Capacity
(kg O2)
Flux In/Out
(kg O2 per year)
Residence Time
(years)
Atmosphere 1.4 * 1018 30,000 * 1010 4,500
Biosphere 1.6 * 1016 30,000 * 1010 50
Lithosphere 2.9 * 1020 60 * 1010 500,000,000


Table 2: Annual gain and loss of atmospheric oxygen (Units of 1010 kg O2 per year)

Gains
Photosynthesis (land)
Photosynthesis (ocean)
Photolysis of N2O
Photolysis of H2O
16,500
13,500
1.3
0.03
Total Gains ~ 30,000
Losses - Respiration and Decay
Aerobic Respiration
Microbial Oxidation
Combustion of Fossil Fuel (anthropologic)
Photochemical Oxidation
Fixation of N2 by Lightning
Fixation of N2 by Industry (anthropologic)
Oxidation of Volcanic Gases
23,000
5,100
1,200
600
12
10
5
Losses - Weathering
Chemical Weathering
Surface Reaction of O3
50
12
Total Losses ~ 30,000

Compounds

File:Oxygen -Molecular-.JPG
A common form of oxygen, O2, which is a gas and consists of 2 oxygen atoms.
Ozone, a form of oxygen, O3, which consists of 3 oxygen atoms.

Due to its electronegativity, oxygen forms chemical bonds with almost all other elements hence the origin of the original definition of oxidation. The only elements to escape the possibility of oxidation are a few of the noble gases. The most famous of these oxides is water (H2O). Other well known examples include compounds of carbon and oxygen, such as carbon dioxide (CO2), alcohols (R-OH), aldehydes, (R-CHO), and carboxylic acids (R-COOH). Oxygenated radicals such as chlorates (ClO3), perchlorates (ClO4), chromates (CrO42−), dichromates (Cr2O72−), permanganates (MnO4), and nitrates (NO3) are strong oxidizing agents in and of themselves. Many metals such as iron bond with oxygen atoms, iron (III) oxide (Fe2O3). Ozone (O3) is formed by electrostatic discharge in the presence of molecular oxygen. A double oxygen molecule (O2)2 is known and is found as a minor component of liquid oxygen. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms.

One unexpected oxygen compound is dioxygen hexafluoroplatinate O2+PtF6. It was discovered when Neil Bartlett was studying the properties of PtF6. He noticed a change in color when this compound was exposed to atmospheric air. Bartlett reasoned that xenon should also be oxidized by PtF6. This led him to the discovery of xenon hexafluoroplatinate Xe+PtF6.

See also Oxygen compounds.


Precautions

Oxygen can be toxic at elevated partial pressures (i.e. high relative concentrations). This is important in some forms of scuba diving, such as with a rebreather.

Certain derivatives of oxygen, such as ozone (O3), singlet oxygen, hydrogen peroxide, hydroxyl radicals and superoxide, are also highly toxic. The body has developed mechanisms to protect against these toxic compounds. For instance, the naturally-occurring glutathione can act as an antioxidant, as can bilirubin which is normally a breakdown product of hemoglobin. To protect against the destructive nature of peroxides, nearly every organism on earth has developed some form of the enzyme catalase, which very quickly disproportionates peroxide into water and dioxygen.

Highly concentrated sources of oxygen promote rapid combustion and therefore are fire and explosion hazards in the presence of fuels. The fire that killed the Apollo 1 crew on a test launchpad spread so rapidly because the capsule was pressurized with pure oxygen as would be usual in an actual flight, but to maintain positive pressure in the capsule, this was at slightly more than atmospheric pressure instead of the 1/3 pressure that would be used in flight. (See partial pressure.) Similar hazards also apply to compounds of oxygen with a high oxidative potential, such as chlorates, perchlorates, and dichromates; they also can often cause chemical burns.

Oxygen derivatives are prone to form free radicals, especially in metabolic processes. Because they can cause severe damage to cells and their DNA, they form part of theories of carcinogenesis and aging.

See also

  • Winkler test for dissolved oxygen for instructions on how to determine the amount of oxygen dissolved in fresh water.
  • Combustion
  • Oxidation
  • Oxygen Catastrophe in geology
  • The role of oxygen as a diving breathing gas
  • Oxygen depletion aquatic ecology
  • Ozone layer
  • Oxygen isotope ratio cycle
  • Aerobic
  • Oxide minerals

References
ISBN links support NWE through referral fees

External links

Commons
Wikimedia Commons has media related to::
General subfields within the Natural sciences
Astronomy | Biology | Chemistry | Earth science | Ecology | Physics

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.