Oxygen
Oxygen is a
chemical element. In the
periodic table it has the symbol
O and
atomic number 8. Oxygen is the second most common element on
Earth composing around 49% of the mass of Earth's crust[
1] and 28% of the mass of Earth as a whole, and is the third most common element in the
universe. On Earth, it is usually
covalently or
ionically bonded to other elements. Unbound oxygen (usually called molecular dioxygen, O
2, a
diatomic molecule) first appeared in significant quantities on Earth during the
Paleoproterozoic era (between 2.5 billion years ago and 1.6 billion years ago) as a product of the
metabolic action of early
anaerobes (
archaea and
bacteria). 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.
At
standard temperature and pressure, oxygen exists as a diatomic molecule with the formula O
2, in which the 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. The word
oxygen derives from two words in
Greek,
οξυς (oxys) (acid, sharp) and
γεινομαι (geinomai) (engender). The name "oxygen" was chosen because, at the time it was discovered in the late 18th century, it was believed that all acids contained oxygen. The
definition of acid has since been revised to not require oxygen in the molecular structure.
Liquid O2 and solid O
2 have a light blue color and both are highly
paramagnetic. Liquid O
2 is usually obtained by the
fractional distillation of liquid air.Liquid and solid O
3 (
ozone) have a deeper color of blue.
A recently discovered
allotrope of oxygen,
tetraoxygen (O
4), is a deep red solid that is created by pressurizing O
2 to the order of 20 GPa. Its properties are being studied for use in
rocket fuels and similar applications, as it is a much more powerful oxidizer than either O
2 or
O3.[
2]
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 was first described by
Michał Sędziwój, a Polish
alchemist and
philosopher in the late 16th century. Sędziwój thought of the gas given off by warm
nitre (saltpeter) as "the elixir of life".
Oxygen was more quantitatively discovered by the
Swedish pharmacist
Carl Wilhelm Scheele sometime before 1773, but the discovery was not published until after the independent discovery by
Joseph Priestley on
August 1,
1774, who called the gas
dephlogisticated air (see
phlogiston theory). Priestley published discoveries in
1775 and Scheele in
1777; consequently Priestley is usually given the credit. Both Scheele and Priestley produced oxygen by heating
manganese oxide to red-hot.
The gas was named by
Antoine Laurent Lavoisier, after Priestley's publication in 1775, from
Greek roots meaning "
acid-former". The name reflects the then-common, but incorrect, belief that all acids contain oxygen.
Oxygen is the most common component of the
Earth's crust (49% by mass)[
3], the second most common component of the Earth as a whole (28.2% by mass), and the second most common component of the
Earth's atmosphere (20.947% by volume).
See also Silicate minerals, Oxide minerals. |
Dioxygen, O2, is a gas and consists of 2 oxygen atoms. Oxygen is most commonly encountered in this form, as it makes up 21% of the atmosphere. |
|
Ozone, O3, is a gas and 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 (H
2O). Other well known examples include compounds of carbon and oxygen, such as
carbon dioxide (CO
2),
alcohols (R-OH),
carbonyls, (R-CO-H( or R
2)), and
carboxylic acids (R-COOH). Oxygenated
radicals such as
chlorates (ClO
3−),
perchlorates (ClO
4−),
chromates (CrO
42−),
dichromates (Cr
2O
72−),
permanganates (MnO
4−), and
nitrates (NO
3−) are strong oxidizing agents in and of themselves. Many metals such as iron bond with oxygen atoms,
iron (III) oxide (Fe
2O
3).
Ozone (O
3) is formed by electrostatic discharge in the presence of molecular oxygen. A double oxygen molecule (O
2)
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 O
2+PtF
6−. 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 PtF
6. This led him to the discovery of
xenon hexafluoroplatinate Xe
+PtF
6−.
See also Oxygen compounds.Oxygen has seventeen known
isotopes with
atomic masses ranging from 12.03 u to 28.06 u. Three are stable,
16O,
17O, and
18O, of which
16O is the most abundant (over 99.7%). The radioisotopes all have half-lives of less than three minutes.
An atomic weight of 16 was assigned to oxygen prior to the definition of the
unified atomic mass unit based upon
12C. Since physicists referred to
16O only, while chemists meant the naturally abundant mixture of isotopes, this led to slightly different atomic weight scales.
Oxygen can be
toxic at elevated
partial pressures (i.e. high relative concentrations). This is important in deep
scuba diving and
surface supplied diving and when using equipment which can provide high concentrations of oxygen such as
rebreathers.
Certain derivatives of oxygen, such as
ozone (O
3),
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.
*
Aerobic*
Breathing gas - the role of oxygen in a breathing gas
*
CombustionThough oxygen promotes combustion in other compounds it is not flamable itself.
*
Hypoxia, a lack of oxygen
*
Hypoxia (environmental) for oxygen depletion in aquatic ecology
*
Oxidation*
Ozone layer*
Oxygen Catastrophe in geology
*
Oxygen isotope ratio cycle*
Oxygen tank*
Winkler test for dissolved oxygen for instructions on how to determine the amount of oxygen
dissolved in fresh water.
*
Los Alamos National Laboratory – Oxygen*
Nist atomic spectra database*
Nuclides and Isotopes Fourteenth Edition: Chart of the Nuclides,
General Electric Company, 1989
*
Priestley Society, Dedicated to Joseph Priestley the man who discovered oxygen*
Los Alamos National Laboratory – Oxygen*
WebElements.com – Oxygen*
It's Elemental – Oxygen*
Oxygen (O2) Properties, Uses, Applications*
Computational Chemistry Wiki*
Oxidizing Agents > Oxygenzh-yue:氧
