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Oxygen is a chemical element; it has 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.
- gas: colorless, liquid and solid: pale blue
- 461000 ppm
- 8
- Overview
- History
- Occurrence and properties
- Allotropy
- Preparative methods
- Commercial production and use
- Chemical properties and reactions
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oxygen (O), nonmetallic chemical element of Group 16 (VIa, or the oxygen group) of the periodic table. Oxygen is a colourless, odourless, tasteless gas essential to living organisms, being taken up by animals, which convert it to carbon dioxide; plants, in turn, utilize carbon dioxide as a source of carbon and return the oxygen to the atmosphere. O...
Oxygen was discovered about 1772 by a Swedish chemist, Carl Wilhelm Scheele, who obtained it by heating potassium nitrate, mercuric oxide, and many other substances. An English chemist, Joseph Priestley, independently discovered oxygen in 1774 by the thermal decomposition of mercuric oxide and published his findings the same year, three years before Scheele published. In 1775–80, French chemist Antoine-Laurent Lavoisier, with remarkable insight, interpreted the role of oxygen in respiration as well as combustion, discarding the phlogiston theory, which had been accepted up to that time; he noted its tendency to form acids by combining with many different substances and accordingly named the element oxygen (oxygène) from the Greek words for “acid former.”
Britannica Quiz
At 46 percent of the mass, oxygen is the most plentiful element in Earth’s crust. The proportion of oxygen by volume in the atmosphere is 21 percent and by weight in seawater is 89 percent. In rocks, it is combined with metals and nonmetals in the form of oxides that are acidic (such as those of sulfur, carbon, aluminum, and phosphorus) or basic (such as those of calcium, magnesium, and iron) and as saltlike compounds that may be regarded as formed from the acidic and basic oxides, as sulfates, carbonates, silicates, aluminates, and phosphates. Plentiful as they are, these solid compounds are not useful as sources of oxygen, because separation of the element from its tight combinations with the metal atoms is too expensive.
Below −183 °C (−297 °F), oxygen is a pale blue liquid; it becomes solid at about −218 °C (−361 °F). Pure oxygen is 1.1 times heavier than air.
During respiration, animals and some bacteria take oxygen from the atmosphere and return to it carbon dioxide, whereas by photosynthesis, green plants assimilate carbon dioxide in the presence of sunlight and evolve free oxygen. Almost all the free oxygen in the atmosphere is due to photosynthesis. About 3 parts of oxygen by volume dissolve in 100 parts of fresh water at 20 °C (68 °F), slightly less in seawater. Dissolved oxygen is essential for the respiration of fish and other marine life.
Natural oxygen is a mixture of three stable isotopes: oxygen-16 (99.759 percent), oxygen-17 (0.037 percent), and oxygen-18 (0.204 percent). Several artificially prepared radioactive isotopes are known. The longest-lived, oxygen-15 (124-second half-life), has been used to study respiration in mammals.
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Oxygen has two allotropic forms, diatomic (O2) and triatomic (O3, ozone). The properties of the diatomic form suggest that six electrons bond the atoms and two electrons remain unpaired, accounting for the paramagnetism of oxygen. The three atoms in the ozone molecule do not lie along a straight line.
Ozone may be produced from oxygen according to the equation:
The process, as written, is endothermic (energy must be provided to make it proceed); conversion of ozone back into diatomic oxygen is promoted by the presence of transition metals or their oxides. Pure oxygen is partly transformed into ozone by a silent electrical discharge; the reaction is also brought about by absorption of ultraviolet light of wavelengths around 250 nanometres (nm, the nanometre, equal to 10−9 metre); occurrence of this process in the upper atmosphere removes radiation that would be harmful to life on the surface of the Earth. The pungent odour of ozone is noticeable in confined areas in which there is sparking of electrical equipment, as in generator rooms. Ozone is light blue; its density is 1.658 times that of air, and it has a boiling point of −112 °C (−170 °F) at atmospheric pressure.
Ozone is a powerful oxidizing agent, capable of converting sulfur dioxide to sulfur trioxide, sulfides to sulfates, iodides to iodine (providing an analytical method for its estimation), and many organic compounds to oxygenated derivatives such as aldehydes and acids. The conversion by ozone of hydrocarbons from automotive exhaust gases to these acids and aldehydes contributes to the irritating nature of smog. Commercially, ozone has been used as a chemical reagent, as a disinfectant, in sewage treatment, water purification, and bleaching textiles.
Production methods chosen for oxygen depend upon the quantity of the element desired. Laboratory procedures include the following:
1. Thermal decomposition of certain salts, such as potassium chlorate or potassium nitrate:
The decomposition of potassium chlorate is catalyzed by oxides of transition metals; manganese dioxide (pyrolusite, MnO2) is frequently used. The temperature necessary to effect the evolution of oxygen is reduced from 400 °C to 250 °C by the catalyst.
2. Thermal decomposition of oxides of heavy metals:
Scheele and Priestley used mercury(II) oxide in their preparations of oxygen.
3. Thermal decomposition of metal peroxides or of hydrogen peroxide:
When required in tonnage quantities, oxygen is prepared by the fractional distillation of liquid air. Of the main components of air, oxygen has the highest boiling point and therefore is less volatile than nitrogen and argon. The process takes advantage of the fact that when a compressed gas is allowed to expand, it cools. Major steps in the operation include the following: (1) Air is filtered to remove particulates; (2) moisture and carbon dioxide are removed by absorption in alkali; (3) the air is compressed and the heat of compression removed by ordinary cooling procedures; (4) the compressed and cooled air is passed into coils contained in a chamber; (5) a portion of the compressed air (at about 200 atmospheres pressure) is allowed to expand in the chamber, cooling the coils; (6) the expanded gas is returned to the compressor with multiple subsequent expansion and compression steps resulting finally in liquefaction of the compressed air at a temperature of −196 °C; (7) the liquid air is allowed to warm to distill first the light rare gases, then the nitrogen, leaving liquid oxygen. Multiple fractionations will produce a product pure enough (99.5 percent) for most industrial purposes.
The steel industry is the largest consumer of pure oxygen in “blowing” high carbon steel—that is, volatilizing carbon dioxide and other nonmetal impurities in a more rapid and more easily controlled process than if air were used. The treatment of sewage by oxygen holds promise for more efficient treatment of liquid effluents than other chemical processes. Incineration of wastes in closed systems using pure oxygen has become important. The so-called LOX of rocket oxidizer fuels is liquid oxygen; the consumption of LOX depends upon the activity of space programs. Pure oxygen is used in submarines and diving bells.
The large values of the electronegativity and the electron affinity of oxygen are typical of elements that show only nonmetallic behaviour. In all of its compounds, oxygen assumes a negative oxidation state as is expected from the two half-filled outer orbitals. When these orbitals are filled by electron transfer, the oxide ion O2− is created. In peroxides (species containing the ion O22−) it is assumed that each oxygen has a charge of −1. This property of accepting electrons by complete or partial transfer defines an oxidizing agent. When such an agent reacts with an electron-donating substance, its own oxidation state is lowered. The change (lowering), from the zero to the −2 state in the case of oxygen, is called a reduction. Oxygen may be thought of as the “original” oxidizing agent, the nomenclature used to describe oxidation and reduction being based upon this behaviour typical of oxygen.
As described in the section on allotropy, oxygen forms the diatomic species, O2, under normal conditions and, as well, the triatomic species ozone, O3. There is some evidence for a very unstable tetratomic species, O4. In the molecular diatomic form there are two unpaired electrons that lie in antibonding orbitals. The paramagnetic behaviour of oxygen confirms the presence of such electrons.
The intense reactivity of ozone is sometimes explained by suggesting that one of the three oxygen atoms is in an “atomic” state; on reacting, this atom is dissociated from the O3 molecule, leaving molecular oxygen.
The molecular species, O2, is not especially reactive at normal (ambient) temperatures and pressures. The atomic species, O, is far more reactive. The energy of dissociation (O2 → 2O) is large at 117.2 kilocalories per mole.
Learn about oxygen, a nonmetallic element essential for life, from Britannica. Find out how oxygen was discovered, how it forms compounds and acids, and how it exists in different forms and isotopes.
Oxygen accounts for about 23% of the atmosphere's mass with pairs of oxygen atoms stuck together to make dioxygen molecules, but it's not just in the air, we breathe. Overall, it's the most abundant element on the earth's surface and the third most abundant in the universe after hydrogen and helium.
Sep 29, 2014 · Density: 0.001429 grams per cubic centimeter. Phase at Room Temperature: Gas. Melting Point: minus 361.82 degrees Fahrenheit (minus 218.79 degrees Celsius) Boiling Point: minus 297.31 degrees F ...
Dec 9, 2021 · Learn about oxygen, medicinal oxygen, oxygen saturation, pulse oximeter, oxygen therapy and COVID-19 from WHO's questions and answers. Find technical specifications, guidance and resources for oxygen sources and distribution.
- What is oxygen?Oxygen (O 2 ) is a chemical element. At standard temperature and pressure, two atoms of oxygen bind together to form a colourless and odourless gas...
- What is medicinal oxygen?Medicinal oxygen is an essential medicine with no substitution. Healthcare professionals use it to treat respiratory diseases like COVID-19 and p...
- What is oxygen saturation?Within the human body oxygen is tightly regulated to prevent hypoxaemia. Hypoxaemia, or low blood oxygen saturation, is a common complication of a...
- How to measure oxygen saturation?The gold standard for measuring arterial oxygen tension and for calculating oxygen saturation is blood gas analysis. This method is invasive and re...
- What is a pulse oximeter and how is it used?Pulse oximetry is the most accurate non-invasive method for detecting hypoxaemia. It is used to measure the percentage of oxygenated haemoglobin in...
- What is the normal oxygen saturation for a healthy individual?Arterial oxygen saturation is referred to as SaO2 when measured by gas analysis and as SpO2 when measured by pulse oximetry. The normal range of Sp...
- What is oxygen therapy?Oxygen therapy or supplemental oxygen is the provision of medical oxygen as a healthcare intervention. Only medicinal-grade oxygen that complies wi...
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Oxygen is the most plentiful element in Earth's crust and the second most abundant in the universe. It is a colourless, odourless, tasteless gas that is essential for life and has many functions in nature and industry. Learn more about its chemical structure, valence, combustion, and availability from Britannica.
Oxygen is the most abundant element in Earth's crust, and after hydrogen and helium, 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 O2.
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