At the top of the Earth's atmosphere isozone (O 3), in theozone layer. It absorbsultraviolet radiation, which means less radiation reaches ground level.
Oxygen gas is used for makingsteel,plastics andtextiles. It also has medical uses and is used for breathing when there is no good air (bydivers andfirefighters, for example), and forwelding. Liquid oxygen and oxygen-rich compounds can be used as arocket propellant.
One of the first knownexperiments on howcombustion needsair was carried out byGreekPhilo of Byzantium in the 2nd century BC. He wrote in his workPneumatica that turning avessel upside down over a burningcandle and putting water around this vessel meant that some water went into the vessel.[3] Philo thought this was because the air was turned into theclassical element fire. This is wrong. A long time after,Leonardo da Vinci worked out that some air was used up during combustion, and this forced water into the vessel.[4]
In the late 17th century,Robert Boyle found that air is needed for combustion.EnglishchemistJohn Mayow added to this by showing that fire only needed a part of air. We now call this oxygen (O2).[5] He found that a candle burning in a closed container made the water rise to replace a fourteenth of the air'svolume before it went out.[6] The same thing happened when a livemouse was put into the box. From this, he worked out that oxygen is used for both respiration and combustion.
J. J. Becher came up with the theory in 1667, andGeorg Ernst Stahl added to it in 1731.[9] Thephlogiston 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.[4]
Materials that leave very littleresidue when they burn, likewood orcoal, were thought to be made mostly of phlogiston. Things thatcorrode, likeiron, were thought to contain very little. Air was not part of this theory.[4]
Polishalchemist,philosopher andphysicianMichael Sendivogius wrote about something in air that he called the "food of life",[10] and this meant what we now call oxygen.[11] Sendivogius found, between 1598 and 1604, that the substance in air is the same as he got by heatingpotassium nitrate. Some people believe this was the discovery of oxygen while others disagree. Some say that oxygen was discovered bySwedish pharmacistCarl Wilhelm Scheele. He got oxygen in 1771 by heatingmercuric oxide and somenitrates.[4][12][13] Scheele called the gas "fire air", because it was the only gas known to allow combustion (gases were called "airs" at this time). He published his discovery in 1777.[14]
On 1 August 1774,British clergymanJoseph Priestley focused sunlight onmercuric oxide in aglass tube. From thisexperiment he got a gas that he called "dephlogisticated air".[13] He found that candles burned more brightly in the gas and a mouse lived longer whilebreathing it. After breathing the gas, Priestley said that it felt like normal air, but his lungs felt lighter and easy afterwards.[7] His findings were published in 1775.[4][15] It is because his findings were published first that he is often said to have discovered oxygen.
French chemistAntoine Lavoisier later said he had discovered the substance as well. Priestley visited him in 1774 and told him about his experiment. Scheele also sent a letter to Lavoisier in that year that spoke of his discovery.[14]
Lavoisier did the first main experiments onoxidation. He was the first person to explain how combustion works.[13] He used these and other experiments to prove the phlogiston theory wrong. He also tried to prove that the substance discovered by Priestley and Scheele was achemical element.
In one experiment, Lavoisier found that there was no increase inweight whentin and air were heated in a closedcontainer. He also found that air rushed in when the container was opened. After this, he found that the weight of the tin had increased by the same amount as the weight of the air that rushed in. He published his findings in 1777.[13] He wrote that air was made up of two gases. One he called "vital air" (oxygen), which is needed for combustion and respiration. The other (nitrogen) he called "azote", which means "lifeless" inGreek. (This is still the name of nitrogen in some languages, includingFrench.)[13]
Lavoisier renamed "vital air" to "oxygène", from Greek words meaning "sour making" or "producer ofacid". He called it this because he thought oxygen was in all acids, which is wrong.[16] Later chemists realised that Lavoiser's name for the gas was wrong, but the name was too common by then to change.[17]
"Oxygen" became the name in theEnglish language, even though English scientists were against it.
John Dalton's theory ofatoms said that all elements had one atom and atoms in compounds were usually alone. For example, he wrongly thought that water (H2O) had the formula of just HO.[18] In 1805,Joseph Louis Gay-Lussac andAlexander von Humboldt showed that water is made up of two hydrogen atoms and one oxygen atom. By 1811,Amedeo Avogadro correctly worked out what water was made of based onAvogadro's law.[19]
By the late 19th century, scientists found that air could be turned into a liquid and the compounds in it could be isolated bycompressing and cooling it.Swiss chemist and physicistRaoul Pictet discoveredliquid oxygen byevaporatingsulfur dioxide to turncarbon dioxide into a liquid. This was then also evaporated to cool oxygen gas in order to turn it into a liquid. He sent atelegram to theFrench Academy of Sciences on 22 December 1877 telling them of his discovery.[20]
Asdioxygen (or justoxygen gas), two oxygenatoms arechemically bound to each other. This bond can be called many things, but simply called acovalentdouble bond. Oxygen gas is veryreactive and can react with many other elements.Oxides are made whenmetal elements react with oxygen, such asiron oxide, which is known asrust. There are a lot of oxide compounds on Earth.
The commonallotrope (type) of oxygen on Earth is called dioxygen (O2). This is the second biggest part of the Earth's atmosphere, after dinitrogen (N2). O2 has a bond length of 121pm and a bondenergy of 498kJ/mol[22] Because of its energy, O2 is used by complex life likeanimals.
Ozone (O3) is very reactive and damages thelungs when breathed in.[23] Ozone is made in the upperatmosphere when O2 combines with pure oxygen made when O2 is split byultraviolet radiation.[16] Ozone absorbs a lot of radiation in the UV part of theelectromagnetic spectrum and so theozone layer in the upper atmosphere protects Earth from radiation.
Above the ozone layer, (inlow Earth orbits), atomic oxygen becomes the most common form.[24]
Tetraoxygen (O4) was discovered in 2001.[25][26] It only exists in extreme conditions when a lot ofpressure is put onto O2.
Oxygendissolves more easily from air into water thannitrogen does. When there is the same amount of air and water, there is one molecule of O2 for every 2 molecules of N2 (aratio of 1:2). This is different to air, where there is a 1:4 ratio of oxygen to nitrogen. It is also easier for O2 to dissolve infreshwater than inseawater.[7][27] Oxygencondenses at 90.20 K (-182.95°C, -297.31 °F) andfreezes at 54.36 K (-218.79 °C, -361.82 °F).[28] Bothliquid and solid O2 are see-through with a light-blue colour.
Oxygen is very reactive and must be kept away from anything that can burn.[29]
Apart fromiron, oxygen is the most common element on Earth (by mass). It makes up nearly half (46% to 49.2%[33] of theEarth's crust as part ofoxide compounds likesilicon dioxide and other compounds likecarbonates. It is also the main part of the Earth'soceans, making up 88.8% by mass. Oxygen gas is the second most common part of the atmosphere, making up 20.95%[34] of its volume and 23.1% of its volume. Earth is strange compared to otherplanets, as a large amount of its atmosphere is oxygen gas.Mars has only 0.1%O 2 by volume, and the other planets have less than that.
The much higher amount of oxygen gas around Earth is caused by the oxygen cycle.Photosynthesis takes hydrogen fromwater usingenergy from sunlight. This gives off oxygen gas. Some of the hydrogen combines withcarbon dioxide to makecarbohydrates.Respiration then takes oxygen gas out of the atmosphere or water and turns it into carbon dioxide and water.[35]
O2 is a very important part ofrespiration. Because of this, it is used in medicine. It is used to increase the amount of oxygen in a person'sblood so more respiration can take place. This can make them become healthy quicker if they are ill.Oxygen therapy is used to treatemphysema,pneumonia, someheart problems, and anydisease that makes it harder for a person to take in oxygen.[36]
Low-pressure O2 is used inspace suits, surrounding the body with the gas. Pure oxygen is used but at a much lower pressure. If the pressure were higher, it would be poisonous.[37][38]
Theoxidation state of oxygen is −2 in nearly everycompound it is in. In a few compounds, the oxidation state is −1, such asperoxides. Compounds of oxygen with other oxygen states are very uncommon.[40]
Water (H 2O) is anoxide ofhydrogen. It is the most common oxide on Earth. All knownlife needs water to live. Water is made of two hydrogen atomscovalent bonded to an oxygen atom (oxygen has a higherelectronegativity than hydrogen).[41] (this is the basic principle of covalent bonding)There are also electrostatic forces (Van de'r Waals forces) between the hydrogen atoms and adjacent molecules' oxygen atoms. These pseudo-bonds bring the atoms around 15% closer to each other than most other simple liquids. This is because Water is apolar molecule (Net asymmetrical distribution of electrons) due to its bent shape, giving it an overall net field direction, mainly due to oxygens 2 non bonding pairs of electrons, pushing the bonding H's further together than the linear arrangement with lower enthalpy (see CO2). This property is exploited by microwaves to oscillate polar molecules, especially water. And its responsible for the extra energy needed to disassociate H20.[42]
Because of oxygen's high electronegativity, it makeschemical bonds with almost all other chemical elements. These bonds giveoxides (for exampleiron reacts with oxygen to giveiron oxide). Mostmetal's surfaces are turned into oxides when inair. Iron's surface will turn torust (iron oxide) when in air for a long time. There are small amounts ofcarbon dioxide (CO 2) in the air, and it is turned intocarbohydrates duringphotosynthesis. Living things give it off duringrespiration.[43]
One hundred million tonnes of O2 are gotten from air for industrial uses every year. Industries use two main methods to make oxygen. The most common method isfractional distillation ofliquefied air. N2evaporates while O2 is left as aliquid.[7] O2 is the second most important industrialgas. Because it is more economical, oxygen is usually stored and transported as aliquid. A small steel tank of 16liters water capacity with a workingpressure of 139bar (2015 psi) holds about 2150 liters of gas and weighs 28kilograms (62 lb) empty. 2150 liters of oxygen weighs about 3 kilograms (6.6 lb).
The other main method of making oxygen is by passing a stream of clean, dry air through a pair ofzeolitemolecular sieves. The zeolite molecular sieves soak up the nitrogen. It gives a stream of gas that is 90% to 93% oxygen.[7]
At highpressures, oxygen gas (O2) can be dangerous toanimals, includinghumans. It can causeconvulsions and otherhealth problems.[a][46] Oxygentoxicity usually begins to occur at pressures more than 50 kilopascals (kPa), equal to about 50% oxygen in the air at standard pressure (air on Earth has around 20% oxygen).[7]
Premature babies used to be placed in boxes with air with a high amount of O2. This was stopped when some babies wentblind from the oxygen.[7]
Breathing pure O2 in space suits causes no damage because there is a lower pressure used.[47]
Concentrated amounts of pure O2 can cause a quickfire. When concentrated oxygen andfuels are brought close together, a slightignition can cause a huge fire.[48] TheApollo 1 crew were all killed by a fire because the air of the capsule had a very high amount of oxygen.[b][50]
↑4.04.14.24.34.4Cook, Gerhard A. & Lauer, Carol M. 1968. "Oxygen". In Clifford A. Hampel (ed.). The Encyclopedia of the Chemical Elements. New York: Reinhold Book Corporation. pp. 499–512. LCCN 68-29938. p499.
↑"Atomic oxygen erosion". Archived fromthe original on June 13, 2007. RetrievedJuly 2, 2020.atomic oxygen, the major component of the low Earth orbit environment
↑Lide, David R. (2003). "Section 4, Properties of the elements and inorganic Compounds; melting, boiling, and critical temperatures of the elements".CRC Handbook of Chemistry and Physics (84th ed.). Boca Raton, Florida: CRC Press.ISBN978-0-8493-0595-5.
↑"Oxygen". Los Alamos National Laboratory. Archived fromthe original on October 26, 2007. RetrievedDecember 16, 2007.
↑Mackenzie, F.T. and J.A. (1995)."Gaseous Composition of Dry Air".Our changing planet. Prentice-Hall. pp. 288–307. Archived fromthe original on 2020-04-13. Retrieved2020-07-02.20.947
↑Canfield, Donald 2014.Oxygen: a four billion year history. Princeton: Princeton University Press.ISBN 978-0-691-14502-0
↑Webb JT; Olson RM; Krutz RW; Dixon G; Barnicott PT (1989). "Human tolerance to 100% oxygen at 9.5 psia during five daily simulated 8-hour EVA exposures".Aviat Space Environ Med.60 (5):415–21.doi:10.4271/881071.PMID2730484.
↑Wade, Mark (2007)."Space Suits". Encyclopedia Astronautica. Archived fromthe original on December 13, 2007. RetrievedDecember 16, 2007.
↑48.048.1Werley, Barry L, ed. (1991).ASTM Technical Professional training.Fire hazards in oxygen systems. Philadelphia: ASTM International Subcommittee G-4.05.
↑Report of Apollo 204 Review Board NASA Historical Reference Collection, NASA History Office, NASA HQ, Washington DC
↑Chiles, James R. (2001).Inviting Disaster: lessons from the edge of technology: an inside look at catastrophes and why they happen. New York: HarperCollins Publishers Inc.ISBN978-0-06-662082-4.
↑SinceO 2's partial pressure is the fraction ofO 2 times the total pressure, elevated partial pressures can occur either from highO 2 fraction in breathing gas or from high breathing gas pressure, or a combination of both.
↑No single ignition source of the fire was conclusively identified, although some evidence points to an arc from an electrical spark.[49]
Emsley, John (2001)."Oxygen".Nature's building blocks: An A-Z guide to the elements. Oxford, England: Oxford University Press. pp. 297–304.ISBN978-0-19-850340-8.
Canfield, Donald 2014.Oxygen: a four billion year history. Princeton: Princeton University Press.ISBN 978-0-691-14502-0
Lane, Nick 2002.Oxygen: the molecule that made the world. Oxford University Press.ISBN 0-19-860783-0
