Oxygen

**Oxygen** is a chemical element. It has the symbol **O** and atomic number 8. It is a colorless, odorless, and tasteless gas at 25 degrees Celsius at Standard atmosphere (a pressure).
Oxygen is part of the air we breathe. It makes up about 21% of Earth’s atmosphere. It is the third most common element in the universe, after hydrogen and helium. Oxygen is part of the chalcogen group on the periodic table. Oxygen is very reactive, meaning it easily combines with other elements. It often forms oxides, like iron oxide (rust) or carbon dioxide (CO₂). It supports combustion, which means it helps things burn, but it does not burn by itself. In its most common form, oxygen exists as a diatomic molecule (O₂), meaning two oxygen atoms are joined together. There is also a special form called ozone (O₃), made of three oxygen atoms. Ozone is more reactive than normal oxygen and helps protect Earth by blocking harmful ultraviolet (UV) rays from the Sun. Oxygen supports respiration, the process living things use to turn food into energy.

Oxygen is one of the most common elements on Earth. It makes up about 21% of the air in the atmosphere, which is the part we breathe. It is also found in the oceans, lakes, and rivers as part of water (H₂O). Oxygen is also found in rocks, minerals, and soil. It makes up almost half of the Earth’s crust by weight, because it is part of many compounds like silicates and oxides. These are the building blocks of rocks and minerals. Plants, algae, and some bacteria make oxygen during photosynthesis. They take in carbon dioxide and sunlight to produce food and release oxygen as a byproduct. Many molecules in living things have oxygen in them, such as proteins, nucleic acids, carbohydrates and fats. Oxygen is also found in space. It can be found in stars and in clouds of gas between stars. Scientists have even found oxygen in the ice and rocks on other planets and moons. Oxygen is also used in industry. It helps with cutting, welding, and melting metals by making flames hotter. In steel factories, oxygen is used to remove unwanted elements from molten metal, making the final product stronger and cleaner. In space and underwater exploration, oxygen is stored in tanks so astronauts and divers can breathe in places where there is no air. Oxygen is also used in airplanes at high altitudes where the air is too thin to breathe normally. Oxygen is used in water treatment plants to help clean water. It is also used in making plastics, fabrics, and other materials. It is even used in rocket engines, where liquid oxygen helps fuel rockets to launch into space.

## History
Oxygen gas () was isolated by Michael Sendivogius before 1604. It is often thought that the gas was discovered in 1773 by Carl Wilhelm Scheele, in Sweden, or in 1774 by Joseph Priestley, in England. Priestley is usually thought to be the main discoverer because his work was published first (although he called it "dephlogisticated air", and did not think it was a chemical element). Antoine Lavoisier gave the name *oxygène* to the gas in 1777. He was the first person to say it was a chemical element. He was also right about how it helps combustion work.

### Early experiments 
One of the first known experiments on how combustion needs air was carried out by Greek Philo of Byzantium in the 2nd century&nbsp;BC. He wrote in his work *Pneumatica* that turning a  upside down over a burning candle and putting water around this vessel meant that some water went into the vessel. Philo thought this was because the air was turned into the classical 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.

In the late 17th&nbsp;century, Robert Boyle found that air is needed for combustion. English chemist John Mayow added to this by showing that fire only needed a part of air. We now call this oxygen (O2). He found that a candle burning in a closed container made the water rise to replace a fourteenth of the air's volume before it went out. The same thing happened when a live mouse was put into the box. From this, he worked out that oxygen is used for both respiration and combustion.

### Phlogiston theory
Robert Hooke, Ole Borch, Mikhail Lomonosov and Pierre Bayen all made oxygen in experiments in the 17th and 18th centuries. None of them thought it was a chemical element. This was probably because of the idea of the *phlogiston theory*. This was what most people believed caused combustion and corrosion.

J. J. Becher came up with the theory in 1667, and Georg Ernst Stahl added to it in 1731. The 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. and this meant what we now call oxygen. Sendivogius found, between 1598 and 1604, that the substance in air is the same as he got by heating potassium nitrate. Some people believe this was the discovery of oxygen while others disagree. Some say that oxygen was discovered by Swedish pharmacist Carl Wilhelm Scheele. He got oxygen in 1771 by heating mercuric oxide and some nitrates.

On 1 August 1774, British clergyman Joseph Priestley focused sunlight on mercuric oxide in a glass tube. From this experiment he got a gas that he called "dephlogisticated air". He found that candles burned more brightly in the gas and a mouse lived longer while breathing it. After breathing the gas, Priestley said that it felt like normal air, but his lungs felt lighter and easy afterwards. It is because his findings were published first that he is often said to have discovered oxygen.

French chemist Antoine 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. Later chemists realised that Lavoiser's name for the gas was wrong, but the name was too common by then to change.

"Oxygen" became the name in the English language, even though English scientists were against it.

### Later history
John Dalton's theory of atoms 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. In 1805, Joseph Louis Gay-Lussac and Alexander 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 on Avogadro's law.

By the late 19th century, scientists found that air could be turned into a liquid and the compounds in it could be isolated by compressing and cooling it. Swiss chemist and physicist Raoul Pictet discovered liquid oxygen by evaporating sulfur dioxide to turn carbon dioxide into a liquid. This was then also evaporated to cool oxygen gas in order to turn it into a liquid. He sent a telegram to the French Academy of Sciences on 22 December 1877 telling them of his discovery.

## Characteristics
### Properties and molecular structure
At standard temperature and pressure, oxygen has no colour, odour or taste. It is a gas with the chemical formula  called dioxygen.

As *dioxygen* (or just *oxygen gas*), two oxygen atoms are chemically bound to each other. This bond can be called many things, but simply called a covalent double bond. Oxygen gas is very reactive and can react with many other elements. Oxides are made when metal elements react with oxygen, such as iron oxide, which is known as rust. There are a lot of oxide compounds on Earth.

### Allotropes
The common allotrope (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 121 pm and a bond energy of 498 kJ/mol Because of its energy, O2 is used by complex life like animals.

Ozone (O3) is very reactive and damages the lungs when breathed in. Ozone is made in the upper atmosphere when O2 combines with pure oxygen made when O2 is split by ultraviolet radiation.

Tetraoxygen (O4) was discovered in 2001. It only exists in extreme conditions when a lot of pressure is put onto O2.

### Physical properties
Oxygen dissolves more easily from air into water than nitrogen does. When there is the same amount of air and water, there is one molecule of O2 for every 2 molecules of N2 (a ratio 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 in freshwater than in seawater. Oxygen condenses at 90.20&nbsp;K (-182.95°C, -297.31&nbsp;°F) and freezes at 54.36&nbsp;K (-218.79&nbsp;°C, -361.82&nbsp;°F). Both liquid 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.

### Isotopes
There are three stable isotopes of oxygen in nature. They are 16O, 17O, and 18O. About 99.7% of oxygen is the 16O isotope.

### Occurrence
Oxygen is the third most common element in the universe, after hydrogen and helium. About 0.9% of the Sun's mass is oxygen. of the Earth's crust as part of oxide compounds like silicon dioxide and other compounds like carbonates. It is also the main part of the Earth's oceans, making up 88.8% by mass. Oxygen gas is the second most common part of the atmosphere, making up 20.95% of its volume and 23.1% of its volume. Earth is strange compared to other planets, as a large amount of its atmosphere is oxygen gas. Mars has only 0.1%  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 from water using energy from sunlight. This gives off oxygen gas. Some of the hydrogen combines with carbon dioxide to make carbohydrates. Respiration then takes oxygen gas out of the atmosphere or water and turns it into carbon dioxide and water.

## Uses
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### Medical
O2 is a very important part of respiration. Because of this, it is used in medicine. It is used to increase the amount of oxygen in a person's blood so more respiration can take place. This can make them become healthy quicker if they are ill. Oxygen therapy is used to treat emphysema, pneumonia, some heart problems, and any disease that makes it harder for a person to take in oxygen.

### Life support
Low-pressure O2 is used in space 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.

### Industrial
Smelting of iron ore into steel uses about 55% of oxygen made by humans. To do this, O2 gas is injected into the ore through a lance at high pressure. This removes any sulfur or carbon from the ore that would not be wanted. They are given off as sulfur oxide and carbon dioxide. The temperature can go as high as 1,700&nbsp;°C because it is an exothermic reaction.

### Oxides and other inorganic compounds
Water () is an oxide of hydrogen. It is the most common oxide on Earth. All known life needs water to live. Water is made of two hydrogen atoms covalent bonded to an oxygen atom (oxygen has a higher electronegativity than hydrogen). (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 a polar 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.

Because of oxygen's high electronegativity, it makes chemical bonds with almost all other chemical elements. These bonds give oxides (for example iron reacts with oxygen to give iron oxide). Most metal's surfaces are turned into oxides when in air. Iron's surface will turn to rust (iron oxide) when in air for a long time. There are small amounts of carbon dioxide () in the air, and it is turned into carbohydrates during photosynthesis. Living things give it off during respiration.

### Organic compounds
Many organic compounds have oxygen in them. Some of the classes of organic compounds that have oxygen are alcohols, ethers, ketones, aldehydes, carboxylic acids, esters, and amides. Many organic solvents also have oxygen, such as acetone, methanol, and isopropanol. Oxygen is also found in nearly all biomolecules that are made by living things.

Oxygen also reacts quickly with many organic compounds at, or below, room temperature when autoxidation happens.

## Industrial production 
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 is fractional distillation of liquefied air. N2 evaporates while O2 is left as a liquid.

### Toxicity
At high pressures, oxygen gas (O2) can be dangerous to animals, including humans. It can cause convulsions and other health problems. Oxygen toxicity usually begins to occur at pressures more than 50 kilopascals&nbsp;(kPa), equal to about 50% oxygen in the air at standard pressure (air on Earth has around 20% oxygen).

### Combustion and other hazards
Concentrated amounts of pure O2 can cause a quick fire. When concentrated oxygen and fuels are brought close together, a slight ignition can cause a huge fire. The Apollo 1 crew were all killed by a fire because the air of the capsule had a very high amount of oxygen.

If liquid oxygen is spilled onto organic compounds, like wood, it can explode.

## Related pages 
*Carbon dioxide
*Atmosphere
*Great Oxygenation Event

## References 
### General references 
* 
*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