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.

The history of oxygen began in the 1700s, during a time when scientists were starting to understand the nature of air. In 1772, Swedish chemist Carl Wilhelm Scheele discovered a gas that made flames burn brighter, but he did not publish his work right away. A few years later, in 1774, British scientist Joseph Priestley also discovered this gas by heating a substance called mercuric oxide. He noticed that candles burned more strongly in the gas, and mice lived longer when breathing it. He called it "dephlogisticated air" based on an old theory of combustion. Soon after, French scientist Antoine Lavoisier gave the gas its modern name, oxygen, which means “acid-former” in Greek. He helped disprove the old phlogiston theory. He also showed that oxygen is an important part of combustion and respiration.

Oxygen can be produced in several ways. The most common method for making large amounts of oxygen is called fractional distillation of liquid air. In this process, air is cooled until it becomes a liquid, then slowly warmed up. Because oxygen boils at a different temperature than other gases, it can be separated and collected. Another way to produce oxygen is by using electrolysis. This method involves passing electricity through water (H₂O) to split it into hydrogen and oxygen gas. Oxygen is also made naturally by plants, algae, and some bacteria through the process of photosynthesis. These organisms take in carbon dioxide and sunlight to make food, and they release oxygen into the air. This natural process is the main source of the oxygen we breathe every day. In hospitals, oxygen can also be created using oxygen concentrators, which pull oxygen out of the air and remove other gases. This helps people who have trouble breathing or need extra oxygen for medical treatments. Oxygen can be stored as a gas under pressure in tanks or stored as a liquid.

Oxygen has many important uses in everyday life, medicine, and industry. One of the most important uses is for breathing. Hospitals give oxygen to patients who have trouble breathing or need extra oxygen, especially during surgery or emergencies. 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 BC. He wrote in his work _Pneumatica_ that turning a vessel 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 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 (O<sub>2</sub>). 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.

Materials that leave very little residue when they burn, like wood or coal, were thought to be made mostly of phlogiston. Things that corrode, like iron, were thought to contain very little. Air was not part of this theory.

### Discovery

Polish alchemist, philosopher and physician Michael Sendivogius wrote about something in air that he called the "food of life", 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. 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.

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. His findings were published in 1775. 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.

### Lavoisier's research

Lavoisier did the first main experiments on oxidation. He was the first person to explain how combustion works. 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 a chemical element.

In one experiment, Lavoisier found that there was no increase in weight when tin and air were heated in a closed container. 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.  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" in Greek. (This is still the name of nitrogen in some languages, including French.)

Lavoisier renamed "vital air" to "oxygène", from Greek words meaning "sour making" or "producer of acid". He called it this because he thought oxygen was in all acids, which is wrong. 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 (H<sub>2</sub>O) 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.

### 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 (O<sub>2</sub>). This is the second biggest part of the Earth's atmosphere, after dinitrogen (N<sub>2</sub>). O<sub>2</sub> has a bond length of 121 pm and a bond energy of 498 kJ/mol Because of its energy, O<sub>2</sub> is used by complex life like animals.

Ozone (O<sub>3</sub>) is very reactive and damages the lungs when breathed in. Ozone is made in the upper atmosphere when O<sub>2</sub> combines with pure oxygen made when O<sub>2</sub> is split by ultraviolet radiation. Ozone absorbs a lot of radiation in the UV part of the electromagnetic spectrum and so the ozone layer in the upper atmosphere protects Earth from radiation.

Above the ozone layer, (in low Earth orbits), atomic oxygen becomes the most common form.

Tetraoxygen (O<sub>4</sub>) was discovered in 2001. It only exists in extreme conditions when a lot of pressure is put onto O<sub>2</sub>.

### 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 O<sub>2</sub> for every 2 molecules of N<sub>2</sub> (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 O<sub>2</sub> to dissolve in freshwater than in seawater. Oxygen condenses at 90.20 K (-182.95°C, -297.31 °F) and freezes at 54.36 K (-218.79 °C, -361.82 °F). Both liquid and solid O<sub>2</sub> 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 <sup>16</sup>O, <sup>17</sup>O, and <sup>18</sup>O. About 99.7% of oxygen is the <sup>16</sup>O 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.

|Z|Element|Mass fraction in parts per million||
| --- | --- | --- | --- |
|1|Hydrogen|739,000|71 × mass of oxygen (red bar)|
|2|Helium|240,000|23 × mass of oxygen (red bar)|
|8|**Oxygen**|||
|6|Carbon|||
|10|Neon|||

Apart from iron, oxygen is the most common element on Earth (by mass). It makes up nearly half (46%
to 49.2% 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.

### Medical

O<sub>2</sub> 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 O<sub>2</sub> 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, O<sub>2</sub> 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 °C because it is an exothermic reaction.

Around 25% of oxygen made by humans is used by chemists. Ethylene is reacted with O<sub>2</sub> to make ethylene oxide. This is then changed to ethylene glycol, which is used to make many products such as antifreeze and polyester (these can then be turned into plastics and fabrics).

The other 20% of oxygen made by humans is used in medicine, metal cutting and welding, rocket fuel, and water treatment.

## Compounds

The oxidation state of oxygen is −2 in nearly every compound it is in. In a few compounds, the oxidation state is −1, such as peroxides. Compounds of oxygen with other oxygen states are very uncommon.

### 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 CO<sub>2</sub>). This property is exploited by microwaves to oscillate polar molecules, especially water. And its responsible for the extra energy needed to disassociate H<sub>2</sub>0.

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 O<sub>2</sub> 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. N<sub>2</sub> evaporates while O<sub>2</sub> is left as a liquid. O<sub>2</sub> is the second most important industrial gas. Because it is more economical, oxygen is usually stored and transported as a liquid. A small steel tank of 16 liters water capacity with a working pressure of 139 bar (2015 psi) holds about 2150 liters of gas and weighs 28 kilograms (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 of zeolite molecular sieves. The zeolite molecular sieves soak up the nitrogen. It gives a stream of gas that is 90% to 93% oxygen.

Oxygen gas can also be made through electrolysis of water into molecular oxygen and hydrogen.

## Safety

Oxygen's NFPA 704 says that compressed oxygen gas is not dangerous to health and is not flammable.

### Toxicity

At high pressures, oxygen gas (O<sub>2</sub>) 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 (kPa), equal to about 50% oxygen in the air at standard pressure (air on Earth has around 20% oxygen).

Premature babies used to be placed in boxes with air with a high amount of O<sub>2</sub>. This was stopped when some babies went blind from the oxygen.

Breathing pure O<sub>2</sub> in space suits causes no damage because there is a lower pressure used.

### Combustion and other hazards

Concentrated amounts of pure O<sub>2</sub> 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

### 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

## Infobox (oxygen)