Oxygen is the chemical element with the symbol O. It is the third-most common element in the universe, after hydrogen and helium. When alone, two oxygen atoms usually bind to make dioxygen (O₂), a colourless gas. It has no taste or smell. It is a pale blue as a liquid and solid. Dioxygen gas makes up 20.8% of the Earth's atmosphere. Oxygen is part of the chalcogen group on the periodic table, and its atomic number is 8. It is a very reactive nonmetal. It also makes oxides with many elements. Oxides make up nearly half of the Earth's crust.

Most life on Earth takes in oxygen gas (O₂) to use in respiration. Many organic molecules in living things have oxygen in them, such as proteins, nucleic acids, carbohydrates and fats. Oxygen is a part of water, which all known life needs to live. Plants make the Earth's dioxygen by photosynthesis, using the Sun's light to separate oxygen from water and carbon dioxide. Ozone (O₃) is at the top of the Earth's atmosphere in the ozone layer. It absorbs ultraviolet radiation, which reduces the radiation that reaches ground level.

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

Oxygen is used for making steel, plastics, textiles, rocket propellant, and for welding.

History

Early experiments

One of the first known experiments on how combustion needs air was carried out by Greek Philo of Byzantium in the 2nd centure 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 was wrong. A long time after, Leonardo da Vinci correctly worked out that air was used up when combustion happened, which forced water into the vessel.

In the late 17th century, Robery 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 (in the form of dioxygen). In one of his experiments, he found that putting a candle in a closed container made the water rise to replace one-fourteenth of the air's volume in the container, before going out. The same thing happened when a mouse was put into the box. From this, he worked out that oxygen is used for 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 it in the year 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.

Very combustible materials that leave only a small amount of residue, like wood or coal, were thought to be made of phlogiston. Things that corrode, like iron, were thought to contain only a small amount. Air was not part of this theory.

Discovery

Polish alchemist, philosopher and physician Michael Sendivogius spoke about a substance in air, calling it the "food of life"., and this substance is oxygen. Sendivogius found, between the years 1598 and 1604, that the substance is the same as what is made during the thermal decomposition of potassium nitrate. Some people believe this was the discovery of oxygen while others disagree.

It is often also said that oxygen was first discovered by Swedish pharmacist Carl Wilhelm Scheele. He made oxygen by heating mercuric oxide and some nitrates in 1771. Scheele called the gas he made "fire air", because it was the only gas known to allow combustion. He published his discovery in 1777.

On 1 August 1774, an experiment carried out by British clergyman Joseph Priestley focused sunlight on mercuric oxide in a glass tube. This made a gas he called "dephlogisticated air". He also found that candles burned brighter in the gas and mouses lived longer while breathing it. When he breathed the gas, he said (simplified) "It felt like normal air, but my lungs felt lighter and easy afterwards." His findings were published in 1775. Because his findings were published first, he is usually said to be the discoverer of oxygen.

French chemist Antoine Lavoisier later said he had discovered the substance as well. Priestly 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 contribution

Lavoisier at the Academy-Louis Ernest Barrias

Lavoisier decomposition air

Lavoisier carried out the first main experiments on oxidation and gave the first right explanation on 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 mass 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 tin had increased in mass by the same amount as 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 he called "azote" (nitrogen), which means "lifeless" in Greek language. This is still the name of nitrogen in some languages, including French.

Lavoisier renamed "vital air" to "oxygène", meaning "producer from acids" in Greek. He called it this because he thought oxygen was in all acids, which was wrong. Many chemists realised that Lavoiser was wrong in his naming, 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 and is a gas with the chemical formula O₂ called dioxygen.

As dioxygen, two oxygen atoms are chemically bound to each other. This bond can be called many things, but simply called a covalent double bond. Dioxygen is very reactive and can react with many other elements. Oxides are made when metal elements react with dioxygen, 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. 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.

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 K (-182.95°C, -297.31 °F) and freezes at 54.36 K (-218.79 °C, -361.82 °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

Ten most common elements in the Milky Way Galaxy estimated spectroscopically
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	10,400	10400
6	Carbon	4,600	4600
10	Neon	1,340	1340

Oxygen is the most common element by mass on Earth. It is the third most common element in the universe, after hydrogen and helium. About 0.9% of the Sun's mass is oxygen. Oxygen makes up 49.2% of the Earth's crust by mass as part of oxide compounds like silicon dioxide. 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 20.8% of its mass and 23.1% of its volume. Earth is strange compared to other known planets, as a large amount of its atmosphere is oxygen gas. Mars has 0.1% O₂ by volume with the rest of the Solar System's planet's having less than that.

The high amount of oxygen gas on Earth is because of the oxygen cycle. This is mainly controlled by photosynthesis, which makes oxygen gas from carbon dioxide, water and the Sun's energy. Respiration then takes the oxygen gas out of the atmosphere and turns it back into carbon dioxide and water. This happens at the same rate, so the amount of oxygen gas and carbon dioxide doesn't change because of it.

Uses

An oxygen concentrator in an emphysema patient's house

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 persons 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.

Safety

O₂

Main symptoms of oxygen toxicity

Oxygen's NFPA 704 (on the right) says that compressed oxygen gas is not dangerous to health and is not flammable.

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 (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 O₂. This was stopped when some babies went blind from the oxygen.

Breathing pure O₂ in space suits causes no damage because there is a lower pressure used.

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 of concentrated oxygen that was used in the air of the capsule.

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

Images for kids

Joseph Priestley is usually given priority in the discovery.
Antoine Lavoisier discredited the phlogiston theory.
Robert H. Goddard and a liquid oxygen-gasoline rocket
An experiment setup for preparation of oxygen in academic laboratories
Orbital diagram, after Barrett (2002), showing the participating atomic orbitals from each oxygen atom, the molecular orbitals that result from their overlap, and the aufbau filling of the orbitals with the 12 electrons, 6 from each O atom, beginning from the lowest-energy orbitals, and resulting in covalent double-bond character from filled orbitals (and cancellation of the contributions of the pairs of σ and σ* and π and π* orbital pairs).
Liquid oxygen, temporarily suspended in a magnet owing to its paramagnetism
Space-filling model representation of dioxygen (O2) molecule
Oxygen discharge (spectrum) tube
Cold water holds more dissolved O₂.
Oxygen and MAPP gas compressed-gas cylinders with regulators
Low pressure pure O₂ is used in space suits.
Most commercially produced O₂ is used to smelt and/or decarburize iron.
Water (H₂O) is the most familiar oxygen compound.
Oxides, such as iron oxide or rust, form when oxygen combines with other elements.
The interior of the Apollo 1 Command Module. Pure O₂ at higher than normal pressure and a spark led to a fire and the loss of the Apollo 1 crew.