oxygen (O)

Oxygen atom
Credit: NASA/World Book

A colorless, odorless, gaseous element, the most abundant in Earth's crust (49.2% by mass) and the second most abundant in the atmosphere (28% by volume). Free oxygen is essential to the survival of all aerobic organisms.

Oxygen was discovered by Joseph Priestley in Leeds in 1774, and independently, and perhaps a year earlier, by Carl Scheele in Uppsala, Sweden. It was named by Antoine Lavoisier from the Greek oxy genes meaning "acid forming".

Chemistry of oxygen

Oxygen lies in group VIA of the periodic table. It is slightly soluble in water and reacts with most elements to form oxides. See also oxidation.

atomic number	8
relative atomic mass	15.9994
electron configuration	1s²2s²2p⁴
atomic radius	66 pm
oxidation state	-2
density	1.429 g/dm³
melting point	-218.4ºC
boiling point	-183.0 ºC

Allotropes of oxygen

The common allotrope of oxygen is the diatomic form known as dioxygen or, simply, oxygen (O₂). There is also a reactive allotrope called ozone (O₃).

Isotopes of oxygen

Oxygen has three stable isotopes: ¹⁶O (99.757%), ¹⁷O (0.038%), and ¹⁸O (0.205%). Variations in the ¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratios (for example, in meteorites) are reported in terms of their deviations (δ¹⁸O and δ¹⁷O) in parts per thousand from a standard. The usual reference is standard mean ocean water (SMOW).

Industrial production and laboratory preparation of oxygen

Oxygen is extracted industrially by the fractional distillation of liquid air. It can also be obtained by the electrolysis of water.

One way to prepare oxygen in the lab is to heat potassium chlorate crystals (KClO₂) with which black manganese dioxide powder has been mixed. The potassium dichlorate decomposes – oxygen is set free and potassium chloride (KCl) is left behind in the test tube. The manganese dioxide acts as a catalyst, reducing the temperature at which the potassium chlorate decomposes.

Uses of oxygen

Oxygen is used in steelmaking (by the Bessemer process), welding, the manufacture of industrial chemicals, and in apparatus for breathing (oxygen masks) and resuscitation (oxygen tents). Liquid oxygen is used as an oxidizer in many large rocket engines.

Oxygen on Earth

After the Earth formed, about 4.5 billion years ago, it was continuously struck by comets and asteroids for hundreds of millions of years. After this incessant rain of impacting objects stopped, life appears to have established itself comparatively quickly, perhaps within 10 to 100 million years. As soon as life began on Earth, it started to disturb the atmosphere's chemical equilibrium. Oxygen built up slowly in the atmosphere over the next three billion years because it was a waste product expelled by some single-celled organisms. Then, around 600 million years ago, multicellular creatures developed and the oxygen content of the atmosphere rose suddenly. This trend was assisted by plants, which established themselves on the continents around 450 million years ago. Concurrently, evolution produced animals that breathed oxygen, instead of excreting it as a waste product. Without life, the oxygen content of the atmosphere would have sunk to extremely low levels.

Oxygen in the universe

Oxygen is the most abundant heavy element in the universe, and the third most abundant element overall, after hydrogen and helium. It is produced by massive stars – those born with a mass of over 8 solar masses – which eject the element into interstellar space when they explode. Oxygen burning is the fusion of oxygen into the even heavier elements silicon and sulfur.

Related category

• INORGANIC CHEMISTRY

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