Oxygen is a chemical element in the periodic table. It has the symbol O and atomic number 8.


Oxygen is a major component of air, produced by plants during photosynthesis, and is necessary for aerobic respiration in animals. Liquid and solid O3 (ozone) have a deeper color of blue.

Liquid O2 and solid O2 have a light blue color and both are highly paramagnetic.

How obtained[]

Liquid O2 is usually obtained by the fractional distillation of liquid air.

A recently discovered allotrope of oxygen, tetraoxygen (O4), is a deep red solid that is created by pressurizing O2 to the order of 20 GPa. Its properties are being studied for use in rocket fuels and similar applications, as it is a much more powerful oxidizer than either O2 or O3.


Liquid oxygen finds use as an oxidizer in rocket propulsion. Oxygen is essential to respiration, so oxygen supplementation has found use in medicine (as oxygen therapy). People who climb mountains or fly in airplanes sometimes have supplemental oxygen supplies (to increase the inspired Oxygen partial pressure nearer to that found at sea-level requires increasing the proportion as a percentage of air). Oxygen is used in welding (such as the oxyacetylene torch), and in the making of steel and methanol.

30% oxygen with 70% Nitrous Oxide is the common basic anaesthetic mixture.


Oxygen is the most common component of the Earth's crust (46.6% by mass), the second most common component of the Earth as a whole (28.2% by mass), and the second most common component of the Earth's atmosphere (20.947% by volume).


Oxygen can be toxic at elevated partial pressures (i.e. high relative concentrations). This is important in some forms of scuba diving, such as with a rebreather.

Highly concentrated sources of oxygen promote rapid combustion and therefore are fire and explosion hazards in the presence of fuels. The fire that killed the Apollo 1 crew on a test launchpad spread so rapidly because the capsule was pressurized with pure oxygen as would be usual in an actual flight, but to maintain positive pressure in the capsule, this was at slightly more than atmospheric pressure instead of the 1/3 pressure that would be used in flight. (See partial pressure.) Similar hazards also apply to compounds of oxygen with a high oxidative potential, such as chlorates, perchlorates, and dichromates; they also can often cause chemical burns.

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