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This is oxygen.

Symbol O
Atomic number 8
Atomic mass 15.9994 amu

Oxygen, discovered in 1774, oxygen comprises 50% by weight of the earth's crust and the oceans and is the most abundant element in the earth's crust. Most of this amount is oxygen found in silicates.

Distinguishing properties

Gaseous oxygen constitutes 21% by volume of the air near the earth's surface, much of which originated from photosynthesis of plants, and water (water contains 88.8% oxygen by weight). Oxygen is absolutely vital for sustaining life on earth. It is a colourless gas with no taste or smell which condenses to a blue liquid at -183°C. Oxygen is prepared industrially by the fractional distillation of liquid air, or by electrolysis of water.


There are two allotropes of oxygen. One composed of two oxygen molecules, O2, and the other composed of three, O3. The latter is known as ozone. Ozone is a very strong oxidizing agent. It is useful as a disinfectant and bleaching agent. It is also used in the treatment of sewage and the manufacture of chemicals. Ozone occurs naturally in the earth's upper atmosphere where it serves as an effective shield against harmful radiation from the sun. Commercial amounts are prepared by passing O2 through an electric spark. The pungent odor often detected around electrical equipment is often that of ozone.


The United States annually produces 15-million tons of liquid oxygen for applications in industry, aerospace, the military and medicine. It is normally supplied under pressure in steel cylinders or as liquid oxygen in insulated containers.

Common compounds

Oxygen reacts with virtually every element to produce oxides. The ten most common compounds in the earth's crust are all oxides.

  • silicon dioxide, SiO2, accounts for nearly half the matter, * magnesium oxide, MgO, accounts for one-third,
  • iron(II) oxide, FeO, accounts for nearly one-fourth,
  • aluminum oxide, Al2O3, calcium oxide, CaO, and iron(III) oxide, Fe2O3, accounts for the remainder.

Most of the lanthanides exist in nature as their oxides. Even the noble gases are not totally immune to forming oxides. Under laboratory conditions, xenon trioxide, XeO3, can be produced.

See also

Periodic table of the elements