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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. 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 oC.
Oxygen is prepared industrially by the fractional
distillation of liquid air, or by electrolysis
of water. 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.
Oxygen reacts with virtually every element to produce oxides. The ten most
common compounds in the earth's crust are all oxides.
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 teh 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.
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.
