O - Oxygen
(Gr. oxys, sharp, acid, and genes, forming; acid former) For many centuries, workersoccasionally realized air was composed of more than one component. The behavior of oxygenand nitrogen as components of air led to the advancement of the phlogiston theory ofcombustion, which captured the minds of chemists for a century. Oxygen was prepared byseveral workers, including Bayen and Borch, but they did not know how to collect it, didnot study its properties, and did not recognize it as an elementary substance.
Priestley is generally credited with its discovery, although Scheele also discovered itindependently.
Its atomic weight was used as a standard of comparison for each of the other elementsuntil 1961 when the International Union of Pure and Applied Chemistry adopted carbon 12 asthe new basis.
Oxygen is the third most abundant element found in the sun, and it plays a part in thecarbon-nitrogen cycle, the process once thought to give the sun and stars their energy.Oxygen under excited conditions is responsible for the bright red and yellow-green colorsof the Aurora.
A gaseous element, oxygen forms 21% of the atmosphere by volume and is obtained byliquefaction and fractional distillation. The atmosphere of Mars contains about 0.15%oxygen. The element and its compounds make up 49.2%, by weight, of the earth's crust.About two thirds of the human body and nine tenths of water is oxygen.
In the laboratory it can be prepared by the electrolysis of water or by heatingpotassium chlorate with manganese dioxide as a catalyst.