OxygenThe most plentiful element in the Earth's crust, about 46.6 percent by weight, oxygen is a nonmetallic element which, in its pure form is a colorless, odorless, tasteless gas. It's most important compound is water.
Oxygen constitutes about 21 percent of the Earth's the atmosphere (by volume) and about 89 percent by weight of sea water. About 3 parts per hundred of the freshwater in rivers and lakes is also oxygen (at 20 degrees centigrade). For over 1 billion years photosynthetic organisms, bacteria, algae and later the plants, have been producing oxygen gas as a waste product of photosynthesis. Almost all the molecular oxygen in our atmosphere today is the result of this process.
Animals, plants and some bacteria take in oxygen and use it as the final acceptor of electrons in respiration. They return carbon dioxide to the atmosphere. Oxygen dissolved in water is essential for respiration of fish and other water dwelling organisms.
PropertiesPure oxygen in the atmosphere consists almost entirely of molecules in which two atoms of oxygen are held together by a double covalent bond. High up in the atmosphere oxygen molecules with three atoms are produced. Called ozone, this molecular arrangement protects life on this planet from the harmful effects of the Sun's ultraviolet radiation. At very low temperatures (about -183 degrees centigrade, oxygen becomes a pale blue liquid and even becomes a solid at about -218 degrees centigrade.
HistoryA Swedish chemist, Carl Wilhelm Scheele, discovered oxygen in 1772 when he was heating potassium nitrate. Joseph Priestley, an English chemist, rediscovered oxygen two years later when he heated mercury(II) oxide. Priestley published his findings in 1774, beating Scheele by three years Antoine Lavoisier, executed in the French Revolution, first recognized the gas was a pure element in the years 1775-80. He invented the name oxygen and went on to explain combustion as a chemical reaction between oxygen and the material being burned.
Chemical propertiesAtomic oxygen has a half filled outermost energy level. It has four electrons in a level which holds eight electrons. Oxygen, therefore, forms a large number of compounds in which it shares electrons (i.e. oxygen readily forms covalent bonds). This sharing, however, it rarely equal. Oxygen has a high affinity for electrons; it is electronegative. The covalent bonds formed by oxygen are polar, with slight negative charges on the oxygen atom and slight positive charges on the other, sharing, atom. Water is a good example of a molecule with polar covalent bonds.