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Oxygen and ozone

Oxygen is a colourless odourless gaseous element belonging to group 16 (formerly VI B) of the periodic table; a.n. is 8; r.a.m. is 15.9994; d. is 1.429 gdm ^-3; m.p. is -218.4°C; b.p. is -183°C. It is the most abundant element in the Earth's crust (49.2% by weight) and is present in the atmosphere (28% by volume). Atmospheric oxygen is of vital importance for all organisms that carry out aerobic respiration. For industrial purposes it is obtained by fractional distillation of liquid air. It is used in metallurgical processes, in high-temperature flames (e.g. for welding) and in breathing apparatus. The common form is diatomic (dioxygen, O₂). There is also a reactive allotrope ozone (O₃). Chemically, oxygen reacts with most other elements forming oxides. The element was discovered by Joseph Priestley in 1774. Ozone (trioxygen) is a colourless gas, O₃, soluble in cold water and in alkalis; m.p. is -192.7°C; b.p. – is 1119°C. Liquid ozone is dark blue in colour and is diamagnetic (dioxygen, O₂, is paramagnetic). The gas is made by passing oxygen through a silent electric discharge and is usually used in mixtures with oxygen. It is produced in the stratosphere by the action of high-energy ultraviolet radiation on oxygen and its presence there acts as a screen for ultraviolet radiation. Ozone is also one of the greenhouse gases. It is a powerful oxidizing agent and is used to form ozonides by reaction with alkenes and subsequently by hydrolysis to carbonyl compounds. Ozone layer (ozonosphere) is a layer of the Earth's atmosphere in which most of the atmosphere's ozone is concentrated. It occurs 15-50 km above the Earth's surface and is virtually synonymous with the stratosphere. In this layer most of the sun's ultraviolet radiation is absorbed by the ozone molecules, causing a rise in the temperature of the stratosphere and preventing vertical mixing so that the stratosphere forms a stable layer. By absorbing most of the solar ultraviolet radiation the ozone layer protects living organisms on the Earth. The fact that the ozone layer is thinnest at the equator is believed to account for the high equatorial incidence of skin cancer as a result of exposure to unabsorbed solar ultraviolet radiation. In the 1980s it was found that depletion of the ozone layer was occurring over both the poles, creating ozone holes. This is thought to have been caused by a series of complex photochemical reactions involving nitrogen oxides produced from aircraft and, more seriously, chlorofluorocarbons (CFCs) and halons. CFCs rise to the stratosphere, where they react with ultraviolet light to release chlorine atoms; these atoms, which are highly reactive, catalyze the destruction of ozone. Use of CFCs is now much reduced in an effort to reverse this human-induced damage to the ozone layer.

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