Occurrence, uses, and properties.
Magnesium is the eighth most abundant element in the Earth's crust (about 2.5 percent), distributed in minerals such as magnesite, dolomite, brucite, serpentine, chrysolite, meerschaum, talc, and most kinds of asbestos. Seawater contains about 0.13 percent magnesium, mostly as the dissolved chloride, which imparts the characteristic bitter taste. Magnesium is about one-sixth as plentiful as potassium in human body cells, where it is required as a CATalyst for enzyme reactions in carbohydrate metabolism.
Magnesium is commercially produced by electrolysis of molten magnesium chloride (MgCl2), processed mainly from seawater and by the direct reduction of its compounds with suitable reducing agents (as from calcined dolomite with ferrosilicon).
At one time, magnesium was used predominantly for photographic flash ribbon and powder, incendiary bombs, and pyrotechnic devices, because in finely divided form it burns in air with an intense white light. Because of its low density (only two-thirds that of aluminum) it has found extensive use in the aerospace industry. A part that would weigh 70 pounds (31.8 kilograms) when made of steel weighs only 15 pounds when made from magnesium. Because the pure metal has low structural strength, alloys have been developed--principally with aluminum, zinc, and manganese--to improve its hardness, tensile strength, and ability to be cast, welded, and machined. Magnesium alloys have a number of appliCATions; they are used for parts of aircraft, spacecraft, machinery, automobiles, portable tools, and household appliances.
Magnesium occurs in nature as a mixture of three isotopes: magnesium-24 (78.70 percent), magnesium-26 (11.17 percent), and magnesium-25 (10.13 percent). It is a very strong reducing agent, reacting with most acids or with boiling water to liberate hydrogen, but is resistant to most alkalies. In compounds it always exhibits a +2 oxidation state because of the loss or sharing of its two 3s electrons.
Magnesium, comprising about 2.5 percent of the Earth's crust, is the eighth most abundant element and, after aluminum and iron, the third most plentiful structural metal; its cosmic abundance is estimated as 9.1 105 atoms (Si = 106 atoms). It occurs as carbonates (magnesite, MgCO3, and dolomite, CaCO3 MgCO3) and in many common siliCATes, including asbestos, talc, and olivine. It also is found as the hydroxide (brucite), chloride (carnallite, KCl MgCl2 6H2O), and sulfate (kieserite).
Magnesium is commercially produced chiefly by the electrolysis of molten magnesium chloride and by the thermal reduction of magnesium oxide with ferrosilicon (see magnesium processing ).
Magnesium is the lightest metal that can be commonly used for structural purposes (its density is 65 percent that of aluminum and 22 percent that of iron), a property widely exploited. Its thermal and electrical conductivity and its melting point are very similar to those of aluminum. Whereas aluminum is attacked by alkalies but is resistant to most acids, magnesium is resistant to most alkalies but is readily attacked by most acids (chromic and hydrofluoric acids are important exceptions). At normal temperatures it is stable in air and water owing to the formation of a thin protective skin of oxide (but burns rapidly when heated in air), and it is attacked by steam. Magnesium is a powerful reducing agent and is used to produce other metals from their compounds (e.g., titanium, zirconium, and hafnium). It reacts directly with many elements; with organic halides in ether solution it forms Grignard reagents. Because of its ready combustibility, magnesium finds appliCATion in explosive and pyrotechnic devices.
Magnesium is mainly used in the form of alloys, which usually contain 10 percent or less of other elements, generally added to increase the strength of the metal. The most important alloys are those of aluminum and zinc. Casting, rolling, extruding, and forging techniques are all employed with the alloys, and further fabriCATion of the resulting sheet, plate, or extrusion is carried out by normal forming, joining, and machining operations. Magnesium is the easiest structural metal to machine and has often been used when a large number of machining operations are required.
Magnesium is essential to all living systems. The photosynthetic function of plants depends upon the action of chlorophyll pigments, which contain magnesium at the centre of a complex, nitrogen-containing ring system (porphyrin). These magnesium compounds enable the energy of light to be used to convert carbon dioxide and water to carbohydrates and oxygen and thus directly or indirectly provide the key to nearly all living processes. Magnesium also takes part in a number of enzyme reactions, which control energy transfer in living cells.
Chemical compounds
Magnesium carbonate
MgCO3, occurs in nature as the mineral magnesite and is an important source of elemental magnesium. It can be produced artificially by the action of carbon dioxide on a variety of magnesium compounds. The odourless white powder has many industrial uses--e.g., as a heat insulator for boilers and pipes and as an additive in food, pharmaceuticals, cosmetics, rubbers, inks, and glass.
Magnesium hydroxide
Mg(OH)2, is a white powder produced in large quantities from seawater by the addition of milk of lime. It is the primary raw material in the production of magnesium metal. In water it forms a suspension known as milk of magnesia, which has long been used as an antacid and a laxative.
The action of hydrochloric acid on magnesium hydroxide produces magnesium chloride, MgCl2, a colourless, deliquescent (water-absorbing) substance employed in magnesium metal production, in the manufacture of a cement for heavy-duty flooring, and as an additive in textile manufacture. Roasting either magnesium carbonate or magnesium hydroxide produces the oxygen compound magnesium oxide, commonly called magnesia, MgO, a white solid used in the manufacture of high-temperature refractory bricks, electrical and thermal insulators, cements, fertilizer, rubber, and plastics. It is used medically as a laxative.
Magnesium sulfate
MgSO4, is a colourless, crystalline substance formed by the reaction of magnesium hydroxide with sulfur dioxide and air. A hydrate form of magnesium sulfate called kieserite, MgSO4H2O, occurs as a mineral deposit. Synthetically prepared magnesium sulfate is sold as Epsom salt, MgSO47H2O. In industry magnesium sulfate is used in the manufacture of cements and fertilizers and in tanning and dyeing; in medicine it serves as a purgative.
Among the organometallic compounds of magnesium are the important Grignard reagents, composed of an organic group (e.g., alkyls and aryls), a halogen atom other than fluorine, and magnesium (see Grignard reagent). These are used in the production of many other kinds of organometallic compounds. Magnesium also is a constituent of chlorophyll, in which it apparently plays a role similar to that of iron in hemoglobin.
atomic number 12 atomic weight 24.312 melting point 651 C boiling point 1,107 C specific gravity 1.74 (20 C) valence 2 electronic config. 2-8-2 or 1s22s22p63s2
History
Magnesium derives its name from magnesite, a magnesium carbonate mineral, and this mineral in turn is said to owe its name to magnesite deposits found in Magnesia, a district in the ancient Greek region of Thessaly. The British chemist Humphry Davy is said to have produced an amalgam of magnesium in 1808 by electrolyzing moist magnesium sulfate, using mercury as a CAThode. The first metallic magnesium, however, was produced in 1828 by the French scientist A.-A.-B. Bussy. His work involved the reduction of molten magnesium chloride by metallic potassium. In 1833 the English scientist Michael Faraday was the first to produce magnesium by the electrolysis of molten magnesium chloride. His experiments were repeated by the German chemist Robert Bunsen.
The first successful industrial production was begun in Germany in 1886 by Aluminium und Magnesiumfabrik Hemelingen, based on the electrolysis of molten carnallite. Hemelingen later became part of the industrial complex IG Farbenindustrie, which, during the 1920s and '30s, developed a process for producing large quantities of molten and essentially water-free magnesium chloride (now known as the IG Farben process) as well as the technology for electrolyzing this product to magnesium metal and chlorine. Other contributions by IG Farben were the development of numerous cast and malleable alloys, refining and protective fluxes, wrought magnesium products, and a vast number of aircraft and automobile appliCATions. During World War II the Dow Chemical Company of the United States and Magnesium Elektron Limited of the United Kingdom began the electrolytic reduction of magnesium from seawater pumped from Galveston Bay, Texas, and the North Sea at Hartlepool, Eng. At the same time in Ontario, Can., L.M. Pidgeon's process of thermally reducing magnesium oxide with silicon in externally fired retorts was introduced.
Following the war, military appliCATions lost prominence. Dow Chemical broadened civilian markets by developing wrought products, photoengraving technology, and surface treatment systems. Extraction remained based on electrolysis and thermal reduction. To these processes were made such refinements as the internal heating of retorts (the Magnetherm process, introduced in France in 1961), extraction from dehydrated magnesium chloride prills (introduced by the Norwegian company Norsk Hydro in 1974), and improvements in electrolytic cell technology from about 1970.
Reference: Encyclopędia Britannica, Inc. 1994-2000 ©
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