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magnesium(măgnē`zēəm, –zhəm), metallic chemical element; symbol Mg; at. no. 12; at. wt. 24.3050; m.p. about 648.8°C;; b.p. about 1,090°C;; sp. gr. 1.738 at 20°C;; valence +2. In 1808, Sir Humphry Davy discovered magnesium in its oxide, although it is not certain that he isolated the metal. Pure magnesium was isolated substantially by A. A. B. Bussy in 1828 by chemical reduction of the chloride. Magnesium was first isolated electrolytically by Michael Faraday in 1833.
Magnesium is a ductile, silver-white, chemically active metal with a hexagonal close-packed crystalline structure. It is malleable when heated. Magnesium is one of the alkaline-earth metalsalkaline-earth metals,
metals constituting Group 2 of the periodic table. Generally, they are softer than most other metals, react readily with water (especially when heated), and are powerful reducing agents, but they are exceeded in each of these properties by the
..... Click the link for more information. in Group 2 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
..... Click the link for more information. . It reacts very slowly with cold water. It is not affected by dry air but tarnishes in moist air, forming a thin protective coating of basic magnesium carbonate, MgCO3·Mg(OH)2. When heated, magnesium powder or ribbon ignites and burns with an intense white light and releases large amounts of heat, forming the oxide, magnesiamagnesia,
common name for the chemical compound magnesium oxide, MgO. It occurs as colorless, cubic crystals. It is refractory, melting at about 2,800°C;. It is very slightly soluble in pure water but is soluble in acids and solutions of ammonium salts.
..... Click the link for more information. , MgO. A magnesium fire cannot be extinguished by water, since water reacts with hot magnesium and releases hydrogen. Magnesium reacts with the halogens and with almost all acids. It is a powerful reducing agent and is used to free other metals from their anhydrous halides.
Magnesium forms many compounds. The oxide, hydroxide, chloride, carbonate, and sulfate are commercially important. They are used in ceramics, cosmetics, fertilizers, insulation, leather tanning, and textile processing. Epsom saltsEpsom salts,
common name for magnesium sulfate heptahydrate, MgSO4·7H2O, a water-soluble bitter-tasting compound that occurs as white or colorless needle-shaped crystals.
..... Click the link for more information. (magnesium sulfate heptahydrate, MgSO4·7H2O), milk of magnesiamilk of magnesia,
common name for the chemical compound magnesium hydroxide, Mg(OH)2. The viscous, white, mildly alkaline mixture that is used medicinally as an antacid and laxative is a suspension of approximately 8% magnesium hydroxide in water.
..... Click the link for more information. (magnesium hydroxide, Mg(OH)2), and citrate of magnesia are used in medicine. Magnesium reacts with organic halides to form the Grignard reagentsGrignard reagent
, any of an important class of extremely reactive chemical compounds used in the synthesis of hydrocarbons, alcohols, carboxylic acids, and other compounds.
..... Click the link for more information. of organic chemistry.
Magnesium is the eighth most abundant element in the earth's crust but does not occur uncombined in nature. It is found in abundance in the minerals brucite, magnesitemagnesite
, mineral, magnesium carbonate, MgCO3, white, yellow, or gray in color. It originates through the alteration of olivine or of serpentine by waters carrying carbon dioxide; through the replacement of calcium by magnesium in calcareous rocks, sometimes
..... Click the link for more information. , dolomitedolomite
. 1 Mineral, calcium magnesium carbonate, CaMg (CO3)2. It is commonly crystalline and is white, gray, brown, or reddish in color with a vitreous to pearly luster. The magnesium is sometimes replaced in part by iron or manganese.
..... Click the link for more information. , and carnalite. It is also found (as the silicate) in asbestos, meerschaum, serpentine, and talc. Magnesium chloride is found in seawater, brines, and salt wells. Mineral waters often contain salts of magnesium; the magnesium ion imparts a bitter flavor. Magnesium is a constituent of the chlorophyll in green plants and is necessary in the diet of animals and humans.
Two methods of producing magnesium commercially are used. The principal method is the electrolysis of fused magnesium chloride, which is used in the extraction of magnesium from seawater (the principal source) and from dolomite. In recovery from seawater, the magnesium is precipitated as magnesium hydroxide by treatment with lime (calcium oxide) obtained from oyster shells. The hydroxide is collected and treated with hydrochloric acid to form the chloride. The chloride is fused and electrolyzed, forming magnesium metal and chlorine gas. The molten metal is cast into ingots for further processing; the chlorine gas is made into hydrochloric acid and is reused to form magnesium chloride. About 1 lb of magnesium is recovered from each 100 gal of seawater; the oceans are a virtually inexhaustible source of this metal. A second method of magnesium production, called the ferrosilicon process, involves the reduction of magnesium oxide (prepared by calcining dolomite) with an iron-silicon alloy.
Magnesium is a commercially important metal with many uses. It is only two thirds as dense as aluminum. It is easily machined, cast, forged, and welded. It is used extensively in alloys, chiefly with aluminum and zinc, and with manganese. Magnesium alloys were used as early as 1910 in Germany. Early structural uses of magnesium alloys were in aircraft fuselages, engine parts, and wheels. They are now also used in jet-engine parts, rockets and missiles, luggage frames, portable power tools, and cameras and optical instruments. Duraluminduralumin
, alloy of aluminum (over 90%) with copper (about 4%), magnesium (0.5%–1%), and manganese (less than l%). Before a final heat treatment the alloy is ductile and malleable; after heat treatment a reaction between the aluminum and magnesium produces increased
..... Click the link for more information. and magnaliummagnalium
, alloy of aluminum and about 5% magnesium. Although weak and soft in the elemental state, magnesium alloys with aluminum, manganese, zinc, tin, zirconium, and cerium to produce alloys useful in engineering materials.
..... Click the link for more information. are alloys of magnesium. The metal is also used in pyrotechnics, especially in incendiary bombs, signals, and flares, and as a fuse for thermitethermite
[from Thermit, a trade name], mixture of powdered or granular aluminum metal and powdered iron oxide. When ignited it gives off large amounts of heat. In wartime it has been used in incendiary bombs. A method for welding using thermite (invented by Dr.
..... Click the link for more information. . It is used in photographic flashbulbs and is added to some rocket and missile fuels. It is used in the preparation of malleable cast iron. An important use is in preventing the corrosion of iron and steel, as in pipelines and ship bottoms. For this purpose a magnesium plate is connected electrically to the iron. The rapid oxidation of the magnesium prevents the slower oxidation and corrosion of the iron.
Mg, a chemical element in Group II of the Mendeleev periodic system. Atomic number, 12; atomic weight, 24.305. Natural magnesium is composed of three stable isotopes: 24Mg (78.60 percent), 25Mg (10.11 percent), and 26Mg (11.29 percent). Magnesium was discovered in 1808 by H. Davy, who carried out electrolysis of moistened magnesia (a well-known substance at that time) on a mercury cathode. Davy obtained an amalgam and then, after distilling off the mercury, a new powdery metal, magnesium. In 1828 the French chemist A. Bussy obtained magnesium in the form of small globules with a metallic glance by reducing molten magnesium chloride with potassium vapor.
Occurrence in nature. Magnesium is a primary component of the earth’s mantle; ultrabasic rocks contain 25.9 percent Mg (by weight). Smaller quantities of magnesium are found in the earth’s crust (average content, 1.87 percent). Magnesium predominates in basic rocks (4.5 percent); lesser amounts (0.56 percent) are found in granite and other acidic rocks. The Mg2+ ion is an analogue of Fe2+ in magmatic processes, a fact which is explainedoby the closeness of their ionic radii (0.74 and 0.80 angstroms [A°], respectively). Both Mg2+ and Fe2+ are components of olivine, pyroxenes, and other magmatic minerals.
Numerous magnesium minerals are known to exist—for example, silicates, carbonates, sulfates, and chlorides. More than half of them were formed in the biosphere (on the bottom of oceans and lakes, in soil, and so on); the remainder are associated with high-temperature processes.
Vigorous migration and differentiation of magnesium are observed in the biosphere. Such physicochemical processes as magnesium dissolution, the deposition of magnesium salts, and the sorption of magnesium by argillaceous matter play the major role here. Magnesium is poorly retained in the biological cycle on land and passes into the ocean with fluvial discharge. The average magnesium content in seawater is 0.13 percent, which is smaller than the sodium content but greater than that of all other metals. Seawater is not saturated with magnesium; therefore, precipitation of magnesium salts does not take place. Magnesium sulfates and chlorides accumulate in deposits together with potassium salts during the evaporation of water from marine lagoons. Dolomite accumulates in the silt of certain lakes (for example, Lake Balkhash). Magnesium is extracted industrially (mainly from dolomites, and also from seawater).
Physical and chemical properties. Solid magnesium is a lustrous silver-white metal that tarnishes upon exposure to air as a result of the formation of an oxide film on its surface. It crystallizes in a hexagonal lattice: a = 3.2028 Å; c = 5.1998 Å. Atomic radius, 1.60 Å; ionic radius of Mg2+, 0.74 Å. Density, 1.739 g/cm3 at 20°C; melting point, 65TC; boiling point, 1107°C; specific heat (at 20°C), 1.04 ×103 joules per (kg.’K), or 0.248 cal/(g.°C); thermal conductivity (at 20°C), 1.55 ×102 watts per (m.°K), or 0.37 cal/(cm.sec.°C); its thermal coefficient of linear expansion for the interval 0°-550°C is deter-mined from the equation 25.0 ×10-6 + 0.0188 t. Specific electric resistance at 20°C, 4.5 ×10-8 ohm-m, or 4.5 microhmcm.
Magnesium is paramagnetic, with a specific magnetic susceptibility of + 0.5 × 10-6. It is relatively soft and ductile, and its mechanical properties depend strictly on the method of treatment. For example, at 20°C the properties of cast and wrought magnesium, respectively, are typified by the following values: Brinell hardness, 29.43 ×107 and 35.32 ×107 newtons per sq m (N/m2), or 30 and 36 kilograms-force per sq mm (kgf/ mm2); yield point, 2.45 ×107 and 8.83 ×107 N/ m2 (2.5 and 9.0 kgf/mm2); tensile strength, 11.28 ×107 and 19.62 ×107 N/m2 (11.5 and 20.0 kgf/mm2); relative elongation, 8.0 and 11.5 percent.
The outer electron configuration for a magnesium atom is 3s2. Magnesium is divalent in all stable compounds. Magnesium is an extremely chemically active metal. Heating to 300°-350°C does not produce significant oxidation of solid magnesium, since its surface is protected by an oxide film. However, at temperatures of 600°-650°C magnesium ignites and burns with a bright flame, yielding magnesium oxide and some magnesium nitride, MgsN2; the latter is also produced by heating magnesium to about 500°C in a nitrogen atmosphere. Magnesium exhibits negligible reactivity with cold water that is not saturated with air, and it gradually displaces the hydrogen from boiling water; reaction with water vapor begins at 400°C. In a humid atmosphere, fused magnesium separates hydrogen from HiO and absorbs it; almost all the hydrogen is removed upon solidification of the metal. Magnesium forms MgH2 at 400°-500°C in a hydrogen atmosphere.
Magnesium displaces most metals from aqueous solutions containing their salts; the standard electrode potential for magnesium at 25°C is 2.38 volts (V). It reacts with cold, dilute inorganic acids but does not dissolve in hydrofluoric acid because of the formation of a protective film composed of insoluble magnesium fluoride, MgF2. Magnesium is virtually insoluble in concentrated H2SO4 and in mixtures of H2SO4 and HNO3. It does not interact with cold aqueous alkali solutions, but it dissolves in solutions of alkali metal hydrocarbonates and ammonia salts. Caustic alkalies precipitate magnesium hydroxide, Mg(OH)2, which exhibits negligible solubility in water, from saline magnesium solutions. The majority of magnesium salts (for example, magnesium sulfate) are readily soluble in water; MgF2, MgCO3, Mga(PO4)2, and certain other double salts are only slightly soluble.
Upon heating, magnesium reacts with halogens to yield halides. Interaction with moist chlorine produces MgCl2 even in the cold. Magnesium sulfide, MgS, can be produced by heating magnesium to 500°-600°C with sulfur or SO2 and H2S; heating with hydrocarbons yields the carbides MgC2 and Mg2Ca. The silicides Mg2Si and MgaSi2, the phosphide MgsP2, and other binary compounds are also known to exist. Magnesium is a strong reducing agent; upon heating, it displaces other metals (beryllium, aluminum, and alkali metals) and nonmetals (boron, silicon, and carbon) from their oxides and halides. It forms numerous organometallic compounds, which determines its significant role in organic synthesis. It alloys with most metals and forms the base material for many light industrial alloys.
Preparation and use. Most of the magnesium used in industry is produced by electrolysis of anhydrous magnesium chloride, MgCl2, or dehydrated carnallite, KCl.MgCl2-6H2O. The electrolyte also contains chlorides of sodium, potassium, and calcium and a small quantity of NaF or CaF2. Not less than 5-7 percent MgCl2 is present in the melt. Electrolysis proceeds at 720°-750°C, and the composition of the cell is adjusted by removing part of the electrolyte and adding MgCb or carnallite. Steel cathodes and graphite anodes are used in this process. The fused magnesium that floats to the surface of the electrolyte is periodically removed from the cathode space, which is separated from the anode by a partition that does not extend to the bottom of the bath. Crude magnesium contains up to 2 percent impurities and must be refined in electric crucible furnaces under a layer of flux and then poured into casting molds. Highest-quality primary magnesium contains 99.8 percent Mg. Subsequent refining is effected by sublimation in vacuum; two to three sublimation cycles raise the purity of magnesium to 99.999 percent. Anode chlorine is then used after purification to prepare anhydrous MgCb from magnesite and titanium tetrachloride, TiCl4, from TiO2.
Other methods for the preparation of magnesium include metallothermy and carbothermy. In the first case, briquettes of dolomite (roasted to full decomposition) and a reducing agent (ferrosilicon or silicoaluminum) are heated at 1280°-1300°C in vacuum (residual pressure, 130-260 N/m2, or 1-2 mm Hg). The magnesium vapors condense at 400°-500°C. The magnesium is refined by remelting under flux or in vacuum and then poured into casting molds. The carbothermic process uses briquettes consisting of a mixture of carbon and magnesium oxide, which are heated in electric furnaces to temperatures above 2100°C. The resultant magnesium vapors are subsequently distilled and condensed. Metallic magnesium is primarily used in the manufacture of magnesium-based alloys. It is also widely used in metallothermic processes for the preparation of poorly reducible and rare metals (titanium, zirconium, hafnium, and uranium), as well as in the deoxidation and desulfuration of metals and alloys. Mixtures of magnesium powder and oxidants serve as illuminating and incendiary compositions. Magnesium compounds are also widely used.
REFERENCESStrelets, Kh. L., A. Iu. Taits, and B. S. Gulianitskii. Metallurgiia magniia, 2nd ed. Moscow, 1960.
Ulbmann Encyklopädie der technischen Chemie, 3rd ed., vol. 12. Munich-Berlin, 1960.
Animals and humans ingest magnesium in food. The daily human magnesium requirement is 0.3-0.5 g; a greater amount is needed during childhood, pregnancy, and lactation. The normal average magnesium level in the blood is 4.3 mg percent. Increased magnesium content induces somnolence, loss of sensitivity, and sometimes paralysis of the skeletal muscles. In the organism, magnesium is concentrated in the liver, from which a large part of the supply passes into the bones and muscles. Magnesium participates in the stimulation of anaerobic carbohydrate metabolism in the muscular system. Calcium is the antagonist of magnesium in the organism. Disruption of the magnesium-calcium balance is observed in cases of rickets, when magnesium passes from the blood into the bones, displacing the calcium in them. Deficiency of magnesium salts in the diet disrupts muscle contraction and the normal excitability of the nervous system. Magnesium deficiency in cattle causes grass tetany (muscular spasms and cessation of growth in the extremities). Magnesium metabolism in animals is controlled by parathyroid hormone, which lowers the magnesium level in the blood, and by prolan, which increases the magnesium content.
Among the magnesium preparations used in medicine are magnesium sulfate (as a sedative, anticonvulsive, spasmolytic, laxative, and cholagogue), magnesium oxide, and magnesium carbonate (as an alkali and mild laxative).
G. IA. ZHIZNEVSKAIA
|Table 1. Chemical composition of magnesium alloys most commonly used in the USSR|
|Main components(percent)||Impurities (percent; maximum)|
|Type of alloy||Al||Zn||Mn||Zr||Nd||Al||Si||Fe||Ni||Cu||Mn||Be||Ca|