Magnesium

Magnesium (chemical symbol - Mg, from Lat. Magnesium) is a chemical element of group 2 (according to the outdated classification - the second group of the main subgroup, IIA), the third period of the periodic system of chemical elements of D. I. Mendeleev, with atomic number 12.

A simple substance magnesium is a light, malleable alkaline earth metal of silver-white color.

MAGNESIUM (Lat. Magnesium), Mg, chemical element of group II periodic. Mendeleev systems, at. n. 12, at. m. 24,305. Natural M. consists of three stable isotopes: 24Mg (78.60%), 25Mg (10.11%) and 26Mg (11.29%). M. was discovered in 1808 by Davy, who subjected moistened magnesia (a long-known substance) to electrolysis with a mercury cathode; Davy received an amalgam, and from it, after distilling mercury, a new powdery metal called magnesium. In 1828, fr. chemist A. Bussy by reducing molten chloride of M. with potassium vapors obtained M. in the form of small balls with metallics. brilliance.

Distribution in nature. m. is a characteristic element of the Earth's mantle, in ultrabasic rocks it contains 25.9% by weight. In the earth's crust, M. is less, the average clark is 1.87%; M. prevails in the main rocks (4.5%), in granites and other acidic rocks it is less (0.56%). In magmatic processes, Mg2+ is an analogue of Fe2+, which is explained by the proximity of their ionic radii (0.74 and 0.80 A, respectively). Mg2+, together with Fe2+, is part of olivine, pyroxenes, and other magmatic. minerals.

Minerals of M. are numerous - silicates, carbonates, sulfates, chlorides, etc. (see Magnesium ores). More than half of them were formed in the biosphere - at the bottom of seas, lakes, in soils, etc.; the rest are associated with high-temperature processes.

In the biosphere, there is a vigorous migration and differentiation of M.; here the main role belongs to physical and chemical processes - dissolution, precipitation of salts, sorption of M. by clays. M. is weakly delayed in the biological cycle on continents and enters the ocean with river runoff. In seawater, on average, 0.13% m.- less than sodium, but more than all other metals. Sea water is not saturated with M. and precipitation of its salts does not occur. When water evaporates in marine lagoons , sulfates and chlorides accumulate in sediments together with potassium salts . Dolomite accumulates in the silts of some lakes (for example, in Lake Balkhash). In industry, M. is obtained mainly from dolomites, as well as from seawater.

Physical and chemical properties. Compact M. is a shiny silvery-white metal that tarnishes in air due to the formation of an oxide film on the surface. M. crystallizes in a hexagonal lattice, a = 3,2028 A, c = 5,1998A. Atomic radius 1.60A, ionic radius Mg2+ 0.74A. Density m. 1,739 g/cm3(20 °C); tpl 651 °C; tkip 1107 ° C. Ud. heat capacity (at 20 °C) 1.04x103 j /(kg * K), i.e. 0.248 cal /(g * °C); thermal conductivity (20 °C) 1.55-102 W /(m *K), i.e. 0.37 cal /(cm * sec* °C); the term, the coefficient of linear expansion in the range 0-550 ° C is determined from the equation 25,0*10-6 + 0,0188 t. Specific electric. the resistance (20 °C) is 4.5*10-8 ohm-m (4.5 mcom-cm). M. is paramagnetic, the specific magnetic susceptibility is +0.5*10-6, M. is a relatively soft and plastic metal; its mechanical. the properties strongly depend on the processing method. For example, at 20 °C, the properties of cast and deformed M, respectively, are characterized by the following values: Brinell hardness 29.43*107 and 35.32*107 n/m2 (30 and 36 kgf/mm2), yield strength 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%.

The configuration of the external electrons of the M. 3s2 atom, In all stable compounds M. is divalent. In chem. In relation to M., it is a very active metal. Heating up to 300-350 ° C does not lead to significant oxidation of compact M., because its surface is protected by an oxide film, but at 600-650 ° C M. ignites and burns brightly, giving magnesium oxide and partly MgsN2 nitride. The latter is obtained also when heated to about 500 ° C in a nitrogen atmosphere. With cold water, not saturated with air, M. almost does not react, slowly displaces hydrogen from boiling water; the reaction with water vapor begins at 400 ° C. Molten M . in a humid atmosphere, releasing hydrogen from H2O, it absorbs it; when the metal solidifies, hydrogen is almost completely removed. In the atmosphere of hydrogen M. at 400-500 ° C forms MgH2.

M. displaces most metals from aqueous solutions of their salts; the standard electrode potential Mg at 25 ° C is 2.38 V. M. interacts with dilute mineral acids in the cold, but does not dissolve in hydrofluoric acid due to the formation of a protective film of insoluble fluoride MgF2. In concentrated H2SO4 and its mixture with NMO3, it is practically insoluble. It does not interact with aqueous solutions of alkalis in the cold, but it dissolves in solutions of alkali metal bicarbonates and ammonium salts. Caustic alkalis are precipitated from solutions of M salts . hydroxide Mg(OH)2, the solubility of k-roy in water is negligible. Most of the salts of M. are highly soluble in water, for example, magnesium sulfate, MgF2, MdCO3 (see Magnesium carbonate), Mg3(PO4)2 and some double salts are slightly soluble.

When heated, M. reacts with halogens, giving halides; with wet chlorine, MgCl2 is formed already in the cold. When heated to 500-600 ° C with sulfur or with SO2 and H2S, MgS sulfide can be obtained, with hydrocarbons - MgC2 and Mg2C3 carbides. Silicides Mg2Si, Mg3Si2, phosphide Mg3P2, and other binary compounds are also known. M. is a strong reducing agent; when heated, it displaces other metals (Be, A1, alkaline) and nonmetals (B, Si, C) from their oxides and halides. M. forms numerous organometallic compounds. compounds that determine its great role in organic. synthesis (see Organomagnesium compounds). m. it is alloyed with most metals and is the basis of many technically important light alloys.

Receipt and application. In the industry, the largest amount of M is obtained by electrolysis of anhydrous chloride MgCl2 or dehydrated carnallite KCl-MgCl2-6H2O (see Magnesium chloride). The electrolyte also includes chlorides Na, K, Ca and a small amount of NaF or CaF2. The MgCl2 content in the melt is at least 5-7%; as the electrolysis proceeds at 720-750 ° C, the composition of the bath is adjusted by removing part of the electrolyte and adding MgCl2 or carnallite. Cathodes are made of steel, anodes are made of graphite. Molten metal floating on the surface of the electrolyte is periodically extracted from the cathode space separated from the anode by a partition that does not reach the bottom of the bath. The composition of rough M. includes up to 2% of impurities; it is refined in crucible electrics. furnaces under a layer of fluxes and poured into molds. The best varieties of primary M. contain 99.8% Mg. Subsequent purification of M. is carried out by sublimation in vacuum: 2-3 sublimations increase the purity of M. up to 99.999%. Anodic chlorine after purification is used to produce anhydrous MgCl2 from magnesite, titanium tetrachloride TiCU from TiO2 dioxide and other compounds.

Other methods of obtaining M.- metalothermic and carbonothermic. According to the first, briquettes of calcined dolomite and reducing agent (ferrosilicon or silicoaluminium) are heated at 1280-1300 ° C in vacuum (residual pressure 130-260 n / m2, i.e. 1-2 mmrt.st.). Vapors of M. condense at 400-500 ° C. To clean it, it is melted under flux or in vacuum, after which it is poured into molds. By ugletermich. according to the method, briquettes from a mixture of coal with oxide of M. are heated in electric furnaces above 2100 ° C; vapors of M. are distilled and condensed.

The most important area of application of metal. M. is the production of alloys based on it (see Magnesium alloys). M. is widely used in metallothermic processes for the production of hard-to-recover and rare metals (Ti, Zr, Hf, U, etc.), M. is used. for deoxidation and de-sulfuration of metals and alloys. Mixtures of powder M. with oxidizing agents serve as lighting and igniting. compositions. M compounds are widely used .

Magnesium in the body. M. is a constant part of plants and animal organisms (in thousandths - hundredths of a percent). The concentrators of M. are some algae accumulating up to 3% of M. (in ash), some foraminifera - up to 3.5%, calcareous sponges - up to 4%. M. is a part of the green pigment of plants - chlorophyll (the total mass of chlorophyll of plants of the Earth contains about 100 billion tons m.), and is also found in all cellular organelles of plants and ribosomes of all living organisms. M. activates mn. enzymes, together with calcium and manganese, ensure the stability of the structure of chromosomes and colloidal systems in plants, participate in maintaining turgor pressure in cells. m. stimulates the intake of phosphorus from the soil and its assimilation by plants, in the form of phosphoric acid salt is part of phytin. The lack of M. in soils causes marbling of the leaf in plants, chlorosis of plants (in such cases, magnesium fertilizers are used). Animals and humans get M. with food. The daily human need for M is 0.3-0.5 g; in childhood, as well as during pregnancy and lactation, this need is higher. The normal content of M. in the blood is about 4.3 mg%; with an increased content, drowsiness, loss of sensitivity, and sometimes paralysis of skeletal muscles are observed. In the body, M. accumulates in the liver, then it means. part of it goes into bones and muscles. In the muscles, M. participates in the activation of the processes of anaerobic carbohydrate metabolism. The antagonist of M. in the body is calcium. A violation of the magnesium-calcium balance is observed in rickets, when M. passes from the blood into the bones, displacing calcium from them. The lack of M. salts in food disrupts the normal excitability of the nervous system, muscle contraction. A large horn. cattle with a lack of M. in feed get sick with so-called herbal tetany (muscle twitching, limb growth arrest). Exchange of M. in animals, it is regulated by the hormone of the parathyroid glands, which lowers the content of M. in the blood, and prolane, which increases the content of M. From the preparations of M. in medical practice, they use: sulfate M. (as a sedative, anticonvulsant, antispasmodic, laxative and choleretic), burnt magnesia (magnesium oxide) and carbonate M. (as an alkali, light laxative). G. Ya. Zhiznevskaya.