Magnesium | Description, Properties, & Compounds

magnesium (Mg) , chemical element , one of the alkaline-earth metals of Group 2 (IIa) of the periodic table , and the lightest structural metal . Its compounds are widely used in construction and medicine , and magnesium is one of the elements essential to all cellular life .

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Occurrence, properties, and uses

Known originally through compounds such as Epsom salts (the sulfate), magnesia or magnesia alba (the oxide), and magnesite (the carbonate), the silvery white element itself does not occur free in nature. It was first isolated in by Sir Humphry Davy, who evaporated the mercury from a magnesium amalgam made by electrolyzing a mixture of moist magnesia and mercuric oxide. The name magnesium comes from Magnesia, a district of Thessaly (Greece) where the mineral magnesia alba was first found.

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Magnesium is the eighth most abundant element in Earth&#;s crust (about 2.5 percent) and is, after aluminum and iron, the third most plentiful structural metal. Its cosmic abundance is estimated as 9.1 × 105 atoms (on a scale where the abundance of silicon = 106 atoms). It occurs as carbonates&#;magnesite, MgCO3, and dolomite, CaMg(CO3)2&#;and in many common silicates, including talc, olivine, and most kinds of asbestos. It also is found as hydroxide (brucite), chloride (carnallite, KMgCl3&#;6H2O), and sulfate (kieserite). It is distributed in minerals such as serpentine, chrysolite, and meerschaum. Seawater contains about 0.13 percent magnesium, mostly as the dissolved chloride, which imparts its characteristic bitter taste.

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&#;e.g., from the reaction of magnesium oxide or calcined dolomite with ferrosilicon (the Pidgeon process). (See magnesium processing.)

At one time, magnesium was used for photographic flash ribbon and powder, because in finely divided form it burns in air with an intense white light; it still finds application in explosive and pyrotechnic devices. Because of its low density (only two-thirds that of aluminum), it has found extensive use in the aerospace industry. However, because the pure metal has low structural strength, magnesium is mainly used in the form of alloys&#;principally with 10 percent or less of aluminum, zinc, and manganese&#;to improve its hardness, tensile strength, and ability to be cast, welded, and machined. 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 alloys have a number of applications: they are used for parts of aircraft, spacecraft, machinery, automobiles, portable tools, and household appliances.

The thermal and electrical conductivity of magnesium 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 to liberate hydrogen (chromic and hydrofluoric acids are important exceptions). At normal temperatures it is stable in air and water because of the formation of a thin protective skin of oxide, but 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.

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Magnesium occurs in nature as a mixture of three isotopes: magnesium-24 (79.0 percent), magnesium-26 (11.0 percent), and magnesium-25 (10.0 percent). Nineteen radioactive isotopes have been prepared; magnesium-28 has the longest half-life, at 20.9 hours, and is a beta emitter. Although magnesium-26 is not radioactive, it is the daughter nuclide of aluminum-26, which has a half-life of 7.2 × 105 years. Elevated levels of magnesium-26 have been found in some meteorites, and the ratio of magnesium-26 to magnesium-24 has been used in determining their age.

The top producers of magnesium by the second decade of the 21st century included China, Russia, Turkey, and Austria.

Magnesium compounds are compounds formed by the element magnesium (Mg). These compounds are important to industry and biology, including magnesium carbonate, magnesium chloride, magnesium citrate, magnesium hydroxide (milk of magnesia), magnesium oxide, magnesium sulfate, and magnesium sulfate heptahydrate (Epsom salts).

Inorganic compounds

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Hydrides, halides and oxo-halides

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Magnesium hydride was first prepared in by the reaction between hydrogen and magnesium under high temperature, pressure and magnesium iodide as a catalyst.[1] It reacts with water to release hydrogen gas; it decomposes at 287 °C, 1 bar:[2]

MgH2 &#; Mg + H2

Magnesium can form compounds with the chemical formula MgX2 (X=F, Cl, Br, I) with halogens. Except for magnesium fluoride, the halides are easily soluble in water, but the solubility of magnesium fluoride is higher than that of other alkaline earth metal fluorides. High-purity magnesium fluoride is produced industrially by the reaction of magnesium sulfate and sodium fluoride, which sublimates at  °C. Magnesium chloride is generally obtained by chlorination of magnesium oxide, or by reacting magnesium chloride hexahydrate with ammonium chloride under dry hydrogen chloride, and then thermally decomposing the resulting magnesium ammonium double salt.[3] Its hydrate will be hydrolyzed, making the solution acidic; direct heating of the hydrate will give the hydrolyzed product:[3]

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[Mg(H2O)6]2+ &#; [Mg(H2O)5(OH)]+ + H3O+ (decomposes in water)

MgCl2·nH2O &#; Mg(OH)Cl + HCl + (n-1)H2O (decomposes when heated)

Magnesium chloride is an ionic compound, which can be electrolysed in a molten state to form magnesium and chlorine gas. The properties of magnesium bromide and magnesium iodide are similar.[citation needed] HMgX (X=Cl,Br,I) can be obtained by reacting the corresponding magnesium halide with magnesium hydride.[3]

Magnesium hypochlorite and magnesium chlorite are unstable compounds, they are easy to hydrolyze, the former generates basic salt Mg(OCl)2·2Mg(OH)2 and the latter generates hydroxide Mg(OH)2; magnesium chlorate can be obtained by reacting magnesium carbonate with chloric acid and crystallizing hexahydrate from solution, which can also be obtained by reacting magnesium hydroxide with chlorine gas and extracted with acetone:[citation needed]

6 Mg(OH)2 + 6 Cl2 &#; 5 MgCl2 + Mg(ClO3)2 + 6 H2O

Magnesium perchlorate is a white powder that is easily soluble in water, which can be obtained by the reaction of magnesium oxide and perchloric acid. The hexahydrate crystallizes from the solution, and then it is dried with phosphorus pentoxide in a vacuum at 200~250 °C to obtain the anhydrous form. It is a commonly used desiccant and can also be used as a Lewis acid or electrophile activator.[4] Magnesium perbromate can also crystallize out of the solution to form the hexahydrate, which can be heated to obtain anhydrous, and the anhydrous is further heated, and it decomposes into magnesium oxide, bromine and oxygen.[5]

Oxides and chalcogenides

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Magnesium oxide is the end product of the thermal decomposition of some magnesium compounds and is usually prepared by igniting carbonates or hydroxides. Magnesium hydroxide is a strong electrolyte, which can be obtained by the reaction of a soluble magnesium salt and sodium hydroxide. Like magnesium oxide, it will generate a basic carbonate when placed in the air.[3] Magnesium sulfide can be produced by the reaction of magnesium and hydrogen sulfide, or by the reaction of magnesium sulfate and carbon disulfide at high temperature:[6]

Mg + H2S &#; MgS + H2

3 MgSO4 + 4 CS2 &#; 3 MgS + 4 COS + 4 SO2

It can be hydrolyzed to Mg(HS)2, and further hydrolyzed to Mg(OH)2 at higher temperatures. A solution of magnesium hydrosulfide can also be prepared by reacting hydrogen sulfide with magnesium oxide in suspension.[7] Magnesium polysulfides have been studied in magnesium-sulfur batteries.[8] Magnesium selenide is more reactive than zinc selenide and decomposes in humid air;[9] the properties of magnesium telluride and magnesium selenide are similar.[10]

Organic compounds

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Grignard reagent

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The name of the Grignard reagent comes from the French chemist Victor Grignard who discovered it. This type of organomagnesium compound has the general formula R&#;Mg&#;X, where R is a hydrocarbon group and X is a halogen. They are usually coordinated with solvent molecules. bit. Grignard reagents can be obtained by reacting magnesium with halogenated hydrocarbons in a solvent. Since there is an oxide film on the surface of magnesium, iodine is generally added to accelerate the reaction.[3] Grignard reagents are commonly used in organic synthesis to extend carbon chains:[11]

Dihydrocarbylmagnesium

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Dihydrocarbylmagnesium is an organic compound with R&#;Mg&#;R&#;, which can be prepared by the reaction of dihydrocarbylmercury and magnesium.[12] Their reactivity is similar to that of Grignard reagents, and they can react with oxygen, water, and ammonia.[13]

Magnesium anthracene is the product obtained from the reaction of magnesium and anthracene in tetrahydrofuran, which can be used to provide C14H102&#; carbanions, which react with electrophiles to obtain di-derivatives of hydrogen anthracene.[14]

Applications

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Magnesium compounds, primarily magnesium oxide (MgO), are used as a refractory material in furnace linings for producing iron, steel, nonferrous metals, glass, and cement. Magnesium oxide and other magnesium compounds are also used in the agricultural, chemical, and construction industries. Magnesium oxide from calcination is used as an electrical insulator in fire-resistant cables.[15] Other applications include:

See also

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References

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External reading

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