magnesium

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.

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 1808 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.

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.

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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.

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.

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

Principal compounds

In compounds, magnesium virtually always exhibits a +2 oxidation state because of the loss or sharing of its two 3s electrons. There are, however, a small number of coordination compounds known with magnesium-magnesium bonds, LMg―MgL, in which the magnesium centres have a formal +1 oxidation state. 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. As magnesium carbonate is both hygroscopic and insoluble in water, it was the original additive used to make table salt free-flowing even in high-humidity conditions.

Magnesium hydroxide, Mg(OH)₂, is a white powder produced in large quantities from seawater by the addition of milk of lime (calcium hydroxide). It is the primary raw material in the production of magnesium metal and has been used as a fire-retardant additive. 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. It is also used to coagulate soy milk in the production of tofu.

Roasting either magnesium carbonate or magnesium hydroxide produces the oxygen compound magnesium oxide, commonly called magnesia, MgO. It is a white solid used in the manufacture of high-temperature refractory bricks, electrical and thermal insulators, cements, fertilizer, rubber, and plastics. It is also used medically as a laxative and antacid.

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, MgSO₄∙H₂O, occurs as a mineral deposit. Synthetically prepared magnesium sulfate is sold as Epsom salt, MgSO₄∙7H₂O. 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. Because of its ability to absorb water readily, the anhydrous form is used as a desiccant (drying agent).

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. These are used in the production of many other kinds of organic and organometallic compounds.

Magnesium is essential to all living cells, as the Mg²⁺ ion is involved with the critically important biological polyphosphate compounds DNA, RNA, and adenosine triphosphate (ATP). Many enzymes depend on magnesium for their functioning. About one-sixth as plentiful as potassium in human body cells, magnesium is required as a catalyst for enzyme reactions in carbohydrate metabolism. Magnesium also is an essential constituent of the green pigment chlorophyll, found in virtually all plants, algae, and cyanobacteria. 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 light energy to drive the conversion of carbon dioxide and water to carbohydrates and oxygen and thus directly or indirectly provide the key to nearly all living processes.