Magnesium
Magnesium is a chemical element; it has symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point, and high chemical reactivity. Like the other alkaline earth metals, it occurs naturally only in combination with other elements and almost always has an oxidation state of +2. It reacts readily with air to form a thin passivation coating of magnesium oxide that inhibits further corrosion of the metal. The free metal burns with a brilliant-white light. The metal is obtained mainly by electrolysis of magnesium salts obtained from brine. It is less dense than aluminium and is used primarily as a component in strong and lightweight alloys that contain aluminium.
In the cosmos, magnesium is produced in large, aging stars by the sequential addition of three helium nuclei to a carbon nucleus. When such stars explode as supernovas, much of the magnesium is expelled into the interstellar medium, where it may recycle into new star systems. Magnesium is the eighth most abundant element in the Earth's crust and the fourth most common element in the Earth, making up 13% of the planet's mass and a large fraction of the planet's mantle. It is the third most abundant element dissolved in seawater, after sodium and chlorine.
This element is the eleventh most abundant element by mass in the human body and is essential to all cells and some 300 enzymes. Magnesium ions interact with polyphosphate compounds such as ATP, DNA, and RNA. Hundreds of enzymes require magnesium ions to function. Magnesium compounds are used medicinally as common laxatives and antacids, and to stabilize abnormal nerve excitation or blood vessel spasm in such conditions as eclampsia.
Characteristics
Physical properties
Elemental magnesium is a gray-white lightweight metal, two-thirds the density of aluminium. Magnesium has the lowest melting and the lowest boiling point of all the alkaline earth metals.Pure polycrystalline magnesium is brittle and easily fractures along shear bands. It becomes much more malleable when alloyed with small amounts of other metals, such as 1% aluminium. The malleability of polycrystalline magnesium can also be significantly improved by reducing its grain size to about 1 μm or less.
Chemical properties
Oxidation
Magnesium is widely used as a reducing agent. Although it oxidises in air, it does not need an inert atmosphere for storage; it forms a thin layer of magnesium oxide that protects the rest of the metal.Direct reaction of magnesium with air or oxygen at ambient pressure forms only the "normal" oxide MgO. However, this oxide may be combined with hydrogen peroxide to form magnesium peroxide, MgO2, and at low temperature the peroxide may be further reacted with ozone to form magnesium superoxide Mg2.
Magnesium reacts with nitrogen in the solid state if it is powdered and heated to just below the melting point, forming magnesium nitride Mg3N2.
Magnesium reacts with water at room temperature, though it reacts much more slowly than calcium, a similar group 2 metal. When submerged in water, hydrogen bubbles form slowly on the surface of the metal; this reaction happens much more rapidly with powdered magnesium. The reaction also occurs faster with higher temperatures. Magnesium's reversible reaction with water can be harnessed to store energy and run a magnesium-based engine. Magnesium also reacts exothermically with most acids such as hydrochloric acid, producing magnesium chloride and hydrogen gas, similar to the HCl reaction with aluminium, zinc, and many other metals. Although it is difficult to ignite in mass or bulk, magnesium metal will ignite.
Magnesium may also be used as an igniter for thermite, a mixture of aluminium and iron oxide powder that ignites only at a very high temperature.
Reaction with water
When finely powdered, magnesium reacts with water to produce magnesium hydroxide and hydrogen gas:However, this reaction is much less dramatic than the reactions of the alkali metals with water, because the magnesium hydroxide builds up on the surface of the magnesium metal and inhibits further reaction.
In addition, when reacting with steam it produces magnesium oxide and hydrogen:
Mg + H2O → MgO + H2
Organic chemistry
Organomagnesium compounds are widespread in organic chemistry. They are commonly found as Grignard reagents, formed by reaction of magnesium with haloalkanes or aryl halides in diethyl ether. Examples of Grignard reagents are phenylmagnesium bromide and ethylmagnesium bromide. The Grignard reagents function as a common nucleophile, attacking the electrophilic group such as the carbon atom that is present within the polar bond of a carbonyl group.A prominent organomagnesium reagent beyond Grignard reagents is magnesium anthracene or magnesocene, which is used as a source of highly active magnesium. First prepared in 1954 by independent groups, one led by Ernst Otto Fischer, the other by Albert Wilkinson, magnesocene is a white to off-yellow pyrophoric powder that violently hydrolyses in water. The related butadiene-magnesium adduct serves as a source for the butadiene dianion.
Complexes of dimagnesium have been observed.
Detection in solution
The presence of magnesium ions can be detected by the addition of ammonium chloride, ammonium hydroxide and monosodium phosphate to an aqueous or dilute HCl solution of the salt. The formation of a white precipitate indicates the presence of magnesium ions.Azo violet dye can also be used, turning deep blue in the presence of an alkaline solution of magnesium salt. The color is due to the adsorption of azo violet by Mg2.
Forms
Alloys
As of 2013, consumption of magnesium alloys was less than one million tonnes per year, compared with 50 million tonnes of aluminium alloys. Their use has been historically limited by the tendency of Mg alloys to corrode, creep at high temperatures, and combust.Corrosion
In magnesium alloys, the presence of iron, nickel, copper, or cobalt strongly activates corrosion. In more than trace amounts, these metals precipitate as intermetallic compounds, and the precipitate locales function as active cathodic sites that reduce water, causing the loss of magnesium. Controlling the quantity of these metals improves corrosion resistance. Sufficient manganese overcomes the corrosive effects of iron. This requires precise control over composition, increasing costs. Adding a cathodic poison captures atomic hydrogen within the structure of a metal. This prevents the formation of free hydrogen gas, an essential factor of corrosive chemical processes. The addition of about one in three hundred parts arsenic reduces the corrosion rate of magnesium in a salt solution by a factor of nearly ten.High-temperature creep and flammability
Magnesium's tendency to creep at high temperatures is greatly reduced by alloying with zinc and rare-earth elements. Flammability is significantly reduced by a small amount of calcium in the alloy. By using rare-earth elements, it may be possible to manufacture magnesium alloys that are able to not catch fire at higher temperatures compared to magnesium's liquidus and in some cases potentially pushing it close to magnesium's boiling point.Compounds
Magnesium forms a variety of compounds important to industry and biology, including magnesium carbonate, magnesium chloride, magnesium citrate, magnesium hydroxide, magnesium oxide, magnesium sulfate, and magnesium sulfate heptahydrate.As recently as 2020, magnesium hydride was under investigation as a way to store hydrogen.
Isotopes
Magnesium has three stable isotopes:, and. All are present in significant amounts in nature. About 79% of Mg is. The isotope is radioactive and in the 1950s to 1970s was produced by several nuclear power plants for use in scientific experiments. This isotope has a relatively short half-life and its use was limited by shipping times.The nuclide has found application in isotopic geology, similar to that of aluminium. is a radiogenic daughter product of aluminium-26|, which has a half-life of 717,000 years. Excessive quantities of stable have been observed in the Ca-Al-rich inclusions of some carbonaceous chondrite meteorites. This anomalous abundance is attributed to the decay of its parent in the inclusions, and researchers conclude that such meteorites were formed in the solar nebula before the had decayed. These are among the oldest objects in the Solar System and contain preserved information about its early history.
It is conventional to plot / against an Al/Mg ratio. In an isochron dating plot, the Al/Mg ratio plotted is /. The slope of the isochron has no age significance, but indicates the initial / ratio in the sample at the time when the systems were separated from a common reservoir.
Production
Occurrence
Magnesium is the eighth-most-abundant element in the Earth's crust by mass and tied in seventh place with iron in molarity. It is found in large deposits of magnesite, dolomite, and other minerals, and in mineral waters, where magnesium ion is soluble.Although magnesium is found in more than 60 minerals, only dolomite, magnesite, brucite, carnallite, talc, and olivine are of commercial importance.
The cation is the second-most-abundant cation in seawater, which makes seawater and sea salt attractive commercial sources for Mg.
Production quantities
World production was approximately 1,100 kt in 2017, with the bulk being produced in China and Russia. The United States was in the 20th century the major world supplier of this metal, supplying 45% of world production even as recently as 1995. Since the Chinese mastery of the Pidgeon process the US market share is at 7%, with a single US producer left as of 2013: US Magnesium, a Renco Group company located on the shores of the Great Salt Lake.In September 2021, China took steps to reduce production of magnesium as a result of a government initiative to reduce energy availability for manufacturing industries, leading to a significant price increase.