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 pass
Chemical element with atomic number 12 (Mg) Be
↑
Mg
↓
Ca sodium ← magnesium → aluminium Atomic number (Z)12Groupgroup 2 (alkaline earth metals)Periodperiod 3Block s-blockElectron configuration[Ne] 3s2Electrons per shell2, 8, 2Physical propertiesPhase at STPsolidMelting point923 K (650 °C, 1202 °F) Boiling point1363 K (1091 °C, 1994 °F) Density (at 20° C)1.737 g/cm3[3]when liquid (at m.p.)1.584 g/cm3 Heat of fusion8.48 kJ/mol Heat of vaporization128 kJ/mol Molar heat capacity24.869 J/(mol·K) [4]Specific heat capacity1023.205 J/(kg·K) [5]Vapor pressure Atomic propertiesOxidation statescommon: +2
0,[6] +1[7]ElectronegativityPauling scale: 1.31 Ionization energies
- 1st: 737.7 kJ/mol
- 2nd: 1450.7 kJ/mol
- 3rd: 7732.7 kJ/mol
- (more)
c = 521.03 pm (at 20 °C)[3]Thermal expansion25.91×10−6/K (at 20 °C)[3][a]Thermal conductivity156[8] W/(m⋅K) Electrical resistivity43.9[9] nΩ⋅m (at 20 °C) Magnetic orderingparamagnetic Molar magnetic susceptibility+13.1×10−6 cm3/mol (298 K)[10]Young's modulus45 GPa Shear modulus17 GPa Bulk modulus35.4[11] GPa Speed of sound thin rod4940 m/s (at r.t.) (annealed)Poisson ratio0.290 Mohs hardness1–2.5 Brinell hardness44–260 MPa CAS Number7439-95-4 HistoryNamingafter Magnesia, Greece[12]DiscoveryJoseph Black (1755[12])First isolationHumphry Davy (1808[12])Isotopes of magnesium
- v
- e
- view
- talk
- edit
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 (group 2 of the periodic table), 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[14] and the fourth most common element in the Earth (after iron, oxygen and silicon), 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.[15]
This element is the eleventh most abundant element by mass in the human body and is essential to all cells and some 300 enzymes.[16] 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 (such as milk of magnesia), and to stabilize abnormal nerve excitation or blood vessel spasm in such conditions as eclampsia.[16]
Characteristics
Physical properties
Elemental magnesium is a gray-white lightweight metal, two-thirds the density of aluminium. Magnesium has the lowest melting (923 K (650 °C)) and the lowest boiling point (1,363 K (1,090 °C)) of all the alkaline earth metals.[17]
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.[18] The malleability of polycrystalline magnesium can also be significantly improved by reducing its grain size to about 1 μm or less.[19]
Chemical properties
OxidationMagnesium is widely used as a reducing agent.[20] 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.[21]
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 Mg(O2)2.[22]
Magnesium reacts with nitrogen in the solid state if it is powdered and heated to just below the melting point, forming magnesium nitride Mg3N2.[23]
Magnesium reacts with water at room temperature, though it reacts much more slowly than calcium, a similar group 2 metal.[21] When submerged in water, hydrogen bubbles form slowly on the surface of the metal; this reaction happens much more rapidly with powdered magnesium.[21] The reaction also occurs faster with higher temperatures (see § Safety precautions). 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 (HCl), producing magnesium chloride and hydrogen gas, similar to the HCl reaction with aluminium, zinc, and many other metals.[24] 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 waterWhen finely powdered, magnesium reacts with water to produce magnesium hydroxide and hydrogen gas:
Mg(s) + 2 H2O(l) → Mg(OH)2(aq) + H2(g)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.[25]
In addition, when reacting with steam it produces magnesium oxide and hydrogen:[25]
Mg(s) + H2O(g) → MgO(aq) + H2(g)
Organic chemistry Further information: Grignard reagentOrganomagnesium 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.[20] 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.[20] The related butadiene-magnesium adduct serves as a source for the butadiene dianion.
Complexes of dimagnesium(I) have been observed.[26]
