Magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg and atomic number 12. Its common oxidation number is +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole. Magnesium is the fourth most common element in the Earth as a whole (behind iron, oxygen and silicon), making up 13% of the planet's mass and a large fraction of the planet's mantle. The relative abundance of magnesium is related to the fact that it easily builds up in supernova stars from a sequential addition of three helium nuclei to carbon (which in turn is made from three helium nuclei). Due to magnesium ion's high solubility in water, it is the third most abundant element dissolved in seawater.

The free element (metal) is not found naturally on Earth, as it is highly reactive (though once produced, it is coated in a thin layer of oxide (see passivation), which partly masks this reactivity). The free metal burns with a characteristic brilliant white light, making it a useful ingredient in flares. The metal is now mainly obtained by electrolysis of magnesium salts obtained from brine. Commercially, the chief use for the metal is as an alloying agent to make aluminium-magnesium alloys, sometimes called magnalium or magnelium. Since magnesium is less dense than aluminium, these alloys are prized for their relative lightness and strength.

In human biology, magnesium is the eleventh most abundant element by mass in the human body. Its ions are essential to all living cells, where they play a major role in manipulating important biological polyphosphate compounds like ATP, DNA, and RNA. Hundreds of enzymes thus require magnesium ions to function. Magnesium compounds are used medicinally as common laxatives, antacids (e.g., milk of magnesia), and in a number of situations where stabilization of abnormal nerve excitation and blood vessel spasm is required (e.g., to treat eclampsia). Magnesium ions are sour to the taste, and in low concentrations they help to impart a natural tartness to fresh mineral waters.

In vegetation, magnesium is the metallic ion at the center of chlorophyll, and is thus a common additive to fertilizers.

Occurrence


Although magnesium is found in over 60 minerals, only dolomite, magnesite, brucite, carnallite, talc, and olivine are of commercial importance.

The Mg2+ cation is the second most abundant cation in seawater (occurring at about 12% of the mass of sodium there), which makes seawater and sea-salt an attractive commercial source of Mg. To extract the magnesium, calcium hydroxide is added to seawater to form magnesium hydroxide precipitate.

MgCl2 + Ca(OH)2 → Mg(OH)2 + CaCl2

Magnesium hydroxide (brucite) is insoluble in water so it can be filtered out, and reacted with hydrochloric acid to obtain concentrated magnesium chloride.

Mg(OH)2 + 2 HCl → MgCl2 + 2 H2O

From magnesium chloride, electrolysis produces magnesium.
In the United States, magnesium is principally obtained by electrolysis of fused magnesium chloride from brines, wells, and sea water. At the cathode, the Mg2+ ion is reduced by two electrons to magnesium metal:

Mg2+ + 2 e− → Mg

At the anode, each pair of Cl− ions is oxidized to chlorine gas, releasing two electrons to complete the circuit:

2 Cl− → Cl2 (g) + 2 e−

The United States has traditionally been the major world supplier of this metal, supplying 45% of world production even as recently as 1995. Today, the US market share is at 7%, with a single domestic producer left, US Magnesium, a Renco Group company in Utah born from now-defunct Magcorp.

As of 2005, China has taken over as the dominant supplier, pegged at 60% world market share, which increased from 4% in 1995. Unlike the above-described electrolytic process, China is almost completely reliant on a different method of obtaining the metal from its ores, the silicothermic Pidgeon process (the reduction of the oxide at high temperatures with silicon).

Symbol		Mg
Atomic Number		12
Atomic Weight		24.305
Oxidation States		+2
Electronegativity, Pauling		1.31
State at RT		Solid, Metal
Melting Point, K		922
Boiling Point, K		1363

Interesting Facts about Magnesium

• At the center of every chlorophyll molecule, in every green plant, there is a magnesium ion.
• Magnesium is one of the two dozen or so elements that are essential for life. Magnesium is vital in human metabolism, needed for over 300 biochemical reactions.
• Magnesium fires must be treated with caution. Adding water to them produces hydrogen, which makes the fire burn even more fiercely.
• If you try to put out a magnesium fire with carbon dioxide, you’ll also find yourself out of luck: magnesium burns in both pure nitrogen and pure carbon dioxide, and a carbon dioxide fire-extinguisher will feed a magnesium fire.
• Mag wheels were once made of a magnesium alloy: magnesium is both light and strong. Mag wheels no longer include magnesium; the wheels did not last well and potentially were a fire hazard.
• Despite the absence of magnesium, the wheels have kept their original name.
• Magnesium is formed in stars with a mass of eight or more Earth suns.
• Near the end of their lives, these stars enter the carbon burning phase, also making oxygen, sodium and neon.
• Magnesium is the second most abundant metal in seawater. (Only sodium is more abundant.)
• Our bodies need the correct amount of magnesium in our diets for us to sleep properly. If it’s too high or too low, we can suffer from sleep disturbance.
• About 13% of our planet’s entire mass comes from magnesium. This means there’s enough magnesium within Earth to make a planet of the same mass as Mars AND have enough magnesium left over to make three more objects of the same mass as our moon.
• There is a significantly higher proportion of magnesium below Earth’s crust than in it.

Appearance and Characteristics

Harmful effects:

Magnesium powder is an explosive hazard.

The bright white light plus ultraviolet from burning magnesium can cause permanent eye damage.

Characteristics:

• Magnesium is a silvery-white, low density, reasonably strong metal that tarnishes in air to form a thin oxide coating. Magnesium and its alloys have very good corrosion resistance and good high temperature mechanical properties.
• The metal reacts with water to produce hydrogen gas.
• When it burns in air, magnesium produces a brilliant white light.

Uses of Magnesium

• The brilliant light it produces when ignited is made use of in photography, flares, pyrotechnics and incendiary bombs.
• With a density of only two-thirds that of aluminum, and just over one-fifth that of iron, magnesium alloys are used in aircraft, car engine casings, and missile construction.
• The metal is widely used in the manufacturing of mobile phones, laptop computers, cameras, and other electronic components.
• Organic magnesium compounds (Grignard reagents) are important in the synthesis of organic molecules.
• Magnesium compounds such as the hydroxide (milk of magnesia, Mg(OH2)), sulfate (Epsom salts), chloride and citrate are used for medicinal purposes.
• Magnesium is the second most important intracellular cation and is involved in a variety of metabolic processes including glucose metabolism, ion channel translocation, stimulus-contraction coupling, stimulus secretion coupling, peptide hormone receptor signal transduction.

Should I use carbon dioxide to extinguish a magnesium fire?

Is it a good idea to extinguish burning magnesium with water?