Magnesium: the essentials

Magnesium is a grayish-white, fairly tough metal. Magnesium is the eighth most abundant element in the earth's crust although not found in it's elemental form. It is a Group 2 element (Group IIA in older labelling schemes). Group 2 elements are called alkaline earth metals. Magnesium metal burns with a very bright light.

Magnesium is an important element for plant and animal life. Chlorophylls are porphyrins based upon magnesium. The adult human daily requirement of magnesium is about 0.3 g day-1.

Table: basic information about and classifications of magnesium.

magnesium rods
Small and large samples of magnesium rod like this, as well as foil and sheet, (and magnesium alloy in foil form) can be purchased from Advent Research Materials via their web catalogue.

Magnesium: historical information

Magnesium was discovered by Sir Humphrey Davy at 1755 in England. Origin of name: from the Greek word "Magnesia", a district of Thessaly.

In 1618 a farmer at Epsom in England attempted to give his cows water from a well. This they refused to drink because of the water's bitter taste. However the farmer noticed that the water seemed to heal scratches and rashes. The fame of Epsom salts spread. Eventually they were recognised to be magnesium sulphate, MgSO4. Black recognized magnesium as an element in 1755. It was isolated by Davy in 1808 who electrolysed a mixture of magnesia (magnesium oxide, MgO) and mercuric oxide (HgO). Davy's first suggestion for a name was magnium but the name magnesium is now used.

Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in series of compounds, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for atoms. Dalton's theory of atoms was published by Thomas Thomson in the 3rd edition of his System of Chemistry in 1807 and in a paper about strontium oxalates published in the Philosophical Transactions. Dalton published these ideas himself in the following year in the New System of Chemical Philosophy. The symbol used by Dalton for magnesium is shown below. [See History of Chemistry, Sir Edward Thorpe, volume 1, Watts & Co, London, 1914.]

Dalton's symbol for magnesium

Magnesium: physical properties

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Magnesium: orbital properties

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Isolation

Isolation: magnesium can be made commercially by several processes and would not normally be made in the laboratory because of its ready availability. There are massive amounts of magnesium in seawater. This can be recovered as magnesium chloride, MgCl2 through reaction with calcium oxide, CaO.

CaO + H2O → Ca2+ + 2OH-

Mg2+ + 2OH- → Mg(OH)2

Mg(OH)2 + 2HCl → MgCl2 + 2H2O

Electrolysis of hot molten MgCl2 affords magnesium as a liquid whih is poured off and chlorine gas.

cathode: Mg2+(l) + 2e- → Mg

anode: Cl-(l) → 1/2Cl2 (g) + e-

The other methos used to produce magnesium is non electrolytic and involves dolomite, [MgCa(CO3)2], an important magnesium mineral. This is "calcined" by heating to form calcined dolomite, MgO.CaO, and this reacted with ferrosilicon alloy.

2[MgO.CaO] + FeSi → 2Mg + Ca2SiO4 + Fe

The magnesium may be distilled out from this mixture of products.

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magnesium atomic number