The chemical element magnesium is classed as an alkali earth metal. It was discovered in 1808 by Humphrey Davy.
|Classification:||Magnesium is an alkali earth metal|
|Melting point:||650 oC, 923 K|
|Boiling point:||1090 oC, 1363 K|
|Neutrons in most abundant isotope:||12|
|Electron configuration:||1s2 2s2 2p6 3s2|
|Density @ 20oC:||1.738 g/cm3|
|Atomic volume:||13.97 cm3/mol|
|Structure:||hcp: hexagonal close packed|
|Specific heat capacity||1.02 J g-1 K-1|
|Heat of fusion||8.48 kJ mol-1|
|Heat of atomization||146 kJ mol-1|
|Heat of vaporization||127.4 kJ mol-1|
|1st ionization energy||737.7 kJ mol-1|
|2nd ionization energy||1450.6 kJ mol-1|
|3rd ionization energy||7732.6 kJ mol-1|
|Electron affinity||78 kJ mol-1|
|Minimum oxidation number||0|
|Min. common oxidation no.||0|
|Maximum oxidation number||2|
|Max. common oxidation no.||2|
|Electronegativity (Pauling Scale)||1.31|
|Polarizability volume||10.6 Å3|
|Reaction with air||vigorous, w/ht ⇒ MgO, Mg3N2|
|Reaction with 15 M HNO3||vigorous ⇒ NOx, Mg(NO3)2|
|Reaction with 6 M HCl||mild ⇒ H2, MgCl2|
|Reaction with 6 M NaOH||none|
|Atomic radius||150 pm|
|Ionic radius (1+ ion)||–|
|Ionic radius (2+ ion)||86 pm|
|Ionic radius (3+ ion)||–|
|Ionic radius (1- ion)||–|
|Ionic radius (2- ion)||–|
|Ionic radius (3- ion)||–|
|Thermal conductivity||156 W m-1 K-1|
|Electrical conductivity||22.4 x 106 S m-1|
|Freezing/Melting point:||650 oC, 923 K|
Discovery of Magnesium
Magnesium and calcium were once thought to be the same substance. In 1755 Scottish chemist Joseph Black showed by experiment that the two were different. Black wrote:
“We have already shewn by experiment, that magnesia alba [magnesium carbonate] is a compound of a peculiar earth and fixed air.” (1)
Magnesium was first isolated by Sir Humphrey Davy in 1808, in London, England. Davy had built a large battery and used it to pass electricity through salts. In doing so, he discovered or isolated for the first time several alkali and alkali earth metals.
Davy made a paste of moist magnesium oxide and red mercury oxide. (2)
He made a depression in the paste and placed about 3.5 grams of mercury metal there to act as the negative electrode. He used platinum as the positive electrode. Davy did the experiment under naphtha (a liquid hydrocarbon under which he had found he could safely store potassium and sodium).
When electricity was passed through the paste, a magnesium-mercury amalgam formed at the mercury electrode. (In later experiments Davy used moist magnesium sulfate instead of the oxide and obtained the amalgam much faster.) (2a)
The mercury was then removed from the amalgam by heating to leave magnesium metal. (2)
In a lecture to the Royal Society in June 1808, Davy described how the magnesium he obtained was not pure because of difficulties in removing the mercury entirely from the magnesium. He was, however, able to observe that in air the metal turned into a white powder, gaining weight as it reacted with oxygen and returned to its oxide form. (2b)
Davy thought the logical name for the new metal was ‘magnesium’ but instead called it ‘magnium.’
He thought the name ‘magnium’ was, “objectionable, but magnesium has been already applied to metallic manganese…”
By 1812, Davy had changed his mind, following the “candid criticisms of some philosophical friends,” and the new metal became known as magnesium, while metallic manganese became known as… manganese. (3)
Magnesium’s name is derived from magnesia, which Davy used in his experiment. Magnesia is the district of Thessaly in Greece where magnesia alba [magnesium carbonate] was found.
In France, in 1830, Antoine Bussy published his work showing how pure magnesium metal could be obtained. Bussy had read Friedrich Wöhler’s 1828 publication of how he had produced pure aluminum by reacting aluminum chloride with potassium. By analogy, Bussy thought he could do something similar to produce pure magnesium from magnesium chloride; he was correct.
Under red heat he reacted magnesium chloride with potassium vapor and obtained pure magnesium. He wrote, “The metal is silvery white, very brilliant, very malleable, flattens into flakes under a hammer… dilute acids attack the metal, releasing hydrogen.” (4)
Appearance and Characteristics
Magnesium powder is an explosive hazard.
The bright white light plus ultraviolet from burning magnesium can cause permanent eye damage.
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, and pyrotechnics.
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. (5)
Abundance and Isotopes
Abundance earth’s crust: 2.3 % by weight, 2.0 % by moles
Abundance solar system: 700 parts per million by weight, 30 parts per million by moles
Cost, pure: $3.7 per 100g
Cost, bulk: $0.29 per 100g
Source: Magnesium is the eighth most abundant element in the Earth’s crust and the sixth most abundant metal. Magnesium is obtained commercially by the ‘Pidgeon’ process. This high temperature method uses silicon as a reducing agent to extract magnesium from minerals such as dolomite (MgCa(CO 3)2) or magnesite (MgCO 3) or saltwater.
Isotopes: Magnesium has 15 isotopes whose half-lives are known with mass ranges from 20 to 34. Naturally occurring magnesium is a mixture of its three stable isotopes and they are found in the percentages shown: 24Mg (79.0%), 25Mg (10.0%) and 26Mg (11.0%).
1. Joseph Black, Experiments upon Magnesia Alba, Quick-Lime, and some other Alkaline Substances (1756)
2. John Davy (Editor), The Collected Works of Sir Humphry Davy, Vol V, 1840, p111 Smith, Elder and Co. Cornhill.
2a. John Davy (Editor), The Collected Works of Sir Humphry Davy, Vol V, 1840, p110 Smith, Elder and Co. Cornhill.
2b. John Davy (Editor), The Collected Works of Sir Humphry Davy, Vol V, 1840, p115 Smith, Elder and Co. Cornhill.
3. Sir Humphry Davy, Elements of Chemical Philosophy., 1812, Part 1, Vol. 1, p198.
4. Gay-Lussac et al, Annals of Chemistry and Physics, 1831, Vol. XLVI, p434-437.
5. Nancy E. Bernhardt, Artur M. Kasko, Nutrition for the Middle Aged and Elderly., (2008) p333. Nova Science Publishers
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