Difference between revisions of "Magnesium"

Revision as of 14:46, 9 July 2018

Magnesium,  ₁₂Mg

Magnesium ribbon stored in mineral oil to prevent oxidation
General properties
Name, symbol	Magnesium, Mg
Appearance	Silvery-white
Magnesium in the periodic table
Be ↑ Mg ↓ Ca Sodium ← Magnesium → Aluminium
Atomic number	12
Standard atomic weight (Ar)	24.305
Group, block	(alkaline earth metals); s-block
Period	period 3
Electron configuration	[Ne] 3s2
per shell	2, 8, 2
Physical properties
Silvery-white
Phase	Solid
Melting point	923 K ​(650 °C, ​​1202 °F)
Boiling point	1363 K ​(1091 °C, ​1994 °F)
Density near r.t.	1.738 g/cm3
when liquid, at	1.584 g/cm3
Heat of fusion	8.48 kJ/mol
Heat of	128 kJ/mol
Molar heat capacity	24.869 J/(mol·K)
pressure
Atomic properties
Oxidation states	+2, +1 ​(a strongly basic oxide)
Electronegativity	Pauling scale: 1.31
energies	1st: 737.7 kJ/mol 2nd: 1450.7 kJ/mol 3rd: 7732.7 kJ/mol
Atomic radius	empirical: 160 pm
Covalent radius	141±7 pm
Van der Waals radius	173 pm
Miscellanea
Crystal structure	​Hexagonal close-packed (hcp)
Speed of sound thin rod	4940 m/s (at ) (annealed)
Thermal expansion	24.8 µm/(m·K) (at 25 °C)
Thermal conductivity	156 W/(m·K)
Electrical resistivity	4.39·10-8 Ω·m (at 20 °C)
Magnetic ordering	Paramagnetic
Young's modulus	45 GPa
Shear modulus	17 GPa
Bulk modulus	45 GPa
Poisson ratio	0.29
Mohs hardness	1–2.5
Brinell hardness	44–260 MPa
CAS Registry Number	7439-95-4
History
Naming	After Magnesia, Greece
Discovery	Joseph Black (1755)
First isolation	Humphry Davy (1808)
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Magnesium is an alkaline earth metal with the symbol Mg and atomic number 12. It is one of the most powerful reducing agents available to the amateur chemist, even more so than zinc. However, it is impossible to plate magnesium out of an aqueous solution, and it is generally not feasible to recover the metal.

Properties

Physical properties

Magnesium is a light, grayish metal. Oxidized pieces are a darker shade of gray, and tend to have a white powder of magnesium oxide on the surface. It has a low melting point of 650 °C, though the metal will ignite in air before it reaches that temperature.

Chemical properties

Magnesium is an extremely powerful reducing agent, though it is relatively stable in air due to the formation of a partial passivation layer. When stored in air, magnesium will slowly oxidize, so it is advisable to store it in a sealed container. In water, magnesium pieces react only slowly to form magnesium hydroxide. It does not react with alkali solutions. However, the metal will react vigorously in dilute acids to form corresponding magnesium salts. Most of these are soluble except for the hydroxide, fluoride, and carbonate. In air, magnesium will ignite with a very hot white flame to form a mixture that consists mostly of magnesium oxide, but also contains some magnesium nitride. The color of the flame is a noticeably purer white than that of titanium or zirconium flames, which appear slightly yellowish.

Availability and sources

Magnesium powder can be purchased from eBay, at varying prices, depending on the particle size.

Fire starting kits often contain magnesium of 95% purity, which is sufficient for most simple reductions.

Some pencil sharpeners, such as those manufactured by KUM and Staedtler are made of magnesium, in case of the latter, 95% pure. A simple test to see if these are made of magnesium or not involves heating one in a blowtorch flame for about 30 seconds (outside!); if it is magnesium, it will catch fire and give off intense white light. A less destructive method involves adding a few drops of aqueous NaOH on the sharpener surface. Magnesium does not react with sodium hydroxide, but aluminium will. These can be found at University of California campuses. They are easily identifiable by their light weight.

Magnesium products of higher purity can be bought from GalliumSource. It is sold as turnings (coarse and fine), ribbon, ingots, rods, and foil.

Magnesium strips can be bought from United Nuclear.

Another good source of magnesium is the sacrificial anodes used in many water heaters. They can be cheaply found at most plumbing stores. One rod generally has around 200 g of magnesium metal and costs around 8-12 $.

Preparation

Elemental magnesium is difficult to prepare, due to it's high reactivity. The industrial method involves the electrolysis of molten magnesium chloride or an eutectic mixture of MgCl₂ and KCl (melting point 450°C), in a Downs cell. This process requires the use of corrosion resistant alloy crucibles, as molten magnesium chloride is very corrosive. The process takes place in an inert atmosphere, either argon or more often sulfur hexafluoride.

Projects

• Make flash powder (water sensitive)
• Make thermite
• Grignard reactions
• Make sodium and potassium metal

Handling

Safety

Magnesium and its compounds are not particularly toxic. Bulk magnesium is not prone to ignition, but magnesium powder and turnings are. Water and Carbon Dioxide extinguishers must NEVER be used to put out magnesium fires, as this accelerates the burning and can produce toxic and/or explosive gasses as a byproduct. Dry sand can be used to fight burning magnesium.

Never consume magnesium or its compounds, when produced in the laboratory, as a supplement.

Storage

Magnesium metal will slowly corrode in air and turn dark gray. To prevent this, storage under mineral oil is sufficient. For long-term storage, ampouling is a viable solution, though rarely necessary. Keep it away from acidic vapors and corrosive gases.

Disposal

No special disposal procedures are required for magnesium and magnesium compounds. Discard them as you wish.

References

Relevant Sciencemadness threads

• Why are pencil sharpners magnesium?
• Mg metal from MgO