Discovered | Name | Characteristics | Reactions | Compounds | Uses

Magmesium is a metal in the 3rd period of Group IIA, the alkaline earth elements. Other elements in this group are beryllium (Be), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

Joseph Black recognized magnesium as an element in 1755. A small quantity of the metal was isolated by Davy in 1808, but it was prepared in quantity in 1831 by Bussy.

The name is derived from the name for the mineral, magnesia (MgO). The name for the mineral probably comes from Magnesia, a district in Thessaly of ancient Greece. The symbol Mg is an abbreviation for the name.

In the solid state the metal has a hexagonal closest packed structure.

Magnesium is the eighth most abundant element in the earth's crust. It does not occur uncombined, but it is found in large deposits in the form of magnesite, dolomite [CaMg(CO3)2], and other minerals.

Magnesium is a light, silvery white, and fairly tough metal. It tarnishes slightly in air, and finely divided magnesium readily ignites upon heating in air and burns with a dazzling white flame.

Magnesium metal reacts with air to form MgO (and some nitride, Mg3N2),

2 Mg(s) + O2 2 MgO(s)

and with acids to give Mg(II) salts and H2 gas. For example,

Mg(s) + 2 HCl(aq) MgCl2(aq) + H2(g)

It can react with water to give Mg(OH)2 and H2 if the mixture is heated.

Mg(s) + 2 H2O Mg(OH)2(s) + H2(g)

This reaction is quite exothermic, one mole of Mg (24g) giving about 355 kJ of heat. This fact has been used by the military to prepare MRE's ("Meals, Ready to Eat") for soldiers in battle. Since the food is certainly better when warmed, the reaction above is used to warm the meals. When in the field, the soldier places a pouch containing the MRE in a cardboard sleeve containing Mg. The soldier adds water, and the evolved heat warms the MRE pouch. One problem is that Mg normally has a thin coating of MgO or Mg(OH)2, making the Mg inert to reaction with water. To allow the reaction, the Mg metal is mixed with a little NaCl (and some Fe powder). The Cl- ion reacts with the coating and gives MgOHCl, which is more soluble in water. This opens the surface to the water, and the strongly exothermic Mg/H2O reaction can occur.

Compounds of magnesium have long been known. Examples are Epsom salts (MgSO4 7H2O), magnesia (MgO), and magnesia alba (MgCO3). The element is also present in such substances as asbestos, meerschaum, serpentine and talc. The metal is now principally obtained in the U.S. by electrolysis of fused magnesium chloride derived from brines,

MgCl2 + electricity Mg(s) + Cl2(g)

from wells and from sea water.

Magnesium readily ignites upon heating in air and is used in flash photography, flares, and pyrotechnics, including incendiary bombs.

Magnesium is one third lighter than aluminum, and Mg-containing alloys are essential for airplane and missile construction. The metal improves the mechanical, fabrication, and welding characteristics of aluminum, when used as an alloying agent. Magnesium is used in producing nodular graphite in cast iron. It is also used as a reducing agent in the production of pure uranium and other metals from their salts. The oxide, chloride, sulfate, and citrate are used in medicine. "Dead-burned" magnesite is employed for refractory purposes, such as bricks and liners in furnaces and converters. Organic magnesium compounds (Grignard reagents) are important.

The green plant pigment, chlorophyll, is a complex of magnesium ion with an organic molecule. This compound is an important catalyst for photosynthesis in plants.