The Chemistry of Magnesium

Physical Properties

Atomic Number:
12
Atomic Weight: 24.305
Electron Configuration: 1s22s22p63s2
Crystal Structure: Hexagonal Close-Packed
Density: 1.74 g/cm3
Melting Point: 649oC
First Ionization Energy: 737.1 kJ/mol
Second Ionization Energy: 1451 kJ/mol
Atomic Radius: 160 pm
Ionic Radius: 66 pm
Standard Reduction Potential: -2.36 V (Mg2+ + 2e-= Mg)

Occurrence and Isolation

Magnesium is the second lightest member of the alkaline earth metals. It is sometimes combined with aluminum to form a lightweight alloy. Magnesium does not occur in nature in elemental form but it is widely distributed in compound form. The most common magnesium minerals include magnesite (magensium carbonate) and dolomite (calcium magnesium carbonate). Additionally, it is the third most abundant element in seawater, behind sodium and chloride. Magnesium is obtained primarily from seawater. This involes precipitation of the magensium as magnesium hydroxide, converting it to the the chloride by reaction with hydrochloric acid, evaporating the resulting solution, and performing electrolysis on the molten salt.

Common Magnesium Compounds

Magnesium forms primarily ionic compounds. Some common magnesium compounds include milk of magnesia, which is a suspension of magnesium hydroxide. Whereas a solution of sodium or potassium hydroxide would butn the mouth and esophagus, magnesium hydroixde can be taken internally because is sparingly insoluble and therefore the pH of the resulting solution is relatively low. Another common household chemical that contains magneisum is Epsom salts, which is magnesium sulfate hepahydrate.

Magnesium can also form covalent bonds with carbon, an example of which is the Grignard reagent phenylmagnesium bromide. Chlorophyll, the compound responsible for the green color of leaves and for photosynthesis, is a coordination compound containing magnesium.

Reactions of Magnesium

Magnesium is one of the least reactive of the alkaline earth metals. At room tempertaure there is very little evidence of reaction with water, but if pgenanolphthalein (an acid-base indictaor) is added, the solutions does turn pink, indicating the presence of hydroxide ions (Figure 3).

Mg + 2H2O = Mg(OH)2 + H2

Magnesium ribbon burns in air with a brilliant white light to form magnesium oxide. In fact, old-fashioned flash bulbs contained magnesium wire in an atmosphere of pure oxygen.

2Mg + O2 = 2 MgO

Magensium oxide adopts the halite structure, with Mg2+ taking the place of Na+ and O2- taking the place of Cl-. It is considered to be a true ionic compound. Like most metal oxides, it is basic, reacting with water to produce magnesium hydroxide.

MgO + H2O = Mg(OH)2

Magnesium will also react with any of the halogens to form the corresponding magnesium halide, and will react with nitrogen to form magnesium nitride at high temperatures. When heated with sulfur, magnesium sulfide is formed. Magnesium reacts readily with acids and displaces the hydrogen from the acid as hydrogen gas.

Mg + 2HCl = MgCl2 + H2
Figure 1. Magnesium metal
Figure 2. The combustion of magnesium metal
Figure 3. The reaction of magnesium wtih water
Figure 4. The structure of magnesium oxide