ALUMINUM

Physical Properties

Atomic Number: 13
Mass Number: 25.98
Electron Configuration: 1s²2s²2p⁶3s²3p¹
Crystal Structure: Face-Centered Cubic
Density: 2.70 g/cm³
Melting Point: 650.4^oC
First Ionization Energy: 577 kJ/mol
Second Ionization Energy: 1816 kJ/mol
Third Ionization Energy: 2744 kJ/mol
Atomic Radius: 143 pm
Ionic Radius: 54 pm
Standard Reduction Potential: -1.66 V (Al³⁺ + 3e^- = Al)

Aluminum is a shiny, hard metal. It is the most abundant metal in the Earth's crust. Aluminum is prepared from the mineral Bauxite, Al₂O₃-nH₂O (n = 1,2, or 3) by the Hall Process. The bauxite is purifed, converted to the hydroxide, and dissolved in molten cryolite, Na₃AlF₃. Pure aluminum metal is obtained by electrolsys of this solution.

The only oxide of aluminum is alumina, Al₂O₃. There are two forms of anhydrous aluminum oxide, the alpha and the gamma form. The alpha form occurs in nature as the mineral conundrum. Conundrum is hard and is used an abrasive. It is resistant to hdyration and to attack by acids. Ruby is essentially conundrum with traces of Cr³⁺ ion. The gamma form of Al₂O₃ readily absorbs water and dissolves in acids.

Aluminum forms only a +3 ion. In solution, the aluminum ion hydrates to form the ion Al(H₂O)₆³⁺. Solutions containing the aluminum ion are acidic due to the reaction of this ion with water to produce Al(H₂O)₅(OH)²⁺ and H⁺. Aluminum forms an insuluble hydroxide Al(OH)₃ in the presence of hydroxide ions and the soluble aluminate ion, Al(OH)₄^-, in the presence of excess hydroxide ions.

Aluminum is a very reactive metal. However, when exposed to air, it quickly develops an oxide coating which inhibits further reaction. In a process called anodizing, a thick oxide coating is applied to an aluminum surface.

An imporant compound of aluminum is alum, or potassium aluminum sulfate. Crystals of alum have a characteristic octahedral shape. Aluminum is also found in aluminosilicate minerals such as feldspars, micas, and clays.