ALUMINUM

Atomic Number: 13
Atomic Weight: 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 metallic element in the Earth's crust; however it is very reactive and is never found in elemetnal form. Aluminum compounds are widely distributed; a few minerals include beryl, cryolite, garnets, and spinels.

Aluminum is widely used in modern society. Just a few uses include the construction industry (door and window frames, siding), the manufacture of aircraft (because it it lightweight), automobile parts, electrical cable, and in food packaging (aluminum foil, aluminum cans).

Aluminum Electrical Cable Electrical cable, particularly larger sizes such as the service entrance cable shown here, is often made of aluminum. Aluminum is not as good a conductor as copper and requires larger cable to carry the same current. However, since aluminum so much lighter that copper, the larger-size aluminum wire is still lighter.

Aluminum is obtained from bauxite. Bauxite is essentially what is left of aluminum-contianing clay minerals ather the more soluble components have been leached away by weathering. Before it can be used bauxite ore must first be purified, a process known as the Bayer process. The bauxite is first treated with hydroxide; the aluminum forms a soluble complex ion, allowing it to be separated from the other compoents of the ore. This is acidified slightly, producing hydrated aluminum oxide, which is heated to drive off the water of hydration. Next, the aluminum metal is obained electrolysis, a process known as the Hall process. The aluminum could be obtained by eelctrolysis of molten aluminum oxide. However, the melting point of aluminum oxide is prohibitively high (over 2000 degrees Celsius). Instead, it is dissolved in cryolite, Na₃AlF₃, the mixture having a much lower melting point than pure aluminum oxide. The cells are operating usign carbon (graphite) electrods, producing aluminum metal at the cathode and carbon dioxide at the anode (from oxidation of the electrode material). Since aluminum is a +3 ion, three moles of electrons must be passed for evert mole of alumimun produced. The producion of aluminum is a major conusmer of electricity.

The only oxide of aluminum is alumina, Al₂O₃. The crystalline form of aluminum oxide is known as corumdum. Structually, it is a hexagonal close-packed array of oxide ions with the aluminum ions in two-thirds of the octahedral holes. It is a very a hard material, and and can scratch most other minerals. It is widely used as an abrasive; sandpaper often contains aluminum oxide.

Several gemstones are based on corundum; examples are ruby and sapphire. Corundum itself is colorless; the colors of these gemstones are due to the traces of transition metals. The red color of rubies is to to the presence of chromium(III) in the lattice. The blue color of sapphire is due to the presence of iron and titanium; one Fe²⁺ and one Ti⁺⁴ can replace two Al³⁺ ions, therefore maining charge balance. Although sapphires are most known for their blue color, they can also be several other colors, including green, yellow, and pink.

Blue Sapphires Sapphires are composed of aluminum oxde. Pure aluminum oxide is colorless; the blue color of sapphire is die to traces of iron and titanium ions. Photo courtesy of R. Weller, Cochise College

Aluminum is actually a vcry reactive metal. However, it quickly devlops a thin oxide coating that protects the surface of the metal and prevents further reaction. The coating is thin enough as not to dull the surface of the metal and is therefore not noticeable. In fact, anodized aluminum is simply aluminum to which has a very thick oxide coating has been electrolytically applied.

Nevertheless aluminum does undergo a number of reactions. Like most of the reactive metals, aluminum will react with hydrochloric acid to yield hydrogen gas.

2Al + 6HCl = 2Al³⁺ + 6Cl^- + 3H₂

Aluminum reacts with hydroxide solutions to liberate hydrogen gas. At first this reaction might seem strange. Several other matals react wtih water to produce hydrogen gas, but not hydroxide. In this case the hydroxide interferes with the formation of the aluminum oxide layer as shown below..

Al₂O₃ + 2OH^- +3H₂O = 2[Al(OH)₄]^-

Next, the aluminum reacts to displace hydrogen from water like the other reactoive metals; this is the reaction that liberates hydrogen gas.

2Al + 6H₂O = 2Al³⁺ + 3H₂ + 6OH^-

This reaction could be written several different ways. Aluminum hydroxide is insoluble, so this might be written Al(OH)₃. Also, in an excess of hydroxide, this could react with additional hydroxide to form [Al(OH)₄]^-.

Many commercial drain cleaners consist of bits of aluminum mixed with lye. The strongly basic solution helps to dissolve grease, and the bubbling produced by the hydrogen gas helps to break up the clog.

Aluminum also reacts with the halogens. For example, aluminum undergoes a spectacular reaction wtih elemental bromine.

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.

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.

An interesting article about aluminum