Aluminum in Periodic Table
Aluminum (Al) or alumium, chemical element, silvery-white, lustrous, low melting, soft metal of Group 13 (IIIA) or boron family of the periodic table uses widely in our everyday life. Aluminum is the most abundant metallic element in the earth’s crust (8 percent by weight) and the third most abundant chemical element after oxygen and silicon. Aluminum differs from the first member of Group 13 (boron) due to its chemical properties, high reactivity, and cationic chemistry in an aqueous solution. It combines most of the nonmetallic elements like nitrogen, sulfur, halogens, and intermetallic aluminum compounds with a large number of metals.
The face-centered cubic crystal lattice (fcc), aluminum has the chemical symbol Al, atomic number 13, atomic weight 26.928, melting point 660 °C, boiling point 2467 °C, density 2.70 gm/cm3, and electron configuration 1s2 2s2 2p6 3s3. The name aluminum coming from the Latin word alumen, which describe the aluminum potassium sulfate, KAl(SO4)2, 12H2O. The common oxidation number or states of metal +1 +2 and +3. Due to decreases of lattice energy with increasing size of cation size, it should be likely to form an Al+3 ion. Due to large ionization energy not compensated by weak lattice energy, aluminum commonly attains +3 state by covalent bonding.
Abundance and Occurrence
The most abundant element, aluminum found in various silicates minerals like mica, feldspar, kaolinite, and clay. The only fluoride mineral is cryolite (Na3AlF6), found in Greenland. The most important minerals found in nature as oxide and anhydrous hydrated oxides. The oxide like Corundum (Al2O3) used in Gems (ruby, sapphire) due to its hardness. The chief commercial source of metallic aluminum is hydroxo oxide like Al2O3, xH2O, or bauxite. A workable bauxite sample contains 40 to 60 percent metal aluminum oxide (Al2O3), 12 to 30 percent water, 1 to 15 percent iron oxide (Fe2O3), and 3 to 4 percent titanium oxide (TiO2). Rich deposits of bauxite are found in Australia, Brazil, Guinea, Jamaica, France, United States, Russia, India, and several African counties.
History and Production
Alum was the compound of aluminum widely used in Rome for the production of different types of medicine and dyes in textile. British chemist Sir Humphry Davy failed to isolate the metal but suggested the name of the element aluminum or alumium. In 1825 Denmark physicist Hans Christian Orsted first isolate the metal by reducing aluminum chloride (AlCl3) with potassium amalgam. Commercial production of the element started in 1954 by electrolysis of NaAlCl4.
Aluminum can be extracted electrolytically from bauxite by electrochemical cells. Purified bauxite (melting point 2050 °C) is dissolved in cryolite and electrolyzed at 950 °C in carbon lined steel cells. A mixture of cryolite, CaF2, AlF3, Al2O3, Li2CO3 in electrolyte used because it has a lower melting point, permits larger current flow, reduces fluorine emission. The electrode reaction for the production of aluminum, cathode: 2Al+3 + 6e → 2Al and anode: 3O-2 → 3O + 6e. The molten aluminum sinks to the bottom of the cell and drained out. The surface of the electrolyte covered with a layer of coke, which oxidized evolving oxygen for the protection of valuable electrodes.
Properties and Chemistry
Aluminum is not attacked by water owing to the presence of a protective coating. If the film is removed by amalgamation, water form [Al(OH)3 + H2]. All the elements of Group 13 have different chemical properties and reactivity. For example, Al, Ga, In dissolve in aqueous HCl giving M+3 ions but Tl does not due to the instability of TlCl. Al scarcely attacked by dilute sulfuric acid but concentrated acid is reduced to sulfur dioxide. Dilute and concentrated nitric acid render aluminum passive by forming a protective oxide layer. Al dissolves in hot aqueous alkali (lithium, sodium, and potassium) to form [Al(OH)4]– and hydrogen.
The electronic structure of aluminum suggested that it forms mostly trivalent chemical compounds. The aluminum compounds in the +3 state are dominantly by covalent chemical bonding but in suitable conditions, it formed +2 and +1 compounds like AlCl, Al2O, AlO. In learning chemistry, the tripositive ion (Al+3) ions may be stabilized in aqueous solutions through strong hydration energy, usually, six water molecules held strongly by the Al+3 ion. The hydride of Al is a colorless involatile solid, thermally unstable above 150-200 °C. It is a strong reducing agent and adducts with strong Lewis bases. The tetrahydro complex of Al like LiAlH4 is a very important chemical compound widely uses in organic chemistry or reduction of hydrocarbon.
All the four trihalides of aluminum have the general formula AlX3, where X = fluorine, chlorine, bromine, and iodine. All the compounds may be prepared by direct combination of the metal with halides. The properties of the trihalides very widely. For example, fluoride is an ionic crystalline solid. Alumina is an important chemical compound of aluminum with two structural forms like α-Al2O3 and ϒ-Al2O3. The α-Al2O3 occurs in nature as corundum which is resistant to water and acids and uses in Gems. The γ-alumina is distorted, less dense, soft with high surface tension in the liquid form. These qualities of γ-alumina an excellent reagent for dehydration, decolorization, and absorption chronograph. β-alumina is now being developed as a ceramic ion conductor useful for high energy batteries. Large quantities of alumina used for the commercial production of aluminum.
Uses of Aluminum
In recent years aluminum acquired a high position among the metals due to its several exclusive properties, it is cheap in price, easily malleable, non-toxic, and fairly corrosion resistant in dry conditions, good thermal and electrical conductors. The corrosion-resistant properties due to the protective oxide layer in metal. Pure aluminum is quite soft and weak but the commercial form contains small amounts of silicon and carbon is hard and strong. The mechanical strength of aluminum significantly increases by alloying with metals like iron, copper, manganese, silicon, magnesium, or zinc. The aluminum alloys are uses in construction work, transportation, containers and packaging, electrical power lines, machinery, and the paint industry.
The metal and its alloys are now extensively used in aircraft, in rocket fuels, trucks, trailers, and car frames, in making windows, doors, building panels, in making machinery, utensils, and other household products including furniture also made of aluminum alloy.