Aluminium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements

13 magnesium ← aluminium → silicon B ↑ Al ↓ Ga Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	aluminium, Al, 13
Chemical series	poor metals
Group, Period, Block	13, 3, p
Appearance	silvery
Atomic mass	26.9815386 (8) g/mol
Electron configuration	[Ne] 3s2 3p1
Electrons per shell	2, 8, 3
Physical properties
Phase	solid
Density (near r.t.)	2.70 g·cm−3
Liquid density at m.p.	2.375 g·cm−3
Melting point	933.47  K (660.32 ° C, 1220.58 ° F)
Boiling point	2792 K (2519 ° C, 4566 ° F)
Heat of fusion	10.71 kJ·mol−1
Heat of vaporization	294.0 kJ·mol−1
Heat capacity	(25 °C) 24.200 J·mol−1·K−1
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 1482 1632 1817 2054 2364 2790
Atomic properties
Crystal structure	face centered cubic, 0.4032 nm
Oxidation states	3 ( amphoteric oxide)
Electronegativity	1.61 (Pauling scale)
Ionization energies ( more)	1st: 577.5 kJ·mol−1
	2nd: 1816.7 kJ·mol−1
	3rd: 2744.8 kJ·mol−1
Atomic radius	125 pm
Atomic radius (calc.)	118 pm
Covalent radius	118 pm
Miscellaneous
Magnetic ordering	paramagnetic
Electrical resistivity	(20 °C) 26.50 nΩ·m
Thermal conductivity	(300 K) 237 W·m−1·K−1
Thermal expansion	(25 °C) 23.1 µm·m−1·K−1
Speed of sound (thin rod)	( r.t.) (rolled) 5000   m·s−1
Young's modulus	70 GPa
Shear modulus	26 GPa
Bulk modulus	76 GPa
Poisson ratio	0.35
Mohs hardness	2.75
Vickers hardness	167 MPa
Brinell hardness	245 MPa
CAS registry number	7429-90-5
Selected isotopes
Main article: Isotopes of aluminium iso NA half-life DM DE ( MeV) DP 26Al syn 7.17×105 y β+ 1.17 26Mg ε - 26Mg γ 1.8086 - 27Al 100% Al is stable with 14 neutrons
References

Aluminium ( IPA: /ˌaljʊˈmɪniəm, -əˈmɪniəm/) or aluminium ( IPA: /əˈluːmɪnəm/, see the "Spelling" section below) is a silvery and ductile member of the poor metal group of chemical elements. In the periodic table it has the symbol Al and atomic number 13.

Aluminium is found primarily in bauxite ore and is remarkable for its resistance to corrosion (due to the phenomenon of passivation) and its light weight. The metal is used in many industries to manufacture a large variety of products and is very important to the world economy. Structural components made from aluminium and its alloys are vital to the aerospace industry and very important in other areas of transportation and building.

Properties

Aluminium is a soft, lightweight metal with normally a dull silvery appearance caused by a thin layer of oxidation that forms quickly when the metal is exposed to air. Aluminium oxide has a higher melting point than pure aluminium. Aluminium is nontoxic (as the metal), nonmagnetic, and nonsparking. It has a tensile strength of about 49 megapascals (MPa) in a pure state and 400 MPa as an alloy. Aluminium is about one-third as dense as steel or copper; it is malleable, ductile, and easily machined and cast. It has excellent corrosion resistance and durability because of the protective oxide layer. Aluminium mirror finish has the highest reflectance of any metal in the 200-400 nm (UV) and the 3000-10000 nm (far IR) regions, while in the 400-700 nm visible range it is slightly outdone by silver and in the 700-3000 (near IR) by silver, gold, and copper. It is the second-most malleable metal (after gold) and the sixth-most ductile. Aluminium is a good heat conductor.

Applications

Whether measured in terms of quantity or value, the use of aluminium exceeds that of any other metal except iron, and it is important in virtually all segments of the world economy.

Pure aluminium has a low tensile strength, but readily forms alloys with many elements such as copper, zinc, magnesium, manganese and silicon (e.g., duralumin). Today almost all materials that claim to be aluminium are actually an alloy thereof. Pure aluminium is encountered only when corrosion resistance is more important than strength or hardness.

When combined with thermo-mechanical processing aluminium alloys display a marked improvement in mechanical properties. Aluminium alloys form vital components of aircraft and rockets as a result of their high strength to weight ratio.

Aluminium is an excellent reflector (approximately 99%) of visible light and a good reflector (approximately 95%) of infrared. A thin layer of aluminium can be deposited onto a flat surface by chemical vapor deposition or chemical means to form optical coatings and mirrors. These coatings form an even thinner layer of protective aluminium oxide that does not deteriorate as silver coatings do. Nearly all modern mirrors are made using a thin coating of aluminium on the back surface of a sheet of float glass. Telescope mirrors are also made with aluminium, but are front coated to avoid internal reflections, refraction, and transparency losses. These first surface mirrors are more susceptible to damage than household back surface mirrors.

Some of the many uses for aluminium are in:

• Transportation (automobiles, aircraft, trucks, railroad cars, marine vessels, bicycles etc.)
• Packaging ( cans, foil, etc.)
• Water treatment
• Treatment against fish parasites such as Gyrodactylus salaris.
• Construction ( windows, doors, siding, building wire, etc.)
• Consumer durable goods (appliances, cooking utensils, etc.)
• Electrical transmission lines (aluminium components and wires are less dense than those made of copper and are lower in price, but also present higher electrical resistance. Many localities prohibit the use of aluminium in residential wiring practices because of its higher resistance and thermal expansion value.)
• Machinery
• MKM steel and Alnico magnets, although non-magnetic itself
• Super purity aluminium (SPA, 99.980% to 99.999% Al), used in electronics and CDs.
• Powdered aluminium, a commonly used silvering agent in paint. Aluminium flakes may also be included in undercoat paints, particularly wood primer — on drying, the flakes overlap to produce a water resistant barrier.
• Anodised aluminium is more stable to further oxidation, and is used in various fields of construction, as well as heat sinking.
• Most electronic appliances that require cooling of their internal devices (like transistors, CPUs - semiconductors in general) have heat sinks that are made of aluminium due to its ease of manufacture and good heat conductivity. Copper heat sinks are smaller although more expensive and harder to manufacture.
• It is used in the blades of weapons (such as swords) designed for stage combat

• Aluminium oxide, alumina, is found naturally as corundum ( rubies and sapphires), emery, and is used in glass making. Synthetic ruby and sapphire are used in lasers for the production of coherent light.

• Aluminium oxidises very energetically and as a result has found use in solid rocket fuels, thermite, and other pyrotechnic compositions.

Aluminium is also a superconductor at low temperatures, with a superconducting critical temperature of 1.2 kelvins.

Engineering use

Aluminium alloys with a wide range of properties are used in engineering structures. Alloy systems are classified by a number system ( ANSI) or by names indicating their main alloying constituents ( DIN and ISO). Selecting the right alloy for a given application entails considerations of strength, ductility, formability, weldability and corrosion resistance to name a few. A brief historical overview of alloys and manufacturing technologies is given in Ref. Aluminium is used extensively in modern aircraft due to its light weight.

Improper use of aluminium can result in problems, particularly in contrast to iron or steel, which appear "better behaved" to the intuitive designer, mechanic, or technician. The reduction by two thirds of the weight of an aluminium part compared to a similarly sized iron or steel part seems enormously attractive, but it should be noted that it is accompanied by a reduction by two thirds in the stiffness of the part. Therefore, although direct replacement of an iron or steel part with a duplicate made from aluminium may still give acceptable strength to withstand peak loads, the increased flexibility will cause three times more deflection in the part.

Where failure is not an issue but excessive flex is undesirable due to requirements for precision of location or efficiency of transmission of power, simple replacement of steel tubing with similarly sized aluminium tubing will result in a degree of flex which is undesirable; for instance, the increased flex under operating loads caused by replacing steel bicycle frame tubing with aluminium tubing of identical dimensions will cause misalignment of the power-train as well as absorbing the operating force. To increase the rigidity by increasing the thickness of the walls of the tubing increases the weight proportionately, so that the advantages of lighter weight are lost as the rigidity is restored.

Aluminium can best be used by redesigning the part to suit its characteristics; for instance making a bicycle of aluminium tubing which has an oversize diameter rather than thicker walls. In this way, rigidity can be restored or even enhanced without increasing weight. The limit to this process is the increase in susceptibility to what is termed " buckling" failure, where the deviation of the force from any direction other than directly along the axis of the tubing causes folding of the walls of the tubing.

The latest models of the Corvette automobile, among others, are a good example of redesigning parts to make best use of aluminium's advantages. The aluminium chassis members and suspension parts of these cars have large overall dimensions for stiffness but are lightened by reducing cross-sectional area and removing unneeded metal; as a result, they are not only equally or more durable and stiff as the usual steel parts, but they possess an airy gracefulness which most people find attractive. Similarly, aluminium bicycle frames can be optimally designed so as to provide rigidity where required, yet have flexibility in terms of absorbing the shock of bumps from the road and not transmitting them to the rider.

The strength and durability of aluminium varies widely, not only as a result of the components of the specific alloy, but also as a result of the particular manufacturing process; for this reason, it has from time to time gained a bad reputation. For instance, a high frequency of failure in many early aluminium bicycle frames in the 1970s resulted in just such a poor reputation; with a moment's reflection, however, the widespread use of aluminium components in the aerospace and automotive high performance industries, where huge stresses are undergone with vanishingly small failure rates, proves that properly built aluminium bicycle components should not be unusually unreliable, and this has subsequently proved to be the case.

Similarly, use of aluminium in automotive applications, particularly in engine parts which must survive in difficult conditions, has benefited from development over time. An Audi engineer commented about the V12 engine, producing over 500 horsepower (370 kW), of an Auto Union race car of the 1930s which was recently restored by the Audi factory, that the aluminium alloy of which the engine was constructed would today be used only for lawn furniture and the like. Even the aluminium cylinder heads and crankcase of the Corvair, built as recently as the 1960s, earned a reputation for failure and stripping of threads in holes, even as large as spark plug holes, which is not seen in current aluminium cylinder heads.

Heat sensitivity

Often, the metal's sensitivity to heat must also be considered. Even a relatively routine workshop procedure involving heating is complicated by the fact that aluminium, unlike steel, will melt without first turning red. Forming operations where a blow torch is used therefore requires some expertise since no visual signs reveal how close the material is to melting.

Aluminium also is subject to internal stresses and strains when it is overheated; the tendency of the metal to creep under these stresses tends to result in delayed distortions. For instance, the warping or cracking of overheated aluminium automobile cylinder heads is commonly observed, sometimes years later, as is the tendency of welded aluminium bicycle frames to gradually twist out of alignment from the stresses of the welding process. Thus, aerospace uses of aluminium avoid heat altogether by joining parts with adhesives or mechanical fasteners. These adhesive junctures were used for some bicycle frames in the 1970s — with unfortunate results when the aluminium tubing corroded slightly, loosening the adhesive and collapsing the frame.

Stresses in overheated aluminium can be relieved by heat-treating the parts in an oven and gradually cooling it — in effect annealing the stresses. Yet these parts can still become distorted, so that heat-treating of welded bicycle frames, for instance, can result in a significant fraction becoming misaligned. If the misalignment is not too severe, the cooled parts can be bent into alignment; of course, if the frame is properly designed for rigidity (see above), that bending will require enormous force.

Aluminium's intolerance to high temperatures has not precluded its use in rocketry; even for use for constructing combustion chambers where gases can reach 3500K. The Agena upper stage engine used a regeneratively cooled aluminium design for some parts of the nozzle, including the thermally critical throat region; in fact the extremely high thermal conductivity of aluminium prevented the throat from reaching the melting point even under massive heat flux, and good reliability and light weight resulted.

Household wiring

Because of its high conductivity and relatively low price compared to copper in the 1960s, aluminium was introduced at that time for household electrical wiring in the United States even though many fixtures had not been designed to accept aluminium wire. But the new use brought some problems:

• The greater coefficient of thermal expansion of aluminium causes the wire to expand and contract relative to the dissimilar metal screw connection, eventually loosening the connection.

• Pure aluminium has a tendency to "creep" under steady sustained pressure (to a greater degree as the temperature rises), again loosening the connection.

• Galvanic corrosion from the dissimilar metals increases the electrical resistance of the connection.

All of this resulted in overheated connections, and fires broke out. Builders then became wary of using the wire, and many jurisdictions outlawed its use in very small sizes in new construction. Yet newer fixtures eventually were introduced with connections designed to avoid loosening and overheating. At first they were marked "Al/Cu", but they now bear a "CO/ALR" coding. Another way to forestall the heating problem is to crimp the aluminium wire to a short " pigtail" of copper wire. A properly done high-pressure crimp by the proper tool is tight enough to eliminate any thermal expansion of the aluminium and to exclude any atmospheric oxygen, thus preventing corrosion between the dissimilar metals. Today, new alloys are used for aluminium wiring in combination with aluminium terminations. Connections made with these products are as safe as those made with copper.

See also: Aluminium wire

History

The Chinese were using aluminium to make things as early as 300 AD. The ancient Greeks and Romans used aluminium salts as dyeing mordants and as astringents for dressing wounds; alum is still used as a styptic. In 1761 Guyton de Morveau suggested calling the base alum alumine. In 1808, Humphry Davy identified the existence of a metal base of alum, which he at first named alumium and later aluminium (see Spelling section, below).

Friedrich Wöhler is generally credited with isolating aluminium (Latin alumen, alum) in 1827 by mixing anhydrous aluminium chloride with potassium. The metal, however, had indeed been produced for the first time two years earlier — but in an impure form — by the Danish physicist and chemist Hans Christian Ørsted. Therefore, Ørsted can also be listed as the discoverer of the metal. Further, Pierre Berthier discovered aluminium in bauxite ore and successfully extracted it. The Frenchman Henri Saint-Claire Deville improved Wöhler's method in 1846 and described his improvements in a book in 1859, chief among these being the substitution of sodium for the considerably more expensive potassium.

Aluminium was selected as the material to be used for the apex of the Washington Monument, at a time when one ounce cost twice the daily wages of a common worker in the project; aluminium was a semiprecious metal at that time.

The American Charles Martin Hall of Oberlin, Ohio applied for a patent (400655) in 1886 for an electrolytic process to extract aluminium using the same technique that was independently being developed by the Frenchman Paul Héroult in Europe. The invention of the Hall-Héroult process in 1886 made extracting aluminium from minerals cheaper, and is now the principal method in common use throughout the world. The Hall-Heroult process cannot produce Super Purity Aluminium directly. Upon approval of his patent in 1889, Hall, with the financial backing of Alfred E. Hunt of Pittsburgh, PA, started the Pittsburgh Reduction Company, renamed to Aluminium Company of America in 1907, later shortened to Alcoa.

Germany became the world leader in aluminium production soon after Adolf Hitler's rise to power. By 1942, however, new hydroelectric power projects such as the Grand Coulee Dam gave the United States something Nazi Germany could not hope to compete with, namely the capability of producing enough aluminium to manufacture sixty thousand warplanes in four years.

Aluminium separation

Although aluminium is the most abundant metallic element in Earth's crust (believed to be 7.5% to 8.1%), it is very rare in its free form, occurring in oxygen-deficient environments such as volcanic mud, and it was once considered a precious metal more valuable than gold. Napoleon III of France had a set of aluminium plates reserved for his finest guests. Others had to make do with gold ones. Aluminium has been produced in commercial quantities for just over 100 years. ^{[ citations needed]}

Recovery of the metal via recycling has become an important facet of the aluminium industry. Recycling involves melting the scrap, a process that uses only five percent of the energy needed to produce aluminium from ore. Recycling was a low-profile activity until the late 1960s, when the growing use of aluminium beverage cans brought it to the public consciousness.

Aluminium is a reactive metal that is difficult to extract from ore, aluminium oxide (Al₂O₃). Direct reduction — with carbon, for example — is not economically viable since aluminium oxide has a melting point of about 2,000 °C. Therefore, it is extracted by electrolysis; that is, the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. By this process, the operational temperature of the reduction cells is around 950 to 980 °C. Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance. Cryolite is a mixture of aluminium, sodium, and calcium fluorides: (Na₃AlF₆). The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process. (Previously, the Deville process was the predominant refining technology.)

The electrolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminium oxide are carbon. Once the ore is in the molten state, its ions are free to move around. The reaction at the cathode — the negative terminal — is

Al³⁺ + 3 e^- → Al

Here the aluminium ion is being reduced (electrons are added). The aluminium metal then sinks to the bottom and is tapped off.

At the positive electrode ( anode), oxygen is formed:

2 O^2- → O₂ + 4 e^-

This carbon anode is then oxidised by the oxygen, releasing carbon dioxide. The anodes in a reduction must therefore be replaced regularly, since they are consumed in the process:

O₂ + C → CO₂

Unlike the anodes, the cathodes are not oxidised because there is no oxygen present at the cathode. The carbon cathode is protected by the liquid aluminium inside the cells. Nevertheless, cathodes do erode, mainly due to electrochemical processes. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.

Aluminium electrolysis with the Hall-Héroult process consumes a lot of energy, but alternative processes were always found to be less viable economically and/or ecologically. The world-wide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminium produced from alumina. (52 to 56 MJ/kg). The most modern smelters reach approximately 12.8 kW·h/kg (46.1 MJ/kg). Reduction line current for older technologies are typically 100 to 200 kA. State-of-the-art smelters operate with about 350 kA. Trials have been reported with 500 kA cells.

Electric power represents about 20% to 40% of the cost of producing aluminium, depending on the location of the smelter. Smelters tend to be situated where electric power is both plentiful and inexpensive, such as South Africa, the South Island of New Zealand, Australia, the People's Republic of China, the Middle East, Russia, Quebec and British Columbia in Canada, and Iceland. (Nearly all the power for aluminium smelting in Iceland comes from the heat vents upon which the island sits. )

In 2004, the People's Republic of China was the top world producer of aluminium.

Isotopes

Aluminium has nine isotopes, whose mass numbers range from 23 to 30. Only ²⁷Al ( stable isotope) and ²⁶Al ( radioactive isotope, t_1/2 = 7.2 × 10⁵ y) occur naturally, however ²⁷Al has a natural abundance of 100%. ²⁶Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of ²⁶Al to ¹⁰Be has been used to study the role of transport, deposition, sediment storage, burial times, and erosion on 10⁵ to 10⁶ year time scales.

Cosmogenic ²⁶Al was first applied in studies of the Moon and meteorites. Meteorite fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial ²⁶Al production. After falling to Earth, atmospheric shielding protects the meteorite fragments from further ²⁶Al production, and its decay can then be used to determine the meteorite's terrestrial age. Meteorite research has also shown that ²⁶Al was relatively abundant at the time of formation of our planetary system. Possibly, the energy released by the decay of ²⁶Al was responsible for the remelting and differentiation of some asteroids after their formation 4.6 billion years ago.

Clusters

In the journal Science of 14 January 2005 it was reported that clusters of 13 aluminium atoms (Al₁₃) had been made to behave like an iodine atom; and, 14 aluminium atoms (Al₁₄) behaved like an alkaline earth atom. The researchers also bound 12 iodine atoms to an Al₁₃ cluster to form a new class of polyiodide. This discovery is reported to give rise to the possibility of a new characterisation of the periodic table: superatoms. The research teams were led by Shiv N. Khanna ( Virginia Commonwealth University) and A. Welford Castleman Jr ( Penn State University).

Precautions

Aluminium is a neurotoxin that alters the function of the blood-brain barrier. It is one of the few abundant elements that appears to have no beneficial function to living cells. A small percent of people are allergic to it — they experience contact dermatitis from any form of it: an itchy rash from using styptic or antiperspirant products, digestive disorders and inability to absorb nutrients from eating food cooked in aluminium pans, and vomiting and other symptoms of poisoning from ingesting such products as Rolaids, Amphojel, and Maalox ( antacids). In other people, aluminium is not considered as toxic as heavy metals, but there is evidence of some toxicity if it is consumed in excessive amounts. The use of aluminium cookware, popular because of its corrosion resistance and good heat conduction, has not been shown to lead to aluminium toxicity in general. Excessive consumption of antacids containing aluminium compounds and excessive use of aluminium-containing antiperspirants are more likely causes of toxicity. In research published in the Journal of Applied Toxicology, Dr. Philippa D. Darby of the University of Reading has shown that aluminium salts increase estrogen-related gene expression in human breast cancer cells grown in the laboratory. These salts' estrogen-like effects have lead to their classification as a metalloestrogen.

It has been suggested that aluminium is a cause of Alzheimer's disease, as some brain plaques have been found to contain the metal. Research in this area has been inconclusive; aluminium accumulation may be a consequence of the Alzheimer's damage, not the cause. In any event, if there is any toxicity of aluminium it must be via a very specific mechanism, since total human exposure to the element in the form of naturally occurring clay in soil and dust is enormously large over a lifetime.^,

Mercury applied to the surface of an aluminium alloy can damage the protective oxide surface film. This may cause further corrosion and weakening of the structure. For this reason, mercury thermometers are not allowed on many airliners, as aluminium is used in many aircraft structures.

Powdered aluminium can react with Fe₂O₃ to form Fe and Al₂O₃. This mixture is known as thermite, which burns with a high energy output. Thermite can be produced inadvertently during grinding operations, but the high ignition temperature makes incidents unlikely in most workshop environments.

Spelling

Etymology/nomenclature history

The earliest citation given in the Oxford English Dictionary for any word used as a name for this element is alumium, which Humphry Davy employed in 1808 for the metal he was trying to isolate electrolytically from the mineral alumina. The citation is from his journal Philosophical Transactions: "Had I been so fortunate as..to have procured the metallic substances I was in search of, I should have proposed for them the names of silicium, alumium, zirconium, and glucium."

By 1812, Davy had settled on aluminium, which, as other sources note, matches its Latin root. He wrote in the journal Chemical Philosophy: "As yet Aluminium has not been obtained in a perfectly free state." But the same year, an anonymous contributor to the Quarterly Review, a British political-literary journal, objected to aluminium and proposed the name aluminium, "for so we shall take the liberty of writing the word, in preference to aluminium, which has a less classical sound."

The -ium suffix had the advantage of conforming to the precedent set in other newly discovered elements of the period: potassium, sodium, magnesium, calcium, and strontium (all of which Davy had isolated himself). Nevertheless, -um spellings for elements were not unknown at the time, as for example platinum, known to Europeans since the 16th century, molybdenum, discovered in 1778, and tantalum, discovered in 1802.

Americans adopted -ium for most of the 19th century, with aluminium appearing in Webster's Dictionary of 1828. In 1892, however, Charles Martin Hall used the -um spelling in an advertising handbill for his new electrolytic method of producing the metal, despite his constant use of the -ium spelling in all the patents he filed between 1886 and 1903. It has consequently been suggested that the spelling on the flier was a simple spelling mistake. Hall's domination of production of the metal ensured that the spelling aluminium became the standard in North America; the Webster Unabridged Dictionary of 1913, though, continued to use the -ium version.

In 1926, the American Chemical Society officially decided to use aluminium in its publications; American dictionaries typically label the spelling aluminium as a British variant.

Present-day spelling

In the UK and other countries using British spelling, only aluminium is used. In the United States, the spelling aluminium is largely unknown, and the spelling aluminium predominates. The Canadian Oxford Dictionary prefers aluminium.

In other English-speaking countries, the spellings (and associated pronunciations) aluminium and aluminium are both in common use in scientific and nonscientific contexts. The spelling in virtually all other languages is analogous to the -ium ending.

The International Union of Pure and Applied Chemistry (IUPAC) adopted aluminium as the standard international name for the element in 1990, but three years later recognized aluminium as an acceptable variant. Hence their periodic table includes both, but places aluminium first. IUPAC officially prefers the use of aluminium in its internal publications, although several IUPAC publications use the spelling aluminium.

Chemistry

Oxidation state one

• AlH is produced when aluminium is heated at 1500°C in an atmosphere of hydrogen.
• Al₂O is made by heating the normal oxide, Al₂O₃, with silicon at 1800°C in a vacuum.
• Al₂S can be made by heating Al₂S₃ with aluminium shavings at 1300°C in a vacuum. It quickly disproportionates to the starting materials. The selenide is made in a parallel manner.
• AlF, AlCl and AlBr exist in the gaseous phase when the tri-halide is heated with aluminium.

Oxidation state two

• Aluminium monoxide, AlO, is present when aluminium powder burns in oxygen.

Oxidation state three

• Fajans rules show that the simple trivalent cation Al³⁺ is not expected to be found in anhydrous salts or binary compounds such as Al₂O₃. The hydroxide is a weak base and aluminium salts of weak acids, such as carbonate, can't be prepared. The salts of strong acids, such as nitrate, are stable and soluble in water, forming hydrates with at least six molecules of water of crystallization.
• Aluminium hydride, (AlH₃)_n, can be produced from trimethylaluminium and an excess of hydrogen. It burns explosively in air. It can also be prepared by the action of aluminium chloride on lithium hydride in ether solution, but cannot be isolated free from the solvent.
• Aluminium carbide, Al₄C₃ is made by heating a mixture of the elements above 1000°C. The pale yellow crystals have a complex lattice structure, and react with water or dilute acids to give methane. The acetylide, Al₂(C₂)₃, is made by passing acetylene over heated aluminium.
• Aluminium nitride, AlN, can be made from the elements at 800°C. It is hydrolysed by water to form ammonia and aluminium hydroxide.
• Aluminium phosphide, AlP, is made similarly, and hydrolyses to give phosphine.
• Aluminium oxide, Al₂O₃, occurs naturally as corundum, and can be made by burning aluminium in oxygen or by heating the hydroxide, nitrate or sulfate. As a gemstone, its hardness is only exceeded by diamond, boron nitride, and carborundum. It is almost insoluble in water.
• Aluminium hydroxide may be prepared as a gelatinous precipitate by adding ammonia to an aqueous solution of an aluminium salt. It is amphoteric, being both a very weak acid, and forming aluminates with alkalis. It exists in various crystalline forms.
• Aluminium sulfide, Al₂S₃, may be prepared by passing hydrogen sulfide over aluminium powder. It is polymorphic.
• Aluminium iodide, (AlI₃)₂, is a dimer with applications in organic synthesis.
• Aluminium fluoride, AlF₃, is made by treating the hydroxide with HF, or can be made from the elements. It consists of a giant molecule which sublimes without melting at 1291°C. It is very inert. The other trihalides are dimeric, having a bridge-like structure.
• Aluminium fluoride/water complexes: When aluminium and fluoride are together in aqueous solution, they readily form complex ions such as AlF(H₂O)₅⁺², AlF₃(H₂O)₃⁰, AlF₆^-3. Of these, AlF₆^-3 is the most stable. This is explained by the fact that aluminium and fluoride, which are both very compact ions, fit together just right to form the octahedral aluminium hexafluoride complex. When aluminium and fluoride are together in water in a 1:6 molar ratio, AlF₆^-3 is the most common form, even in rather low concentrations.
• Organo-metallic compounds of empirical formula AlR₃ exist and, if not also giant molecules, are at least dimers or trimers. They have some uses in organic synthesis, for instance trimethylaluminium.
• Alumino-hydrides of the most electropositive elements are known, the most useful being lithium aluminium hydride, Li[AlH₄]. It decomposes into lithium hydride, aluminium and hydrogen when heated, and is hydrolysed by water. It has many uses in organic chemistry, particularly as a reducing agent. The aluminohalides have a similar structure.