Encyclopedia
Aluminium or
aluminum is the chemical element in the periodic table that has the symbol
Al and atomic number 13. It is a silvery and ductile member of the poor metal group of
chemical elements. Aluminium is found primarily as the ore
bauxite and is remarkable for its resistance to corrosion and its light weight. Aluminium is used in many industries to make millions of different 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 in which light weight, durability, and strength are needed.
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 is nontoxic , nonmagnetic, and nonsparking. It has a tensile strength of about 49 megapascals 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 and the 3000-10000 nm regions, while in the 400-700 nm visible range it is slightly outdone by
silver and in the 700-3000 by
silver,
gold, and
copper. It is the second-most malleable metal 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 . 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 of
visible light and a good reflector 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
- Packaging
- Water treatment
- Treatment against fish parasites such as Gyrodactylus salaris.
- Construction
- Consumer durable goods
- Electrical transmission lines
- Machinery
- MKM steel and Alnico magnets, although non-magnetic itself
- Super purity aluminium , 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 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 designed for stage combat
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 or by names indicating their main alloying constituents . 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. Aluminum 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 , 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.
Often, aluminium's sensitivity to heat must also be considered. Even a relatively routine workshop procedure involving heating is complicated by the fact that aluminium, as opposed to steels, 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 will accumulate internal stresses and strains under conditions of overheating; while not immediately obvious, the tendency of the metal to "creep" under sustained stresses results in delayed distortions, for instance the commonly observed warping or cracking of aluminium automobile cylinder heads after an engine is overheated, sometimes as long as years later, or the tendency of welded aluminium bicycle frames to gradually twist out of alignment from the stresses accumulated during the welding process. For this reason, many uses of aluminium in the aerospace industry avoid heat altogether by joining parts using
adhesives; this was also used for some of the early aluminium bicycle frames in the 1970s, with unfortunate results when the aluminium tubing corroded slightly, loosening the bond of the adhesive and leading to failure of the frame. Stresses from overheating aluminium can be relieved by heat-treating the parts in an oven and gradually cooling, in effect annealing the stresses; this can also result, however, in the part becoming distorted as a result of these stresses, so that such heat-treating of welded bicycle frames, for instance, results in a significant fraction becoming misaligned. If the misalignment is not too severe, once cooled they can be bent back into alignment with no negative consequences; of course, if the frame is properly designed for rigidity , this 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 at the time, aluminium was introduced for household electrical wiring to a large degree in the United States in the 1960s. Unfortunately, many of the wiring fixtures at the time were not designed to accept aluminium wire. More specifically:
- 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 , again producing a degree of looseness in an initially tight connection.
- Galvanic corrosion from the dissimilar metals increases the electrical resistance of the connection.
In combination, these properties caused connections between electrical fixtures and aluminium wiring to overheat which resulted in several fires. As a result, aluminium household wiring has become unpopular, and in many jurisdictions it is not permitted in very small sizes in new construction. However, aluminium wiring can be safely used with fixtures whose connections are designed to avoid loosening and overheating. Older fixtures of this type are marked "Al/Cu", and newer ones are marked "CO/ALR". Otherwise, aluminium wiring can be terminated by crimping it to a short "
pigtail" of copper wire, which can be treated as any other copper wire. A properly done crimp, requiring high pressure produced by the proper tool, is tight enough not only to eliminate any thermal expansion of the aluminium, but also to exclude any atmospheric
oxygen and thus prevent corrosion between dissimilar metals. New alloys are used for aluminium building wire today in combination with aluminium terminations. Connections made with these standard industry products are as safe and reliable as copper connections.
- See also: Aluminium wire
History
The
Chinese were using aluminum 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
aluminum .
Friedrich Wöhler is generally credited with isolating aluminium 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 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 Aluminum 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 , it is very rare in its free form, occurring in oxygen deficient environments such as volcanic mud, and 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.
Aluminium was, when it was first discovered, extremely difficult to separate from its ore. Aluminium is among the most difficult metals on Earth to refine, despite the fact that it is one of the planet's most common. The reason is that aluminium is oxidised very rapidly and that its oxide is an extremely stable compound that, unlike rust on iron, does not flake off. Ironically, the very reason for which aluminium is used in many applications is why it is so hard to produce.
Recovery of this metal from scrap has become an important component of the aluminium industry. Recycling involves simply melting the metal, which is far less expensive than creating it from ore. Refining aluminium requires enormous amounts of
electricity; recycling it requires only 5% of the energy used to produce it. A common practice since the early
1900s, aluminium recycling is not new. It was, however, a low-profile activity until the late 1960s when the exploding popularity of aluminium
beverage cans finally placed recycling into the public consciousness. Other sources for recycled aluminium include automobile parts, windows and doors, appliances, containers and other products.
Aluminium is a reactive metal and it is hard to extract it from its ore, aluminium oxide . Direct reduction, with
carbon for example, is not economically viable since aluminium oxide has a melting point of about 2000 °C. Therefore, it is extracted by
electrolysis — the aluminium oxide is dissolved in molten
cryolite and then reduced to the pure metal. By this process, the actual operational temperature of the reduction cells is around 950 to 980 °C. Cryolite was originally found as a mineral on Greenland, but has been replaced by a synthetic cryolite. Cryolite is a mixture of aluminium,
sodium, and calcium fluorides: . The aluminium oxide is obtained by refining
bauxite, which is red since it contains 30 to 40% iron oxide. This is done using the so-called 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 where a negative terminal is applied to is
- Al3+ + 3 e- ? Al
Here the aluminium ion is being
reduced . The aluminium metal then sinks to the bottom and is tapped off.
At the positive electrode oxygen gas is formed:
- 2 O2- ? O2 + 4 e-
This carbon
anode is then oxidised by the oxygen. 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 during the operation, since there is no oxygen present at the cathode. The carbon cathode is protected by the liquid aluminium inside the cells. However, cathodes do erode, mainly due to electrochemical processes. After 5 to 10 years, depending on the current used in the electrolysis, a cell has to be reconstructed completely, because the cathodes are completely worn.
A summary of the above process broken down into its simplest form. Aluminium is produced through a process known as electrolysis or electrometallurgy. Bauxite is mined and then refined into aluminium oxide. Aluminium oxide is combined with electricity in a cell containing a molten electrolyte. A current of electricity is added separating aluminium oxide into molten aluminium metal and Carbon dioxide. The molten aluminium collects at the bottom of the cell and is then formed into a crucible and cast into ingots.
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. . The most modern smelters reach approximately 12.8 kW·h/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 aluminium smelter. Smelters tend to be located where electric power is plentiful and inexpensive, such as
South Africa, the
South Island of
New Zealand,
Australia, the
People's Republic of China,
Middle East,
Russia,
Iceland and
Quebec in
Canada.
In 2004, the People's Republic of China was the top world producer of aluminium.
Suriname depends on aluminium exports for 70% of its export earnings.
See also .Isotopes
Aluminium has nine isotopes, whose mass numbers range from 23 to 30. Only
27Al and
26Al 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 10
5 to 10
6 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 had been made to behave like an iodine atom; and, 14 aluminium atoms behaved like an alkaline earth atom. The researchers also bound 12 iodine atoms to an Al
13 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 and A. Welford Castleman Jr .
Precautions
Aluminium is one of the few abundant elements that appears to have no beneficial function in living cells, but a few 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 . 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 Al
2O
3. 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
Word expert Michael Quinlon, who says he "writes about International English from a British viewpoint," notes that in 1807
Humphry Davy proposed the name
alumium for the metal Davy was trying to isolate electrolytically from the mineral
alumina. In 1812, Davy finally settled on
aluminum, which, as other sources note, matches its
Latin root. The same year, an anonymous contributor to the
Quarterly Review, a British political-literary journal, objected to
aluminum and proposed the name
aluminium:
Aluminium, for so we shall take the liberty of writing the word, in preference to aluminum, which has a less classical sound.
This had the advantage of conforming to the
-ium suffix precedent set by other newly discovered elements of the period:
potassium,
sodium,
magnesium, calcium, and strontium . Nevertheless,
-um spellings for elements were not unknown at the time.
Platinum, which had been known to Europeans since the 16th century, molybdenum, which was discovered in 1778, and tantalum, which was discovered in 1802, all have spellings ending in
-um.
The United States adopted the
-ium for most of the 19th century with
aluminium appearing in
Webster's Dictionary of 1828. However, in 1892
Charles Martin Hall used the
-um spelling in an advertising handbill for his new efficient electrolytic method for the production of aluminium, despite using the
-ium spelling in all of his patents filed between 1886 and 1903. It has consequently been suggested that the spelling on the flier was a simple spelling mistake rather than a deliberate choice to use the
-um spelling. Hall's domination of production of the metal ensured that the spelling
aluminum became the standard in North America, even though the
Webster Unabridged Dictionary of 1913 continued to use the
-ium version.
In 1926, the American Chemical Society officially decided to use
aluminum in its publications, and 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
aluminum predominates. The Canadian Oxford Dictionary prefers
aluminum.In other
English-speaking countries, the spellings
aluminium and
aluminum 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 adopted
aluminium as the standard international name for the element in 1990, but three years later recognized
aluminum 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
aluminum.
Chemistry
Oxidation state one
- AlH is produced when aluminium is heated at 1500°C in an atmosphere of hydrogen
| Triple point [i] || 13.8033 K, 7.042 kPa
...
.
- 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 bases, 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, 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, Al23, 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
|-
| align="center" colspan="2" | |-
...
. 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
...
, Al
2S
3, may be prepared by passing
hydrogen sulfide over aluminium powder. It is polymorphic.
- Aluminium iodide, 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 AlF5+2, AlF330, 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 concentrat
