Aluminum is the metallic chemical element of atomic number 13. Its symbol is Al; its atomic weight is 26.98; its specific gravity is 2.70; its melting point is 1,220.5°F (660°C); and its boiling point is 4,566.2°F (2,519°C).
Aluminum is a metal in group 13 of the periodic table. Its atoms consist of a single stable isotope, 27Al. Known as aluminium in other English-speaking countries, it was named after alum, one of its salts that has been known for thousands of years and was used by the Egyptians, Greeks, and Romans as a mordant—a chemical that helps dyes stick to cloth.
Aluminum is a light-weight, silvery metal, familiar to every household in the form of pots and pans, beverage cans, and aluminum foil. It is attractive, nontoxic, corrosion-resistant, non-magnetic, and easy to form, cast, or machine into a variety of shapes. Humans have devised a wide variety of uses for aluminum and produce 44 million tons of it each year.
Pure aluminum is a relatively soft metal, but when melted together with other elements such as copper, manganese, silicon, magnesium, and zinc, it forms alloys with a wide range of useful properties. Aluminum alloys are used in airplanes, highway signs, bridges, storage tanks, and buildings. Aluminum is being used more and more in automobiles because it is only one-third as heavy as steel and therefore decreases fuel consumption.
Where aluminum comes from
Aluminum is the third most abundant element in Earth's crust, after only oxygen and silicon, and it is the most abundant of all metals. It constitutes 8.1% of the crust by weight and 6.3% of all the atoms in the crust. Because it is a very active metal, aluminum is never found in the metallic form, but only in a wide variety of earthy and rocky minerals, including bauxite, feldspar, mica, granite, and clay. Kaolin is an especially fine, white aluminum-containing clay that is used in making porcelain.
Aluminum oxide, Al2O3, often called alumina, does not melt until over 3,632°F (2,000°C), and is used to line furnaces. Other forms of alumina are corundum and emery, which are very hard and are used as abrasives. Among the many other mineral forms in which aluminum is found are several semiprecious gemstones, including garnet (Fe3Al2Si3O12), beryl (Be3Al2Si6O18), and ruby and sapphire, which are Al2O3 containing impurities of chromium and iron, respectively. Artificially made rubies and sapphires are used in lasers.
How aluminum is obtained
As a highly reactive metal, aluminum is very difficult to separate from the other elements with which it is combined in its minerals and compounds. In spite of its great abundance on Earth, the metal itself remained unknown for centuries. In 1825, some impure aluminum metal was finally isolated by H. C. Oersted (1777–1851) by treating aluminum chloride, AlCl3, with potassium amalgam—potassium dissolved in mercury. Then in 1827, Friedrich Wöhler (1800–1882) obtained pure aluminum by the reaction of metallic potassium with AlCl3. He is generally given credit for the discovery of this element.
At this time, it was still very expensive to produce aluminum metal in any quantity, and for a long time it remained a rare and valuable metal. In 1852, aluminum was selling for about $545 a pound. The big breakthrough came in 1886, when Charles M. Hall, a 23-year-old student at Oberlin College in Ohio, and Paul L-T. Héroult, another college student in France, independently invented what is now known as the Hall or Hall-Héroult process. It consists of dissolving alumina in melted cryolite, Na3AlF6, a common aluminum-containing mineral, and then passing an electric current through the hot liquid. Molten aluminum metal collects at the cathode (negative electrode) in a process called electrolysis. Not long after the development of this process, the price of aluminum metal plummeted to around 30 cents a pound.
In the production of aluminum today by the Hall-Héroult process, the aluminum oxide is dissolved in a molten mixture of sodium, calcium, and aluminum fluorides, which melts at a lower temperature than cryolite. The aluminum oxide is in the form of bauxite, a white, brown, or red earthy clay; it was first found near Les Baux, France in 1821 and is now the main source of all aluminum. It is mined in various parts of Africa and in France, Surinam, Jamaica, and the United States. The world's supply of bauxite appears to be immense enough to last for centuries at the rate it is being mined today.
In spite of the fact that aluminum is very active chemically, it does not corrode in moist air the way iron does. Instead, it quickly forms a thin, hard coating of aluminum oxide. Unlike iron oxide or rust, which flakes off, the aluminum oxide sticks tightly to the metal and protects it from further oxidation. The oxide coating is so thin that it is transparent, so the aluminum retains its silvery metallic appearance. Sea water, however, will corrode aluminum unless it has been given an unusually thick coating of oxide by the anodizing process.
When aluminum is heated to high temperatures in a vacuum, it evaporates and condenses onto any nearby cool surface such as glass or plastic. When evaporated onto glass, it makes a very good mirror, and aluminum has largely replaced silver for that purpose because it does not tarnish and turn black, as silver does when exposed to impure air. Many food-packaging materials and shiny plastic novelties are made of paper or plastic with an evaporated coating of bright aluminum. The silver-colored helium balloons seen at birthday parties and other celebrations are made of a tough plastic called Mylar, covered with a thin, evaporated coating of aluminum metal.
Aluminum conducts electricity about 60% as well as copper, which is still very good among metals. Because it is also light in weight and highly ductile (can be drawn out into thin wires), it is used instead of copper in almost all of the high-voltage electric transmission lines in the United States.
Aluminum is used to make kitchen pots and pans because of its high heat conductivity. It is handy as an air- and water-tight food wrapping because it is very malleable; it can be pressed between steel rollers to make foil (a thin sheet) less than a thousandth of an inch thick. Claims are occasionally made that aluminum is toxic and that aluminum cookware is therefore dangerous, but no evidence for this belief has ever been found. Many widely used antacids in the drug store contain thousands of times more aluminum (in the form of aluminum hydroxide) than a person could ever get from eating food cooked in an aluminum pot. Aluminum is the only light element that has no known physiological function in the human body. In fact, aluminum salts do not have any known use by any organism.
Chemistry and compounds
Aluminum is an unusual metal in that it reacts not only with acids, but with bases as well. Like many active metals, aluminum dissolves in strong acids to evolve hydrogen gas and form salts. In fact, cooking even weakly acidic foods such as tomatoes in an aluminum pot can dissolve enough aluminum to give the dish a “metallic” taste. Aluminum also dissolves in strong bases such as sodium hydroxide, commonly known as lye. Most oven cleaners, which are designed to work on steel and porcelain, contain sodium or potassium hydroxide; the user must take care not to get it on any aluminum parts of the range because it will cause adverse effects. Some commercial drain cleaners contain lye mixed with shavings of aluminum metal; the aluminum dissolves in the sodium hydroxide solution to produce bubbles of hydrogen gas, which add a mechanical clog-breaking action to the grease-dissolving action of the lye.
Hydrated aluminum chloride, AlCl3•H2O, also called aluminum chlorohydrate, is used in antiperspirants because, like alum (potassium aluminum sulfate), it has an astringent effect—a tissue-shrinking effect—that closes up the sweat-gland ducts and stops perspiration.
Over one million tons of aluminum sulfate, Al2(SO4)3, are produced in the United States each year by dissolving aluminum oxide in sulfuric acid, H2SO4. It is used in water purification because when it reacts with lime (or any base), it forms a sticky precipitate of aluminum hydroxide that sweeps out tiny particles of impurities. Sodium aluminum sulfate, NaAl(SO4)2•12H2O, a kind of alum, is used in “double-acting” baking powders. It acts as an acid, reacting at oven temperatures with the sodium bicarbonate in the powder to form bubbles of carbon dioxide gas.