The transition elements, or transition metals, are the 35 elements that make up Groups 3 through 12 of the periodic table, in the so-called d-block. The transition metals include some of the best-known names on the periodic table—iron, gold, silver, copper, mercury, zinc, nickel, chromium, titanium, and platinum.
The transition elements are all metals. They all have a shiny, lustrous, metallic appearance that may range from silver to gold to white to gray. Except for mercury, the only liquid metal, they all have relatively high melting and boiling points.
All the transition elements are in the d-block of the periodic table of the elements. This means that they have partially filled d subshells, with varying electron configurations.
The transition metals tend to form complexes, compounds in which a group of atoms clusters around a single metal atom. Ordinary copper sulfate, for example, normally occurs in a configuration that includes four water molecules surrounding a single copper ion. They also tend to form colored compounds, some of which are striking and beautiful. Copper compounds tend to be blue or green; chromium compounds are yellow, orange, or green; nickel compounds are blue, green, or yellow; and manganese compounds are purple, black, or green.
Iron is the fourth most abundant element in Earth’s crust, following oxygen, silicon, and aluminum. Earth’s core is believed to consist largely of iron. Iron rarely occurs in an uncombined form and is usually found as a mineral such as hematite [iron(III) oxide], magnetite [lodestone, a mixture of iron(II) and iron(III) oxides], limonite [hydrated iron(III) oxide], pyrite [iron sulfide], and siderite [iron(II) carbonate].
Iron is a silver-white or gray metal with a melting point of 2,795 degrees Fahrenheit (1,535 degrees Celsius) and a boiling point of 4,982 degrees Fahrenheit (2,750 degrees Celsius). Its chemical symbol, Fe, is taken from the Latin name for iron, ferrum. It is both malleable and ductile. Malleability is a property common to most metals, meaning that a substance can be hammered into thin sheets. Many metals are also ductile, meaning that they can be drawn into a fine wire.
In a pure form, iron is relatively soft and slightly magnetic. When hardened, it becomes much more magnetic. Iron is the most widely used of all metals. Prior to its use, however, it must be treated in some way to improve its properties, or it must be combined with one or more other metals to form an alloy such as steel. (An alloy is a mixture of two or more metals with properties different from those of either metal.)
One of the most common forms of iron is pig iron, produced by smelting iron ore with coke (nearly pure carbon) and limestone in a blast furnace. (Smelting is the process of obtaining a pure metal from its ore.) Pig iron is approximately 90 percent pure iron and is used primarily in the production of cast iron and steel.
Cast iron is a term used to describe various forms of iron that also?contain carbon and silicon ranging in concentrations from 0.5 to 4.2 percent of the former and 0.2 to 3.5 percent of the latter. Cast iron has a vast array of uses in products ranging from thin rings to massive turbine bodies. Wrought iron contains small amounts of a number of other elements, including carbon, silicon, phosphorus, sulfur, chromium, nickel, cobalt, copper, and molybdenum. Wrought iron can be fabricated into a number of forms and is widely used because of its resistance to corrosion.
How Iron Is Obtained
Iron is one of the handful of elements that was known to ancient civilizations. Originally it was prepared by heating a naturally occurring ore of iron with charcoal in a very hot flame. The charcoal was obtained by heating wood in the absence of air. There is some evidence that this method of preparation was known as early as 3000 BC, but the secret of ore smelting was carefully guarded within the Hittite civilization of the Near East for almost 2,000 years.
Then, when that civilization fell in about 1200 BC, the process of iron ore smelting spread throughout eastern and southern Europe. Ironsmiths were soon making ornamental objects, simple tools, and weapons from iron. So dramatic was the impact of this new technology on human societies that the period following 1200 BC is generally known as the Iron Age.
A major change in the technique for producing iron from its ores occurred around 1709. As trees (and therefore the charcoal made from them) grew increasingly scarce in Great Britain, English inventor Abraham Darby (c. 1678–1717) discovered a method for making coke from soft coal. Since coal was abundant in the British Isles, Darby’s technique provided for a consistent and dependable method of converting iron ores to the pure metal.
The modern production of iron involves heating iron ore with coke and limestone in a blast furnace, where temperatures range from 400 degrees Fahrenheit (200 degrees Celsius) at the top of the furnace to 3,600 degrees Fahrenheit (2,000 degrees Celsius) at the bottom. Some blast furnaces are as tall as 15-story buildings and can produce 2,400 tons (2,200 metric tons) of iron per day.
Inside a blast furnace, a number of chemical reactions occur. One of these involves the reaction of coke (nearly pure carbon) with oxygen to form carbon monoxide. This carbon monoxide then reacts with iron ore to form pure iron and carbon dioxide. Limestone is added to the reaction mixture to remove impurities in the iron ore. The product of this reaction, known as slag, consists primarily of calcium silicate. The iron formed in a blast furnace exists in a molten form (called pig iron) that can be drawn off at the bottom of the furnace. The slag also is molten but less dense than the iron. It is drawn off from taps just above the outlet from which the molten iron is removed.
Early efforts to use pig iron for commercial and industrial applications were not very successful. The material proved to be quite brittle, and objects made from it tended to break easily. Cannons made of pig?iron, for example, were likely to blow apart when they fired a shell. By 1760 inventors had begun to find ways of toughening pig iron. These methods involved remelting the pig iron and then burning off the carbon that remained mixed with the product. The most successful early device for accomplishing this step was the Bessemer converter, named after its English inventor, Henry Bessemer (1813–1898). In the Bessemer converter, a blast of hot air is blown through molten pig iron. The process results in the formation of stronger forms of iron: cast and?wrought iron. More importantly, when additional elements such as manganese and chromium are added to the converter, a new product—steel—is formed.
Later inventions improved on the production of steel by the Bessemer converter. In the open hearth process, for example, a mix of molten pig iron, hematite, scrap iron, and limestone is placed into a large?brick container. A blast of hot air or oxygen is then blown across the?surface of the molten mixture. Chemical reactions within the molten?mixture result in the formation of either pure iron or, with the addition of alloying metals such as manganese or chromium, a high grade of steel.
An even more recent variation on the Bessemer converter concept is the basic oxygen process (BOP). In the BOP, a mixture of pig iron, scrap iron, and scrap steel is melted in a large steel container and a blast of pure oxygen is blown through the container. The introduction of alloying metals makes possible the production of various types of steel with many different properties.
Uses of Iron
Alloyed with other metals, iron is the most widely used of all metallic elements. The way in which it is alloyed determines the uses to which the final product is put. Steel, for example, is a general term used to describe iron alloyed with carbon and, in some cases, with other elements. The American Iron and Steel Institute recognizes 27 standard types of steel. Three of these are designated as carbon steels that may contain, in addition to carbon, small amounts of phosphorus and/or sulfur. Another 20 types of steel are made of iron alloyed with one or more of the following elements: chromium, manganese, molybdenum, nickel, silicon, and vanadium. Finally, four types of stainless and heat-resisting steels contain some combination of chromium, nickel, and manganese alloyed with iron.
Steel is widely used in many types of construction. It has at least six times the strength of concrete, another traditional building material, and about three times the strength of special forms of high-strength concrete. A combination of these two materials—called reinforced concrete—is one of the strongest of all building materials available to architects. The strength of steel has made possible some remarkable feats of construction, including very tall buildings (skyscrapers) and bridges with very wide spans. It also has been used in the manufacture of automobile bodies, ship hulls, and heavy machinery and machine parts.
Metallurgists (specialists in the science and technology of metals) have invented special iron alloys to meet very specific needs. Alloys of cobalt and iron (both magnetic materials themselves) can be used in the manufacture of very powerful permanent magnets. Steels that contain the element niobium (originally called columbium) have unusually great strength and have been used, among other places, in the construction of nuclear reactors. Tungsten steels are also very strong and have been used in the production of high-speed metal cutting tools and drills. The alloying of aluminum with iron produces a material that can be used in AC (alternating current) magnetic circuits since it can gain and lose magnetism very quickly.
Metallic iron has other applications as well. Its natural magnetic properties make it suitable for both permanent magnets and electromagnets. It is also used in the production of various types of dyes, including blueprint paper and certain inks, and in the manufacture of abrasives.
Iron is essential to the survival of all vertebrates. Hemoglobin, the molecule in blood that transports oxygen from the lungs to an organism’s cells, contains a single iron atom buried deep within its complex structure. When humans do not take in sufficient amounts of iron in their daily diets, they may develop a disorder known as anemia. Anemia is characterized by a loss of skin color, a weakness and tendency to faint, palpitation of the heart, and a general sense of exhaustion.
Iron also is important to the good health of plants. It is found in a group of compounds known as porphyrins (pronounced POUR-fuh-rinz) that play an important role in the growth and development of plant cells. Plants that lack iron have a tendency to lose their color, become weak, and die.
Copper is one of only two metals with a distinctive color (the other being gold). Copper is often described as having a reddish-brown hue. It has a melting point of 1,985 degrees Fahrenheit (1,085 degrees Celsius) and a boiling point of 4,645 degrees Fahrenheit (2,563 degrees Celsius). Its chemical symbol, Cu, is derived from the Latin name for the element, cuprum.
Copper is one of the elements that is essential to life in tiny amounts (often referred to as trace elements), although larger amounts can be toxic. About 0.0004 percent of the weight of the human body is copper. It can be found in such foods as liver, shellfish, nuts, raisins, and dried beans.
Copper also is found in an essential biochemical compound known?as hemocyanin. Hemocyanin is chemically similar to the red hemoglobin found in human blood, which has an iron atom in the?center of its molecule. By contrast, hemocyanin contains an atom of copper rather than iron in its core. Lobsters and other large crustaceans have blue blood whose color is caused by the presence of hemocyanin.
History of Copper
Copper was one of the first metals known to humans. One reason for this fact is that copper occurs not only as ores (compounds that must be converted to metal) but occasionally as native copper, a pure form of the element found in the ground. In prehistoric times an early human could actually find a chunk of pure copper in the earth and hammer it into a tool with a rock.
Native copper was mined and used in the Tigris-Euphrates valley (modern Iraq) as long as 7,000 years ago. Copper ores have been mined for at least 5,000 years because it is fairly easy to get the copper out of the ore. For example, if an ore of copper oxide is heated in a wood fire, the carbon in the charcoal reacts with oxygen in the oxide and converts it to pure copper metal.
Making Pure Copper
Extremely pure copper (greater than 99.95 percent purity) is generally called electrolytic copper because it is made by the process known as electrolysis. Electrolysis is a reaction by which electrical energy is used to bring about some kind of chemical change. The high purity is needed because most copper is used to make electrical equipment. Small amounts of impurities present in copper can seriously reduce its ability to conduct electricity. Even 0.05 percent of arsenic as an impurity, for example, will reduce copper’s conductivity by 15 percent. Electric wires must be made of very pure copper, especially if the electricity is to be carried for many miles through high-voltage transmission lines.
Uses of Copper
By far the most important use of copper is in electrical wiring. It is an excellent conductor of electricity (second only to silver), it can be made extremely pure, it corrodes very slowly, and it can be formed easily into thin wires.
Copper is also an important ingredient of many useful alloys. Brass is an alloy of copper and zinc. If the brass contains mostly copper, it is a golden yellow color; if it contains mostly zinc, it is pale yellow or silvery. Brass can be cast or machined into everything from candlesticks to cheap, gold-imitating jewelry, though brass jewelry often turns human skin green—the copper reacts with salts and acids in the skin to form green copper chloride and other compounds.
Several other copper alloys include bronze, which is mainly copper plus tin; German silver and sterling silver, which consist of silver plus copper; and silver tooth fillings, which contain about 12 percent copper.
Probably the first alloy ever to be made and used by humans was bronze. Archaeologists broadly divide human history into three periods. The Bronze Age (c. 4000–3000 BC) is the second of these periods, occurring after the Stone Age and before the Iron Age. During the Bronze Age, both bronze and pure copper were used for making tools and weapons.
Because it resists corrosion and conducts heat well, copper is widely used in plumbing and heating applications. Copper pipes and tubing are used to distribute hot and cold water through houses and other buildings. Copper’s superior ability to conduct heat also makes it useful in the manufacture of cooking utensils such as pots and pans. An even temperature across the pan bottom is important for cooking so food does not burn or stick to hot spots. The insides of the pans must be coated with tin, however, to keep excessive amounts of copper from seeping into the food.
Copper corrodes only slowly in moist air—much more slowly than iron rusts. First, it darkens in color because of the formation of a thin layer of black copper oxide. Then, as the years goes by, the copper oxide is converted into a bluish-green patina (a surface appearance that comes with age) of basic copper carbonate. The green color of the Statue of Liberty, for example, was formed in this way.
Mercury, the only liquid metal, has a beautiful silvery color. Its chemical symbol, Hg, comes from the Latin name of the element, hydrargyrum, for “liquid silver.” Mercury has a melting point of -36 degrees Fahrenheit (-38 degrees Celsius) and a boiling point of 673 degrees Fahrenheit (352.5 degrees Celsius). Its presence in Earth’s crust is relatively low compared to other elements, equal to about 0.08 parts per million. Mercury is not considered to be rare, however, because it is found in large, highly concentrated deposits.
Nearly all mercury exists in the form of a red ore called cinnabar, or mercury(II) sulfide. Sometimes shiny globules of mercury appear among outcrops of cinnabar, which is probably the reason that mercury was discovered so long ago. The metal is relatively easy to extract from the ore. In fact, the modern technique for extracting mercury is nearly identical in principle to the method used centuries ago. Cinnabar is heated in the open air. Oxygen in the air reacts with sulfur in the cinnabar, producing pure mercury metal. The mercury metal vaporizes and is allowed to condense on a cool surface, from which it can be collected.
Mercury does not react readily with air, water, acids, alkalis, or most other chemicals. It has a surface tension six times greater than that of water. Surface tension refers to the tendency of a liquid to form a tough “skin” on its surface. The high surface tension of mercury explains its tendency not to “wet” surfaces with which it comes into contact.
No one knows exactly when mercury was discovered, but many ancient civilizations were familiar with this element. As long ago as Roman times, people had learned to extract mercury from ore and used it to purify gold and silver. Ore containing gold or silver would be crushed and treated with mercury to form a mercury alloy, called an amalgam. When the amalgam is heated, the mercury vaporizes, leaving pure gold or silver behind.
Mercury and all of its compounds are extremely poisonous. The element also has no known natural function in the human body. Classified as a heavy metal, mercury is difficult for the body to eliminate. This means that even small amounts of the metal can act as a cumulative poison, collecting over a long period of time until it reaches a dangerous level.
Humans can absorb mercury through any mucous membrane and through the skin. Its vapor can be inhaled, and mercury can be ingested in foods such as fish, eggs, meat, and grain. In the body, mercury affects the nervous system, liver, and kidneys. Mercury poisoning symptoms include tremors, tunnel vision, loss of balance, slurred speech, and unpredictable emotions. (Tunnel vision is a narrowing of the visual field so that peripheral vision—the outer part of the field of vision that encompasses the far right and far left sides—is completely eliminated.) The phrase “mad as a hatter” owes it origin to symptoms of mercury poisoning that afflicted hat makers in the 1800s, when a mercury compound was used to prepare beaver fur and felt materials.
Until recently, scientists thought that inorganic mercury was relatively harmless. As a result, industrial wastes containing mercury were routinely discharged into large bodies of water. Then, in the 1950s, more than 100 people in Japan were poisoned by fish containing mercury. Forty-three people died, dozens more were horribly crippled, and babies born after the outbreak developed irreversible damage. It was found that inorganic mercury in industrial wastes had been converted to a much more harmful organic form known as methyl mercury. As this substance works its way up the food chain, its quantities accumulate to dangerous levels in larger fish. Today, the dumping of mercury-containing wastes has been largely banned, and many of its industrial uses have been halted.
Mercury is used widely in a variety of measuring instruments and devices, such as thermometers, barometers, hydrometers, and pyrometers. It also is used in electrical switches and relays, in mercury arc lamps, and for the extraction of gold and silver from amalgams. A small amount is still used in the preparation of amalgams for dental repairs.
The largest single use of mercury today, however, is in electrolytic cells, in which sodium chloride is converted to metallic sodium and gaseous chlorine. The mercury is used to form an amalgam with sodium in the cells.