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Material hierarchy: Metal
Metals used in manufacturing are usually alloys, which are composed of two or more elements, with at least one being a metallic element. Metals can be divided into two basic groups: Ferrous and Nonferrous.
Metals have crystalline structures in the solid state, almost without exception. The unit cells of these crystal structures are almost always BCC, FCC, or HCP. The atoms of the metals are held together by metallic bonding, which means that their valence electrons can move about with relative freedom (compared with the other types of atomic and molecular bonding). These structures and bonding generally make the metals strong and hard. Many of the metals are quite ductile (capable of being formed, which is useful in manufacturing), especially the FCC metals. Other general properties of metals related to structure and bonding include: high electrical and thermal conductivity, opaqueness (impervious to light rays), and reflectivity (capacity to reflect light rays).
Metals have relatively high moduli. Most, when pure, are soft and easily deformed. They can be made strong by alloying and by mechanical and heat treatment, but they remain ductile, allowing them to be formed by deformation processes. Certain high-strength alloys (spring steel, for instance) have ductilities as low as 1 percent, but even this is enough to ensure that the material yields before it fractures and that fracture, when it occurs, is of a tough, ductile type. Partly because of their ductility, metals are prey to fatigue and of all the classes of material, they are the least resistant to corrosion.
Nearly 80 percent of the known elements are metals. Metals are important in all aspects of construction and manufacturing. Industries use metals and combinations of metals called alloys to build cars and a wide variety of machinery. Compounds that contain metals are used in drugs, batteries, and many other products.
Metals have certain properties that distinguish them from other elements. Metals reflect light and have a shiny appearance. They also are good conductors of electricity and heat. Most metals are malleable--that is, they can be hammered into thin sheets. Most metals are also ductile, which means that they can be drawn out into wires.
In a chemical reaction with a nonmetal, a metal atom gives up one or more electrons to the nonmetal. For example, the metal sodium (Na) reacts with chlorine (Cl) to form the compound sodium chloride (NaCl). In this chemical reaction, each sodium atom gives up one electron, which has a negative charge, to form a positive sodium ion (Na). The chlorine takes on this electron to become a chloride ion (Cl). The oppositely charged ions bond to form NaCl--common table salt.
The properties of a pure metal are different from the properties of a compound containing the metal. For example, in the metallic state, sodium is shiny and highly malleable, and it is extremely reactive with air. But the compound sodium chloride is colorless and brittle, and is stable in air.
In the earth's crust, most of the metallic elements occur in compounds and not in the metallic state. For example, the earth's crust includes about 8 percent aluminum, 5 percent iron, and 4 percent calcium. All these are present in compounds. A few of the rare and least reactive metals may be found in the metallic state in the earth's crust. These metals include copper, gold, mercury, and platinum. Scientists think the earth's core is mainly made up of iron in the metallic state.
Combinations of metals retain the properties of metals. These combinations include bronze, bell metal, gun metal, and type metal. The alloys and metals that do not contain iron are referred to as nonferrous alloys and metals.
Metals Through the Ages. Ancient people knew and used many native metals. Gold was used for ornaments, plates, jewelry, and utensils as early as 3500 B.C. Gold objects showing a high degree of culture have been excavated at the ruins of the ancient city of Ur in Mesopotamia. Silver was used as early as 2400 B.C., and many ancient people considered it to be more valuable than gold, because it was rarer in the native state. Native copper also was used at an early date for making tools and utensils, because it was found near the surface of the ground and could be easily worked and shaped.
Since about 1000 B.C., iron and steel have been the chief metals for construction. Today, supplies of the best iron ore for steelmaking are being exhausted. The same is true for copper, lead, and zinc deposits. The earth's supply of aluminum is almost unlimited.
Magnesium, another light, strong metal, has also become important. It is extracted from sea water and the common rock called dolomite. The radioactive metal uranium is used as a fuel in nuclear reactors.