Metal
From Encyclopædia
Metals are a large group of chemical elements that have certain characteristics in common, such as the ability to conduct heat and
electricity. They range from the familiar (
iron, aluminum,
Copper,
lead) and the precious (gold, silver,
platinum) to the rare (
promethium,
terbium,
ytterbium) and the radioactive (
uranium,
plutonium). In fact about three fourths of all the known elements are true metals. They occupy, in whole or part, all of the groups in the
periodic table except for Group VIIA (
fluorine and the other halogens) and Group 0 (helium and the other noble gases).The development of knowledge about metals and how to use them was
Central to the development of the modern
world (see
metallurgy). Because of their useful properties, many hundreds of millions of tons of metals are used every year in a wide range of industrial applications.CharacteristicsMetals are defined by the physical and chemical properties that they share to some degree. The most distinguishing
property, as noted, is their good conductivity of
electricity and heat. Another very important characteristics is that most metals can be worked. That is, they will change shape rather than shattering when they are placed under
pressure or impact. Those that can be hammered or beaten into sheets are called malleable, and those that can be drawn into
wire are called ductile. Metals also generally show a high reflectivity of
light when they are in a form that exhibits a polished surface. This is known as metallic
luster. At the atomic level many metals are found to have a notably simple
crystal structure, either hexagonal (honeycomb) or cubic.Various other properties, such as hardness, are not shared by all metals. Some of them are soft enough to scratched by a fingernail or deformed by
hand. One familiar metal, mercury, is even a liquid at ordinary temperatures.Metals show similar behavior in various kinds of chemical reactions (see REACTION, chemical). For example, their oxides react with water to form basic, or alkaline, solutions. Metals also characteristically combine with nonmetals, such as the halogens, to form ionic compounds (see ION AND IONIZATION). In such compounds the metal ion is always positive and the nonmetal ion is always negative (see ELECTROMOTIVE SERIES).AlloysDifferent metals can often be fused with each other to yield new metallic substances called alloys (see ALLOY). Alloys of two or more metals can be formulated in a wide variety of compositions and usually have physical properties differing considerably from their components. By careful choice of composition, alloys can be made that have great hardness, toughness, mechanical strength, and resistance to corrosion. Common alloys include brass (
Copper and
zinc), bronze (
Copper and tin), stainless steel (
iron,
chromium,
nickel, and
carbon), and plumber's solder (
lead and tin). The latter is formulated for specific mechanical properties. Electrician's solder uses the same elements, but is formulated for the lowest possible melting temperature.MetalloidsElements falling on the dividing line between metals and nonmetals in the
periodic table are called metalloids, or semimetals, and have physical and chemical properties between the two extremes. The metalloids are the elements boron,
silicon, germanium, arsenic, antimony,
selenium, and
tellurium. They often appear shiny and metallic to the eye, but are brittle and differ radically in their electrical properties.
silicon and germanium are
semiconductors (see
semiconductor) and have electrical conductivities that lie between the extremes of metals and nonmetal insulators. Unlike true metals,
semiconductors become more conductive at high temperatures and are extremely sensitive to trace levels of impurities, a fact of
Central importance to the use of these substances in solid-state electronic devices.Atomic PropertiesAtoms of the metallic elements differ in several important ways from those of the nonmetals and the metalloids. Metal
atoms in general have a rather low affinity for
electrons. They have the lowest ionization potentials and the lowest
electron-negativities of all the elements. Ionization potential is a measure of how tightly an
atom holds its
electrons. It is the quantity of work required to separate an
electron from the
atom. The term electronegativity refers to an
atom's attraction for
electrons when the
atom is combined with other elements. It is the power of an
atom in a
molecule to attract
electrons to itself. A closely related
property is the relatively larger atomic diameters of metals compared to nonmetals and metalloids. Since the
electrons of a metal
atom are held more weakly (lower ionization potential), they are allowed a greater range of motion. This gives the
atom a larger effective size in proportion to its
electron population.These atomic properties help to explain much of the characteristic chemical behavior of metals. Metal oxides are crystalline substances built up of positive metal ions and negative ions of
oxygen. The oxide ion is itself a powerful base that, when free, invariably combines with water to produce hydroxide ions and, consequently, basic (alkaline) solutions (see ACIDS AND BASES).Although the metallic elements vary greatly in their reactivity, all can be induced to give up
electrons and form positive ions with relative ease. That is, almost all metals can be oxidized under mild conditions.The Metallic BondThe bonding in solid metals is very different from that of covalent and ionic substances (see
CHEMICAL BOND). Most of the distinguishing properties of metals stem from these differences.The simplest theory of the metallic bond is called the free-
electron of
electron-gas model. The metal
atoms are imagined to be positive ions immersed in a negatively charged gas or sea of VALENCE
electrons, giving the entire structure electrical
neutrality. The valence
electrons in the sea are not associated with any?given
atoms and are free to move throughout the entire body of the solid metal.This simple model accounts for many of the characteristic properties of metals. Metals can conduct
electricity, which is simply a flow of
electrons, because of the mobility of the free valence
electrons. The electrical resistance of metals increases with temperature because the heightened vibrational motion of the metal
atoms impedes
electron flow. Layers of metal
atoms can be shifted or displaced with respect to each other without disrupting the
electron sea, with the result that metals are plastic under
pressure or impact, allowing them the properties of malleability and ductility. Molten metals conduct electric current almost as well as the solids, because in the liquid state the positive metal ions, now mobile, are still immersed in the
conducting free-
electron sea. Finally, the free-
electron metallic bond is nonspecific, which means that one type of metal ion can be substituted for another without changing the overall bonding in a major way, so that metals can form an incredible variety of
solid solutions, or alloys.Philip C. KellerBibliography: Alexander, W., and Street, A., Metals in the Service of Man (1972); Cottrell, Alan, Introduction to the Modern Theory of Metals (1988); Edwards, P. P., and Rao, C. N., eds., The Metallic and Nonmetallic States of Matter (1985); Maddin, Robert, The Beginning of the Use of Metals and Alloys (1988); Martin, J. W., Elementary
science of Metals (1974); Parrish, R. V., The Metallic Elements (1977); Raymond, Robert, Out of the Fiery Furnace (1986); Sharpe, Alan G., Inorganic Chemistry (1986); Smithells, C. J., ed., Metals Reference Book, 5th ed. (1976).
