Metals

• Type of physical science: Chemistry
• Field of study: Chemistry of solids

Dense, lustrous solids have been known since prehistoric times, when they were first used to make tools and weapons. In the ages since then, humans have learned to understand metals and to control their properties.

Overview

Metals are elements, alloys, or compounds, usually solid, that are recognizable by a set of characteristic properties that include high luster and superior electrical and thermal conductivities. Although there are exceptions, most metals possess densities greater than 1 gram per milliliter and can be hammered easily into useful shapes or drawn into thin wires.

Of the ninety or so elements found naturally on earth, about two-thirds are classified as metals and include such well-known materials as copper, gold, silver, aluminum, iron, and mercury. More exotic elements, such as osmium, cesium, niobium, and a host of others, are known, including all the elements first discovered through their production in artificial nuclear reactions.

The elemental metals are often divided on the basis of density into light metals and heavy metals. Most light metals are found in the first two columns of the periodic table of elements and manifest somewhat similar properties. These metals are rather reactive toward atmospheric oxygen and water and are therefore not found uncombined in nature. Most are fairly soft, being able to be cut with a knife, and have relatively low melting points. Several metals possess densities less than that of water, which means that they will float.

Potassium is a typical light metal. With a density of 0.86 grams per milliliter, potassium floats when placed on water, but does not remain still. Because of its high reactivity, the metal skitters about the surface, generating hydrogen gas and sufficient heat to ignite the gas. The metal itself appears silvery and shiny when not covered with an oxide coating, conducts heat and electricity efficiently, and melts at 63.7Celsius (146.6 degrees Fahrenheit), a temperature below the boiling point of water.

The heavy metals exhibit a wide range of properties while still displaying metallic characteristics. Osmium is a hard, lustrous, silvery metal that melts at 3,054 degrees Celsius (5,529 degrees Fahrenheit) and conducts electricity more readily than iron or tin. The solid is more than twenty times denser than water. In contrast, lead is a soft metal that melts at 600 degrees Celsius (1,112 degrees Fahrenheit). It conducts electricity about half as readily as osmium, yet more than fifty times more efficiently than graphite, which is considered a good conductor. Lead is about half as dense as osmium. Compared to the light metals, both lead and osmium are rather unreactive. Osmium is inert enough to be found uncombined in nature, though it is rare; lead reacts slowly with moist air to form the oxide or with sulfur-containing materials to form the sulfide. Other heavy metals behave in a similar fashion: Some metals are lighter, some heavier; some are inert, some reactive; some melt at low temperatures, some at high temperatures. They all, however, share the properties that make them recognizable as metals--they are good conductors and are lustrous.

Metal alloys are homogeneous solid-state mixtures that contain at least one metallic element. Thus, bronze consists of copper and tin; brass contains zinc and copper; and steel contains iron, carbon, and frequently other substances. Although alloys occur naturally, in meteorites for example, the great majority are man-made, and their compositions are controlled to produce desired properties. Thus, the hardness of a bronze can be adjusted by changing the tin content, the temper of a steel by changing the carbon content, and so on. As a result of varied uses and the myriad combinations and proportions of materials that can be used in their production, alloys exhibit physical properties that span broad ranges. Therefore, an alloy of mercury and sodium can be made to melt in the vicinity of room temperature, while an alloy of gold and platinum can be made that melts above 1,700 degrees Celsius (3,092 degrees Fahrenheit). Similarly, alloys have been designed that are hard or soft, conductive or resisting, malleable or brittle.

While most combinations of metals form homogeneous solid-state solutions, several actually form compounds. When the molten metals are mixed, zinc and magnesium combine at a fixed ratio of two zinc atoms to one magnesium atom to produce the compound MgZn₂.

Similarly, bismuth and gold, lead and magnesium, and other combinations of metals form distinct new substances as opposed to alloys.

More interesting, perhaps, are the compounds that exhibit metallic behavior, even though their constituent elements are nonmetals. Polythiazil is a polymeric compound consisting of equal numbers of nitrogen and sulfur atoms arranged in long chains. It is a bronze-colored, lustrous solid that exhibits high electrical conductivity. Indeed, at sufficiently low temperatures, this material behaves as a superconductor. Polythiazil behaves like a metal even though nitrogen and sulfur are classic nonmetals. Similar kinds of metals have been made in the laboratory from carbon in combination with other nonmetallic elements, and the investigation of these so-called organic metals constitutes an active area of research in chemistry.

The properties of metallic solids depend upon their structures. Metal crystals consist of atoms arranged in a regular three-dimensional repeating array known as a crystal lattice. In contrast to other kinds of solids, metals have a tendency to crystallize in only a few arrangements--lattices that are very efficient in filling space. Consequently, metals tend to have rather high densities.

In a metallic crystal, the units that occupy the lattice sites are the atomic cores that remain from atoms that have lost their outermost electrons. Potassium, for example, exhibits a lattice in which singly charged positive ions are found. In calcium, doubly charged positive ions occupy the lattice sites, and so on for other metals. The electrons that are freed from each atom are free to move throughout the entire crystal and can be thought of as a mobile electron gas in which the positive atomic cores are immersed. This mobile gas or "sea of electrons," as it is often called, serves as a sort of glue that holds the metallic lattice together and constitutes a metallic bond.

This model of a metallic solid can be used to explain many of the characteristic properties of metals. The ability to conduct electricity and the high luster of metals are related to the ability of electrons to move freely. Insulators, such as wood or diamond, possess no mobile charges. Similarly, nonlustrous materials have few or no unbound electrons. The facility with which metallic crystals can be deformed without fracture is likewise explained in terms of metallic bonding. Once a crystal has changed shape because of a stress that moves its atomic cores, the mobile electrons move to new positions and maintain the solid's cohesion. Thus, metals are malleable and ductile rather than brittle. This behavior contrasts with that of covalent solids, which are extremely hard, and ionic solids, which are brittle; it also arises from the nondirectional character of metallic bonding.

Differences among metals can also be related to this model of a solid. Metals that crystallize in more open lattices tend to have lower densities; metals that crystallize in more congested lattices have higher densities. Thus, potassium forms a body-centered cubic lattice in which each atom is surrounded by four nearest neighbors. Calcium, on the other hand, crystallizes in a face-centered cubic lattice in which each atom is surrounded by twelve nearest neighbors. This difference is reflected in the respective densities of the two metals: Calcium at 1.54 grams per milliliter is almost twice as dense as potassium (0.86) even though individual atoms of the two metals have almost the same mass.

Variations in the strength of metallic bonding are explained by this model. Since the cohesive strength of a metal depends upon the mobile electrons of the lattice, it might be expected that properties that depend upon cohesion would vary systematically as the number of electrons contributed per atom to the mobile sea increased. In broad outline, this trend is observed. Other factors being about the same, the melting point and the hardness of a metal increase with the tendency of the metallic atom to lose electrons. Thus, potassium, which loses one electron per atom, melts at 63.7 degrees Celsius (146.6 degrees Fahrenheit) and is softer than calcium, which loses two electrons per atom and melts at 839 degrees Celsius (1,542 degrees Fahrenheit).

Applications

Because of their desirable physical and chemical properties, metallic solids have found use in myriad applications, some of which date back to ancient times. Indeed, testimony to the technological importance of metals and some indication of their role in transforming society can be found in historians' designations for the periods following the new Stone Age: the Copper Age, the Bronze Age, and the Iron Age.

Gold and silver have long been prized as precious metals and used for the production of jewelry and coins. While the striking appearance of these metals is apparent, other, less obvious properties suit these metals for such uses. Both of these solids display good malleability. That is, they can be hammered into different shapes rather easily. Gold can be pounded into a foil thin enough to be translucent. Additionally, both elements are remarkably unreactive. The beautifully polished surface of a gold coin or of a silver plate exhibits a lesser tendency to become discolored through attack by atmospheric or other environmental materials than would a similar artifact fashioned from iron or copper. This tendency to be unreactive also means that gold or silver can be melted and reshaped repeatedly without attrition of the material. These properties, prized by medieval alchemists, led them to describe gold and silver as "noble metals," terminology that persists to this day as a description for an unreactive metal. In contrast, reactive metals such as lead or iron were called "base metals."

In comparison to gold or silver, copper is not nearly so unreactive, though it is often considered a noble metal. Upon exposure to atmospheric gases, copper tends to corrode, but because the products of the corrosion reaction form a coating that protects the underlying metal, copper can be used as a building material where great strength is not required. Thus, one finds copper roofing, gutters, and rainspouts on many older buildings. Copper is recognizable on such structures not from the shiny, reddish-brown appearance of the metal but from the greenish color of the protective layer that covers it. In a similar fashion, copper-containing alloys such as bronze also form protective coatings or patinas that change the appearance of the underlying metal and contribute to the aesthetic appeal of objects of art.

Copper is also noted for another set of properties. Among the elemental metals, it trails only silver in its ability to conduct electrical current. Of the common metals, copper is perhaps the most easily joined by soldering. As a result, huge quantities of copper are produced annually for use in fabricating electrical wire and in manufacturing tubing to be used for plumbing and other kinds of pipes. Copper is also an excellent conductor of heat (again second only to silver among metallic elements). Consequently, copper cladding is applied to items such as pots or kettles, where an even distribution of heat is desired. Alternatively, copper alloys or the metal itself can be used to fashion kitchen vessels, though pure copper is somewhat less desirable because of its relative softness and tendency to be easily deformed.

While pure elemental metals are employed in common applications, the uses of alloys are much more widespread. Of particular importance is the iron-based alloy steel. The term "steel" does not, in fact, designate a single alloy but rather a whole family of metals. Iron that is obtained from mineral ores (pig iron) is commonly contaminated with up to about 5 percent of impurities, principally carbon. In steelmaking, the carbon content of pig iron is reduced and metals chosen to impart desired properties to the final product are added. Therefore, a tough steel might contain manganese as an alloying additive. This type of steel has found use in rock-breaking machinery and as the rails that carry trains. A stainless steel would contain chromium, and perhaps nickel and molybdenum. This is the type of steel found in cutlery or in other applications where corrosion resistance is particularly important. Some idea of the range of alloying possibilities should be apparent from the variety of elements that have been used as additives: nickel, chromium, manganese, vanadium, boron, cobalt, tungsten, titanium, aluminum, and others.

Although the alloys of steel are extensively used, many other alloys are more widely known. Most people can identify brass or pewter, have seen eighteen-carat gold or white gold, and are familiar with silver solder. Cars, cookware, the change carried in pockets, and airplanes are all composed in large part of one or more kind of metal alloy.

Context

Though metals have been known since prehistoric times, it is only in the last hundred years that an understanding of their behavior and an aggressive exploitation of their properties has emerged.

Many historians attribute to the ancients a knowledge of seven metals: gold, silver, copper, tin, lead, iron, and mercury. These elements have in common the property that they are sufficiently unreactive to be found uncombined in nature. In addition, most of these metals can be recovered from the mineral ores that contain them by heating the ore to only moderate temperatures.

Copper was apparently the first metal to be widely used for tools, weapons, and containers. Its comparatively low melting point of 1,083 degrees Celsius (1,985 degrees Fahrenheit) and relative softness made it an accessible and useful material for societies emerging from the new Stone Age.

Evidence of copper implements dating back to about 7000 BC has been found, and it appears that the element was being obtained from its ore malachite by 5000 BC in Iran (Persia) and Afghanistan.

As useful as copper was to early societies, its malleability contributed to its shortcomings. Weapons or tools fashioned from copper were easily bent or nicked, and sharpened edges quickly dulled. About 4000 BC, however, artisans in the Middle East discovered that small additions of tin would produce the alloy bronze, from which more durable implements could be fashioned.

The Iron Age began about two thousand years later. Though iron was probably known from meteorite deposits, the metal was not widely available because the technology for extracting the metal from its ore requires rather high temperatures. Iron working requires a fire into which air is blown. By about 1500 BC, the ability to extract and work iron was known in the Middle East, particularly among the Hittites, and within five hundred years, the ability to make steel had been developed in India. In the centuries that followed, craftsmen, and later scientists, have developed metallurgy to a high state.

Between the time the first craftsman extracted copper from malachite and the time when chemists began to synthesize organic metals, countless workers contributed to the knowledge of these useful materials. A good part of that knowledge arose from the futile attempts of alchemists in medieval and Renaissance times to convert base metals into gold. In the course of their attempts, they learned of the effects of acids on various metals, they cataloged the properties of metallic oxides, and they investigated transformations affected by heat.

Another impetus to develop metal technology grew from the Industrial Revolution.

With the invention of the steam engine and other devices, the need for high-quality steel increased. To build bridges and railroad tracks, locomotives and steamships, required a dependable supply of steel of consistent quality. In response, the early steelmaking craft discovered in India and later fostered at places such as Damascus and Toledo, Spain, was transformed into a science at industrial centers. In the United States, an enormous steel industry flourished about cities such as Pittsburgh and Johnstown in Pennsylvania. About the same time, aluminum began to come into widespread use. In 1886, Paul-Louis-Toussaint Heroult in France and Charles Martin Hall in the United States independently developed a process for obtaining aluminum from its bauxite ore. As a result, the price of aluminum fell to levels that made the metal a reasonable choice for structural applications where lighter materials were required.

Improved methods of copper production and lead smelting also evolved in the nineteenth century, and toward the end of the century, Ludwig Mond devised a method of producing nickel efficiently.

An understanding of the physical basis of metallic behavior lagged behind the technology. In 1861, Henry Clifton Sorby in Sheffield first used microscopy in a systematic fashion to examine metals. Soon afterward, many workers were combining metallography with studies of melting point, hardness, tensile strength, and electrical conductivity. Application of thermodynamic principles to metallurgy was greatly aided by Hendrik Willem Bakhuis Roozeboom's demonstration that the phase rule of Josiah Willard Gibbs could be fruitfully applied to the study of alloys.

In the early part of the twentieth century, a molecular-level understanding of metals began to emerge. In 1916, Hendrik Antoon Lorentz proposed the forerunner of the modern model of a metal, suggesting that metals consisted of closely packed atomic spheres with free electrons in interstitial spaces. Wolfgang Pauli then applied the principles of quantum mechanics to compute the energies of these free electrons in 1927. Subsequently, Arnold Sommerfeld, Hans Albrecht Bethe, and others refined the quantum mechanical approach to metallic solids as part of the discipline of solid-state physics.

Principal terms

ALLOY: a metal consisting of two or more metals or a metal and a nonmetal

CRYSTAL: a solid in which external surfaces are flat and make definite angles with each other; crystals exhibit long range order at the molecular level

ION: a charged particle; starting from a neutral molecule, positive ions result from removing one or more electrons, negative ions from adding one or more electrons

LATTICE: a repeating spatial pattern; the long-range order of crystals consists of structural units in a three-dimensional lattice

METALS: a class of elements that are usually lustrous solids and are good conductors of heat and electricity

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