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metal

metal, chemical element displaying certain properties by which it is normally distinguished from a nonmetal, notably its metallic luster, the capacity to lose electrons and form a positive ion, and the ability to conduct heat and electricity. The metals comprise about two thirds of the known elements (see periodic table). Some metals, including copper, tin, iron, lead, gold, silver, and mercury, were known to the ancients; copper is probably the oldest known metal.

Physical Properties

Metals differ so widely in hardness, ductility (the potentiality of being drawn into wire), malleability, tensile strength, density, and melting point that a definite line of distinction between them and the nonmetals cannot be drawn. The hardest elemental metal is chromium; the softest, cesium. Copper, gold, platinum, and silver are especially ductile. Most metals are malleable; gold, silver, copper, tin, and aluminum are extremely so. Some metals exhibiting great tensile strength are copper, iron, and platinum. Three metals (lithium, potassium, and sodium) have densities of less than one gram per cubic centimeter at ordinary temperatures and are therefore lighter than water. Some heavy metals, beginning with the most dense, are osmium, iridium, platinum, gold, tungsten, uranium, tantalum, mercury, hafnium, lead, and silver.

For many industrial uses, the melting points of the metals are important. Tungsten fuses, or melts, only at extremely high temperatures (3,370°C.), while cesium has a melting point of 28.5°C. The best metallic conductor of electricity is silver. Copper, gold, and aluminum follow in the order named. All metals are relatively good conductors of heat; silver, copper, and aluminum are especially conductive. The radioactive metal uranium is used in reactor piles to generate steam and electric power. Plutonium, another radioactive element, is used in nuclear weapons and nuclear reactors as well as in pacemakers. Some of the radioactive metals not found in nature, e.g., fermium and seaborgium, are produced by nuclear bombardment.

Some elements, e.g., arsenic and antimony, exhibit both metallic and nonmetallic properties and are called metalloids. Furthermore, although all metals form crystals, this is also characteristic of certain nonmetals, e.g., carbon and sulfur.

Chemical Properties

Chemically, the metals differ from the nonmetals in that they form positive ions and basic oxides and hydroxides. Upon exposure to moist air, a great many undergo corrosion, i.e., enter into a chemical reaction; e.g., iron rusts when exposed to moist air, the oxygen of the atmosphere uniting with the metal to form the oxide of the metal. Aluminum and zinc do not appear to be affected, but in fact a thin coating of the oxide is formed almost at once, stopping further action and appearing unnoticeable because of its close resemblance to the metal. Tin, lead, and copper react slowly under ordinary conditions. Silver is affected by compounds such as sulfur dioxide and becomes tarnished when exposed to air containing them. The metals are combined with nonmetals in their salts, as in carbides, carbonates, chlorides, nitrates, phosphates, silicates, sulfides, and sulfates.

The Electromotive Series

On the basis of their ability to be oxidized, i.e., lose electrons, metals can be arranged in a list called the electromotive series, or replacement series. Metals toward the beginning of the series, like cesium and lithium, are more readily oxidized than those toward the end, like silver and gold. In general, a metal will replace any other metal, or hydrogen, in a compound that it precedes in the series, and under ordinary circumstances it will be replaced by any metal, or hydrogen, that it follows.

Metals in the Periodic Table

Metals fall into groups in the periodic table determined by similar arrangements of their orbital electrons and a consequent similarity in chemical properties. Groups of similar metals include the alkali metals (Group 1 in the periodic table), the alkaline-earth metals (Group 2 in the periodic table), and the rare-earth metals (the lanthanide and actinide series of Group 3). Most metals other than the alkali metals and the alkaline earth metals are called transition metals (see transition elements). The oxidation states, or valence, of the metal ions vary from +1 for the alkali metals to as much as +7 for some transition metals.

Sources and Uses

Although a few metals occur uncombined in nature, the great majority are found combined in their ores. The separation of metals from their ores is called extractive metallurgy. Metals are mixed with each other in definite amounts to form alloys; a mixture of mercury and another metal is called an amalgam. Bronze is an alloy of copper and tin, and brass contains copper and zinc. Steel is an alloy of iron and other metals with carbon added for hardness.

Since metals form positive ions readily, i.e., they donate their orbital electrons, they are used in chemistry as reducing agents (see oxidation and reduction). Finely divided metals or their oxides are often used as surface catalysts. Iron and iron oxides catalyze the conversion of hydrogen and nitrogen to ammonia in the Haber process. Finely divided catalytic platinum or nickel is used in the hydrogenation of unsaturated oils. Metal ions orient electron-rich groups called ligands around themselves, forming complex ions. Metal ions are important in many biological functions, including enzyme and coenzyme action, nucleic acid synthesis, and transport across membranes.

For the uses of specific metals, see separate articles.

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metal

metal Element that is a good conductor of heat and electricity. Its atoms bond together within crystals in a unique way. Mixtures of such elements (alloys) are also metals. About three-quarters of known elements are metals. Most are hard, shiny materials that form oxides. Malleability and ductility are further metallic characteristics. Some metals have very high melting points and various high-temperature applications: tungsten, with the highest melting point of all at 3410°C (6170°F), is employed for incandescent-lamp filaments. Aluminium and iron are the two most abundant and useful metals. Titanium, although rarely seen as a metal, is more commonly distributed than the more familiar copper, zinc, and lead. Other metals of economic importance, because they undergo nuclear fission, are uranium and plutonium.

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metal

met·al / ˈmetl/ • n. 1. a solid material that is typically hard, shiny, malleable, fusible, and ductile, with good electrical and thermal conductivity (e.g., iron, gold, silver, copper, and aluminum, and alloys such as brass and steel): vessels made of ceramics or metal | being a metal, aluminum readily conducts heat. ∎  Heraldry gold and silver (as tinctures in blazoning). 2. Brit. (also road met·al) broken stone for use in making roads. 3. molten glass before it is blown or cast. 4. heavy metal or similar rock music. • v. (met·aled , met·al·ing ; chiefly Brit. met·alled, met·al·ling) [tr.] [usu. as adj.] (metaled) make out of or coat with metal: metaled key rings.

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metal

metal metals were traditionally divided into noble or precious metals (gold, silver, and platinum, which resist corrosion) and base or imperfect metals (such as lead). In heraldry, metal is used for the tinctures or (gold) and argent (silver).

Recorded from Middle English, the word comes via Old French or Latin, from Greek metallon ‘mine, quarry, metal’.

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metal

metal XIII. — (O)F. métal, † metail or its source L. metallum mine, quarry, metal — synon. Gr. métallon.
So metallic XVI. metalline XV. — F. Hence metallize XVI. See METTLE.

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metal

metalbattle, cattle, chattel, embattle, prattle, rattle, Seattle, tattle •fractal •cantle, covenantal, mantel, mantle, Prandtl •pastel • Fremantle • tittle-tattle •startle, stratal •Nahuatl •fettle, kettle, metal, mettle, nettle, petal, Popocatépetl, settle •dialectal, rectal •dental, gentle, mental, Oriental, parental, rental •transeptal •festal, vestal •gunmetal •antenatal, fatal, hiatal, natal, neonatal, ratel •beetle, betel, chital, decretal, fetal •blackbeetle •acquittal, belittle, brittle, committal, embrittle, it'll, kittle, little, remittal, skittle, spittle, tittle, victual, whittle •edictal, rictal •lintel, pintle, quintal •Bristol, Chrystal, crystal, pistol •varietal • coital • phenobarbital •orbital • pedestal • sagittal • vegetal •digital • skeletal • Doolittle •congenital, genital, primogenital, urogenital •capital • lickspittle • hospital • marital •entitle, mistitle, recital, requital, title, vital •subtitle • surtitle •axolotl, bottle, dottle, glottal, mottle, pottle, throttle, wattle •fontal, horizontal •hostel, intercostal, Pentecostal •greenbottle • bluebottle • Aristotle •chortle, immortal, mortal, portal •Borstal •anecdotal, sacerdotal, teetotal, total •coastal, postal •subtotal •brutal, footle, pootle, refutal, rootle, tootle •buttle, cuttle, rebuttal, scuttle, shuttle, subtle, surrebuttal •buntal, contrapuntal, frontal •crustal • societal • pivotal •hurtle, kirtle, myrtle, turtle

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Metal

Metal

Crystallography of metals

Survey of the periodic table

A material, in chemistry, is called a metal based on the way it reacts to other elements. Metallic elements characteristically form positive ions when their compounds are in solution. Their oxides form hydroxides rather than acids with water. Nearly three-fourths of the elements in each group of the periodic table are metals except for the Group 17 (halogen) and Group 18 (noble gas) elements. Most metals form crystalline solids, and most are good conductors of electricity; most have rather high chemical reactivities. Many metals are quite hard, with high physical strength. When polished, metals tend to be good reflectors of light. Some of the more commonly known metals are aluminum, copper, gold, iron, lead, nickel, silver, titanium, uranium, and zinc.

Metals easily form alloys with other metals. The presence of even a small amount of another element in a metal severely affects its properties, as in the case of carbon in iron. Mercury, cesium, and gallium exist as liquids at room temperature.

The behavior of metals as atoms or ions deeply affects the electrochemical reactions they undergo, and similarly affects the metabolism of plants and animals. Iron, copper, cobalt, potassium, and sodium are examples of metals that are essential to biological function. Some metals such as cadmium, mercury, lead, barium, chromium, and beryllium are highly toxic.

Crystallography of metals

Metals usually differ from nonmetals by their excellent thermal and electrical conductivities, and by their great mechanical strengths and ductilities. These properties follow directly from the nonlocalized electronic bonds in these materials. The electrons in metals are mobile; in a true metal, there are no underlying directed bonds.

With the exception of manganese and uranium, all true metals have one of the following crystal structures: body-centered cubic (sodium, potassium, molybdenum), iron face-centered cubic (copper, silver, gold), iron close-packed hexagonal (beryllium, magnesium, zirconium).

The origins of metallic behavior may be understood by considering the first and simplest of these three structures. There are eight nearest neighbors in a body-centered cubic structure. The number of next nearest atoms is six. The one valence electron of a body-centered cubic element like sodium clearly cannot furnish 14 or even eight covalent bonds with its neighbors. Thus, the single valence electron is shared.

The elements on the left-hand side of the periodic table readily pool their valence electrons, as they have low ionization potentials. Their large de-localization energies result in net binding. As one moves to the right of Group 1 in the periodic table, the metallic properties of the elements become weaker, and the tendency to form covalent bonds increases. As a result, thermal and electrical conductivities diminish, densities decrease, and the materials become hard, but brittle.

Carbon in Group 14, for example, does not allow its valence electrons to escape, but readily shares them with four neighbors. Graphitic carbon is made up of well separated layer planes with high conductivities along the planes but weak conductivities at right angles to these planes; consequently graphite is a two dimensional metal. In diamond, the electron bonds are tetrahedral and highly directed; this has the effect of making diamond brittle. Silicon, germanium, and gray tin also have diamond-like structures, and their bonding is largely covalent.

Survey of the periodic table

The first element of the periodic table, hydrogen, is a nonmetal. In the case of the alkali metals of Group 1, however, one finds that lithium, sodium, potassium, rubidium, cesium, and francium all exhibit to a high degree typically metallic properties. Each of these atoms has one electron in the outermost energy level. The energies required to pull off these single valence electrons are relatively small; on the other hand, the energies required to pull off a second electron are many times higher.

Group 2 of the periodic table includes the elements beryllium, magnesium, calcium, strontium, and radium. These elements are known as the alkaline earth metals. In each of the Group 2 elements, there are two electrons in the outer-most energy level. Going down the group from beryllium to radium, one finds decreasing ionization potentials with increasing ionic radius. In general, the larger atoms hold their electrons less tightly than do the smaller atoms. Although the first two electrons are removed relatively easy, removal of a third electron from the Group 2 elements requires very high energies.

Groups 3 through 12 in the periodic table are known as the transition elements. The most characteristic property of the transition elements is that they are all metals. This is because the outermost electron shells of these elements contain very few electrons. Unlike the Group 1 and 2 elements, the transition metals tend to be hard, brittle, and high melting. The difference is due in part to the relatively small size of the transition element radii, and partly to the existence of some covalent bonding between the ions.

The Group 13 elements have the same relationship to the alkaline earth elements that the alkaline earth elements have to the alkali metals, that is, the group properties are modified by the presence of a third valence electron. The elements of Group 13 are boron, aluminum, gallium, indium, and thallium. Except for boron, which may be classified as a semi-metal, these elements tend to show metallic properties.

Group 14 elements include carbon, silicon, germanium, tin, and lead. As already noted, carbon forms a solid of complex structure that does not exhibit metallic properties. The second and third members of the

KEY TERMS

Valence electrons The electrons in the outermost shell of an atom that determine an elements chemical properties.

group, silicon and germanium, cannot be classified as metals either; they are only semimetals.

In Group 15, there is a complete change of properties from nonmetallic to metallic in going down the group. The lighter members, nitrogen and phosphorous, are typically nonmetals. The middle members, arsenic and antimony, are semimetals. The heaviest member, bismuth, is a metal.

The Group 16 elements include oxygen, sulfur, selenium, tellurium, and polonium. As would be expected from their location on the far right of the periodic table, the Group 16 elements have high ionization potentials, and metallic properties are difficult to observe. However, in going down the group, electrons are less tightly held; so there is some suggestion of metallic behavior in the heavier Group 16 elements.

The Group 17 elements, i.e., fluorine, chlorine, bromine, iodine, and astatine, all have high electron negativities and consequently show practically no metallic properties. Iodine, however, does show some metallic characteristics. Astatine may have some metallic properties, but it is a short-lived radioactive element, and measurements of its properties are difficult to carry out.

The Group 18 elements, or noble gases, consist of six gases: helium, neon, argon, krypton, xenon, and radon. The noble gases are nonmetals.

See also Alloy; Electrical conductivity; Element, chemical; Metallurgy.

Randall Frost

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Metal

Metal

A material is called a metal based on the way it reacts to other elements. Metallic elements characteristically form positive ions when their compounds are in solution . Their oxides form hydroxides rather than acids with water . Nearly three-fourths of the elements in each group of the periodic table are metals except for the Group 17 (halogen) and Group 18 (noble gas) elements. Most metals form crystalline solids, and most are good conductors of electricity ; most have rather high chemical reactivities. Many metals are quite hard, with high physical strength. When polished, metals tend to be good reflectors of light .

Metals easily form alloys with other metals. The presence of even a small amount of another element in a metal severely affects its properties, as in the case of carbon in iron . Mercury, cesium, and gallium exist as liquids at room temperature .

The behavior of metals as atoms or ions deeply affects the electrochemical reactions they undergo, and similarly affects the metabolism of plants and animals. Iron, copper , cobalt, potassium, and sodium are examples of metals that are essential to biological function. Some metals such as cadmium, mercury, lead , barium , chromium, and beryllium are highly toxic.


Crystallography of metals

Metals usually differ from nonmetals by their excellent thermal and electrical conductivities, and by their great mechanical strengths and ductilities. These properties follow directly from the nonlocalized electronic bonds in these materials. The electrons in metals are mobile; in a true metal, there are no underlying directed bonds.

With the exception of manganese and uranium , all true metals have one of the following crystal structures: body-centered cubic (sodium, potassium, molybdenum), iron face-centered cubic (copper, silver, gold), iron close-packed hexagonal (beryllium, magnesium , zirconium).

The origins of metallic behavior may be understood by considering the first and simplest of these three structures. There are eight nearest neighbors in a body-centered cubic structure. The number of next nearest atoms is six. The one valence electron of a body-centered cubic element like sodium clearly cannot furnish 14 or even eight covalent bonds with its neighbors. Thus, the single valence electron is shared.

The elements on the left-hand side of the periodic table readily pool their valence electrons, as they have low ionization potentials. Their large de-localization energies result in net binding. As one moves to the right of Group 1 in the periodic table, the metallic properties of the elements become weaker, and the tendency to form covalent bonds increases. As a result, thermal and electrical conductivities diminish, densities decrease, and the materials become hard, but brittle.

Carbon in Group 14, for example, does not allow its valence electrons to escape, but readily shares them with four neighbors. Graphitic carbon is made up of well separated layer planes with high conductivities along the planes but weak conductivities at right angles to these planes; consequently graphite is a two dimensional metal. In diamond , the electron bonds are tetrahedral and highly directed; this has the effect of making diamond brittle. Silicon, germanium, and grey tin also have diamond-like structures, and their bonding is largely covalent.


Survey of the periodic table

The first element of the periodic table, hydrogen , is a nonmetal . In the case of the alkali metals of Group 1, however, one finds that lithium , sodium, potassium, rubidium, cesium, and francium all exhibit to a high degree typically metallic properties. Each of these atoms has one electron in the outermost energy level. The energies required to pull off these single valence electrons are relatively small; on the other hand, the energies required to pull off a second electron are many times higher.

Group 2 of the periodic table includes the elements beryllium, magnesium, calcium , strontium, and radium. These elements are known as the alkaline earth metals . In each of the Group 2 elements, there are two electrons in the outer-most energy level. Going down the group from beryllium to radium, one finds decreasing ionization potentials with increasing ionic radius. In general, the larger atoms hold their electrons less tightly than do the smaller atoms. Although the first two electrons are removed relatively easy, removal of a third electron from the Group 2 elements requires very high energies.

Groups 3 through 12 in the periodic table are known as the transition elements. The most characteristic property of the transition elements is that they are all metals. This is because the outermost electron shells of these elements contain very few electrons. Unlike the Group 1 and 2 elements, the transition metals tend to be hard, brittle, and fairly high melting. The difference is due in part to the relatively small size of the transition element radii, and partly to the existence of some covalent bonding between the ions.

The Group 13 elements have the same relationship to the alkaline earth elements that the alkaline earth elements have to the alkali metals, that is, the group properties are modified by the presence of a third valence electron. The elements of Group 13 are boron, aluminum , gallium, indium, and thallium. Except for boron, which may be classified as a semimetal, these elements tend to show metallic properties.

Group 14 elements include carbon, silicon, germanium, tin, and lead. As already noted, carbon forms a solid of complex structure that does not exhibit metallic properties. The second and third members of the group, silicon and germanium, cannot be classified as metals either; they are only semimetals.

In Group 15, there is a complete change of properties from nonmetallic to metallic in going down the group. The lighter members, nitrogen and phosphorous, are typically nonmetals. The middle members, arsenic and antimony, are semimetals. The heaviest member, bismuth, is a metal.

The Group 16 elements include oxygen , sulfur , selenium, tellurium, and polonium. As would be expected from their location on the far right of the periodic table, the Group 16 elements have high ionization potentials, and metallic properties are difficult to observe. However, in going down the group, electrons are less tightly held; so there is some suggestion of metallic behavior in the heavier Group 16 elements.

The Group 17 elements, i.e., fluorine, chlorine , bromine, iodine, and astatine, all have high electronegativities and consequently show practically no metallic properties. Iodine, however, does show some metallic characteristics. Astatine may have some metallic properties, but it is a short-lived radioactive element, and measurements of its properties are difficult to carry out.

The Group 18 elements, or noble gases, consist of six gases: helium, neon, argon, krypton, xenon, and radon . The noble gases are nonmetals.

See also Alloy; Electrical conductivity; Element, chemical; Metallurgy.

Randall Frost

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Valence electrons

—The electrons in the outermost shell of an atom that determine an element's chemical properties.

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