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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

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

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Valence electrons

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

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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 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 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 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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