metal


metal
metallike, adj.
/met"l/, n., v., metaled, metaling or (esp. Brit.) metalled, metalling.
n.
1. any of a class of elementary substances, as gold, silver, or copper, all of which are crystalline when solid and many of which are characterized by opacity, ductility, conductivity, and a unique luster when freshly fractured.
2. Chem.
a. such a substance in its pure state, as distinguished from alloys.
b. an element yielding positively charged ions in aqueous solutions of its salts.
3. an alloy or mixture composed wholly or partly of such substances, as brass.
4. an object made of metal.
5. formative material; stuff.
6. mettle.
7. Print.
a. See type metal.
b. the state of being set in type.
8. molten glass in the pot or melting tank.
9. Brit. See road metal.
v.t.
10. to furnish or cover with metal.
11. Brit. to pave or surface (a road) with broken stone.
[1250-1300; ME ( < OF) < L metallum quarry, metal < Gk métallon mine, quarry, metal]

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I
Any of a class of substances with, to some degree, the following properties: good heat and electricity conduction, malleability, ductility, high light reflectivity, and capacity to form positive ions in solution and hydroxides rather than acids when their oxides meet water.

About three-quarters of the elements are metals; these are usually fairly hard and strong crystalline (see crystal) solids with high chemical reactivity that readily form alloys with each other. Metallic properties increase from lighter to heavier elements in each vertical group of the periodic table and from right to left in each row. The most abundant metals are aluminum, iron, calcium, sodium, potassium, and magnesium. The vast majority are found as ores rather than free. The cohesiveness of metals in a crystalline structure is attributed to metallic bonding: The atoms are packed close together, with their very mobile outermost electrons all shared throughout the structure. Metals fall into the following classifications (not mutually exclusive and most not rigidly defined): alkali metals, alkaline earth metals, transition elements, noble (precious) metals, platinum metals, lanthanide (rare earth) metals, actinide metals, light metals, and heavy metals. Many have essential roles in nutrition or other biochemical functions, often in trace amounts, and many are toxic as both elements and compounds (see mercury poisoning, lead poisoning).
II
(as used in expressions)

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 any of a class of substances characterized by high electrical and thermal conductivity as well as by malleability, ductility, and high reflectivity of light.

      Approximately three-quarters of all known chemical elements are metals. The most abundant varieties in the Earth's crust are aluminum, iron, calcium, sodium, potassium, and magnesium. The vast majority of metals are found in ores (mineral-bearing substances), but a few such as copper, gold, platinum, and silver frequently occur in the free state because they do not readily react with other elements.

      Metals are usually crystalline solids. In most cases, they have a relatively simple crystal structure distinguished by a close packing of atoms and a high degree of symmetry. Typically, the atoms of metals contain less than half the full complement of electrons in their outermost shell. Because of this characteristic, metals tend not to form compounds with each other. They do, however, combine more readily with nonmetals (e.g., oxygen and sulfur), which generally have more than half the maximum number of valence electrons. Metals differ widely in their chemical reactivity. The most reactive include lithium, potassium, and radium, whereas those of low reactivity are gold, silver, palladium, and platinum.

      The high electrical and thermal conductivities of the simple metals (i.e., the non-transition metals of the periodic table) are best explained by reference to the free-electron theory. According to this concept, the individual atoms in such metals have lost their valence electrons to the entire solid, and these free electrons that give rise to conductivity move as a group throughout the solid. In the case of the more complex metals (i.e., the transition elements), conductivities are better explained by the band theory, which takes into account not only the presence of free electrons but also their interaction with so-called d electrons.

      The mechanical properties of metals, such as hardness, ability to resist repeated stressing (fatigue strength), ductility, and malleability, are often attributed to defects or imperfections in their crystal structure. The absence of a layer of atoms in its densely packed structure, for example, enables a metal to deform plastically, and prevents it from being brittle.

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Universalium. 2010.

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