Difference between revisions of "Metal" - New World Encyclopedia

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==Alloys==
 
==Alloys==
  
An [[alloy]] is a [[mixture]] with metallic properties that contains at least one metal element. Examples of alloys are [[steel]] ([[iron]] and [[carbon]]), [[brass]] (copper and zinc), [[bronze]] ([[copper]] and tin), and [[duralumin]] (aluminium and copper). Alloys specially designed for highly demanding applications, such as [[jet engine]]s, may contain more than ten elements.
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An '''alloy''' is a combination of two or more [[chemical elements]], at least one of which is a metal, and where the resulting material has metallic properties. Examples of alloys are [[steel]]* ([[iron]] and [[carbon]]), [[brass]]* ([[copper]] and [[zinc]]), [[bronze]]* (copper and [[tin]]), and [[duralumin]] ([[aluminum]] and copper). The resulting metallic substance generally has properties significantly different from those of its components.
  
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An alloy with two components is called a ''binary alloy''; one with three is a ''ternary alloy''; one with four is a ''quaternary alloy''. Alloys specially designed for highly demanding applications, such as [[jet engine]]s, may contain more than 10 elements.
An '''alloy''' is a combination, either in [[solution]] or [[chemical compound|compound]], of two or more [[chemical element|elements]], at least one of which is a [[metal]], and where the resulting material has [[metallic]] properties.  An alloy with two components is called a binary alloy; one with three is a ternary alloy; one with four is a quaternary alloy. The resulting metallic substance generally has properties significantly different from those of its components.
 
  
Alloys are usually designed to have properties that are more desirable than those of their components. For instance, [[steel]] is stronger than [[iron]], one of its main elements. It 'inherits' the characteristics of the elements it was made from.
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An alloy "inherits" the characteristics of the elements it was made from, but it is usually designed to have properties more desirable than those of its components. For instance, steel is stronger than iron, one of its main components.
  
Unlike pure metals, most alloys do not have a single [[melting point]]. Instead, they have a melting range in which the material is a mixture of [[solid]] and [[liquid]] phases. The temperature at which melting begins is called the [[solidus]], and that at which melting is complete is called the [[liquidus]]. However, for most pairs of elements, there is a particular ratio which has a single melting point, and this is called a [[eutectic]] mixture.
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Unlike pure metals, most alloys do not have a sharp [[melting point]]*. Instead, they have a melting range in which the material becomes a mixture of [[solid]]* and [[liquid]]* phases. The temperature at which melting begins is called the ''solidus'', and that at which melting is complete is called the ''liquidus''. For most pairs of elements, however, there is a particular ratio that has a single melting point, and this is called a ''eutectic'' mixture.
  
In practice, some alloys are used so predominantly with respect to their base metals that the name of the primary constituent is also used as the name of the alloy. For example, 14 [[Carat (purity)|carat]] (58%) [[gold]] is an alloy of gold with other elements. Similarly, the [[silver]] used in [[jewellery]] and the [[aluminium]] used as a structural building material are also alloys.
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In practice, some alloys are used so predominantly with respect to their primary metals that the alloy is named after the primary constituent. For example, 14 carat (58%) gold is an alloy of gold with other elements. Similarly, the silver used in [[jewelry]]* and the aluminum used as a structural material are alloys.
 
 
The term "alloy" is sometimes used in everyday speech as a synonym for "aluminium alloy"; one example of such usage is the "alloy wheels" which might be fitted to an automobile. (This usage is obviously inaccurate, since all steels and most other metals in practical use are also alloys.)
 
  
 
== Base metals ==
 
== Base metals ==
  
In [[chemistry]], the term '''base metal''' is used informally to refer to a [[metal]] that [[oxidation|oxidizes]] or [[corrosion|corrode]] relatively easily, and react variably with dilute [[hydrochloric acid]] (HCl) to form [[hydrogen]]. Examples include [[iron]], [[nickel]], [[lead]] and [[zinc]]. [[Copper]] is considered a base metal as it oxidizes relatively easily, although it does not react with HCl. It is commonly used in opposition to [[noble metal]].
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In [[alchemy]], the term '''base metal''' was used as a designation for common and inexpensive metals, to draw a contrast with '''[[#Precious metals|precious metals]]''' such as gold and silver. A long-cherished goal of the alchemists was the transmutation of base metals into precious metals.
  
In [[alchemy]], a '''base metal''' was a common and inexpensive metal, as opposed to [[precious metal]]s, mainly [[gold]] and [[silver]]. A longtime goal of the alchemists was the [[transmutation]] of base metals into precious metals.
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In chemistry today, the term ''base metal'' is used informally to refer to a metal that oxidizes or corrodes relatively easily and reacts variably with dilute [[hydrochloric acid]]* (HCl) to form [[hydrogen]]. Examples include [[iron]], [[nickel]], [[lead]], and [[zinc]]. [[Copper]], too, is considered a base metal because it oxidizes relatively easily, although it does not react with HCl. Metals that resist oxidation or corrosion are called '''noble metals''', which also tend to be [[#Precious metals|precious metals]].
  
In [[numismatics]], coins used to derive their value primarily from the [[precious metal]] content. Most modern currencies are [[fiat currency]], allowing the coins to be made of '''base metal'''.
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In the past, coins used to derive their value primarily from their precious metal content. Most modern currencies, however, are fiat currency—money that is given legal tender status. This approach allows coins to be made of base metals.
  
 
== Precious metals ==
 
== Precious metals ==

Revision as of 21:34, 15 July 2006

Hot metal work from a blacksmith

Traditionally, the term metal ([from the [Greek language|Greek]] word metallon) has been applied to a chemical element that has a shiny surface and is a good conductor of heat and electricity. These properties, however, can vary from one metal to the next. More recently, chemists have recognized that the main distinguishing features of a metal are (a) the ability of its atoms to lose some of their outermost electrons to form cations, and (b) the bonding of its atoms by what are called metallic bonds.

Metals form one of three groups of elements—the other two being nonmetals and metalloids. These groups are distinguished by their ionization and bonding properties. On the periodic table, a diagonal line drawn from boron (B) to polonium (Po) separates the metals from the nonmetals. Elements on this line are metalloids, sometimes called semi-metals; elements to the lower left are metals; elements to the upper right are nonmetals. In nature, nonmetals are more abundant than metals, but most elements in the periodic table are metals. Some well-known metals are aluminum, calcium, copper, gold, iron, lead, platinum, silver, titanium, uranium, and zinc.


  • A more modern definition of metals is that they have overlapping conduction bands and valence bands in their electronic structure. This definition opens up the category for metallic polymers and other organic metals, which have been made by researchers and employed in high-tech devices. These synthetic materials often have the characteristic silvery-grey reflectiveness of elemental metals.


Properties

All metals, except mercury, are solids at room temperature. In addition, the colors of metals tend to range from silvery white to gray—the two exceptions are copper and gold. As mentioned above, metals tend to be lustrous (shiny) and good conductors of heat and electricity. They also conduct sound well. Furthermore, they are usually ductile and malleable—that is, they can be readily drawn into wires and beaten into sheets. Solid nonmetals, on the other hand, are generally brittle, lack luster, and are insulators.

Metals are usually thought of as being hard substances, high in density and melting point. It should be noted, however, that there are metals that are soft, low in density, and low in melting point, but they are also quite reactive, and we rarely encounter them in their elemental, metallic form.

The property of electrical conductivity is mainly because each atom exerts only a loose hold on its outermost electrons, called the valence electrons). Thus, a pure metal may be thought of as a lattice of positively charged ions (cations) surrounded by a cloud of delocalized electrons.

Most metals are chemically unstable, reacting with oxygen in the air over varying time scales. The alkali metals (in Group 1 of the periodic table) react quickest, followed by the alkaline earth metals (in group 2). The transition metals—such as iron, copper, zinc, nickel—take much longer to oxidize. For example, potassium burns in seconds, but iron rusts over periods of months and years.

Other metals—such as palladium, platinum, and gold—do not react with the atmosphere at all. Some metals form a barrier layer of oxide on their surface which cannot be penetrated by further oxygen molecules and thus retain their shiny appearance and good conductivity for many decades (like aluminium, some steels, and titanium). The oxides of metals are basic (as opposed to those of nonmetals, which are acidic).

Alloys

An alloy is a combination of two or more chemical elements, at least one of which is a metal, and where the resulting material has metallic properties. Examples of alloys are steel (iron and carbon), brass (copper and zinc), bronze (copper and tin), and duralumin (aluminum and copper). The resulting metallic substance generally has properties significantly different from those of its components.

An alloy with two components is called a binary alloy; one with three is a ternary alloy; one with four is a quaternary alloy. Alloys specially designed for highly demanding applications, such as jet engines, may contain more than 10 elements.

An alloy "inherits" the characteristics of the elements it was made from, but it is usually designed to have properties more desirable than those of its components. For instance, steel is stronger than iron, one of its main components.

Unlike pure metals, most alloys do not have a sharp melting point. Instead, they have a melting range in which the material becomes a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. For most pairs of elements, however, there is a particular ratio that has a single melting point, and this is called a eutectic mixture.

In practice, some alloys are used so predominantly with respect to their primary metals that the alloy is named after the primary constituent. For example, 14 carat (58%) gold is an alloy of gold with other elements. Similarly, the silver used in jewelry and the aluminum used as a structural material are alloys.

Base metals

In alchemy, the term base metal was used as a designation for common and inexpensive metals, to draw a contrast with precious metals such as gold and silver. A long-cherished goal of the alchemists was the transmutation of base metals into precious metals.

In chemistry today, the term base metal is used informally to refer to a metal that oxidizes or corrodes relatively easily and reacts variably with dilute hydrochloric acid (HCl) to form hydrogen. Examples include iron, nickel, lead, and zinc. Copper, too, is considered a base metal because it oxidizes relatively easily, although it does not react with HCl. Metals that resist oxidation or corrosion are called noble metals, which also tend to be precious metals.

In the past, coins used to derive their value primarily from their precious metal content. Most modern currencies, however, are fiat currency—money that is given legal tender status. This approach allows coins to be made of base metals.

Precious metals

A gold nugget

A precious metal is a rare metallic chemical element of high, durable economic value.

Chemically, the precious metals are less reactive than most elements, have high luster, and have higher melting points than other metals. Historically, precious metals were important as currency, but are now regarded mainly as investment and industrial commodities. Gold, silver, platinum and palladium each have an ISO 4217 currency code.

The best-known precious metals are gold and silver. While both have industrial uses, they are better known for their uses in art, jewelry, and coinage. Other precious metals include the Platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum, of which platinum is the most widely traded. Plutonium and uranium could also be considered precious metals.

The demand for precious metals is driven not only by their practical use, but also by their role as investments and a store of value. Palladium is, as of May 8 2006, valued ($371 USD per ounce) at a little under half the price of gold ($682 USD/ounce), and platinum ($1,188 USD/ounce) at around twice that of gold. Silver is substantially less expensive ($13.93 USD/ounce) than these metals, presently at 1/49 the price of gold, but is often traditionally considered a precious metal for its role in coinage and jewelry.

Bullion

A 500 gram silver bullion bar produced by Johnson Matthey

Precious metals in bulk form are known as bullion, and are traded on commodity markets. Bullion metals may be cast into ingots, or minted into coins. The defining attribute of bullion is that it is valued by its mass and purity rather than by a face value as money.

American Platinum Eagle bullion coin.

Many nations mint bullion coins, of which the most famous is probably the gold South African Krugerrand. Although nominally issued as legal tender, these coins' face value as currency is far below that of their value as bullion. For instance, the United States mints a gold bullion coin (the Gold Eagle) at a face value of $50 containing 1 troy ounce (31.1035 g) of gold — as of January 2006, this coin is worth about $550 as bullion. Bullion coins' minting by national governments gives them some numismatic value in addition to their bullion value, as well as certifying their purity. The level of purity varies from country to country, with some bullion coins of as pure as 99.99% available, such as the Canadian Gold Maple Leaf. Note that a 100% pure bullion is not possible, as absolute purity in extracted and refined metals can only be asymptotically approached.

One of the largest bullion coins in the world is a 10,000 Australian dollar coin minted in Australia which consists of a full kilogram of 99.9% pure gold; however China has produced coins in very limited quantities (less than 20 pieces minted) that exceed 260 troy ounces (8 kg) of gold.

Gold as an investment and silver as an investment are often seen as a hedge against both inflation and economic downturn. Silver bullion coins have become popular with coin collectors due to their relative affordability, and unlike most gold and platinum issues which are valued based upon the markets, silver issues are more often valued as collectables, far higher than their actual bullion value.

Precious metal status

Gold bars from the Bank of Sweden

A given metal is precious if it is rare. If mining or refining processes improve, or new supplies are discovered and exploited, the value of such a metal declines.

An interesting case of a precious metal going common is that of aluminium. Aluminium was, when it was first discovered, extremely difficult to separate from the ore it was part of and, since the whole of the Earth's aluminium was bound up in the form of compounds, the most difficult metal on earth to get, despite the fact that it is one of the planet's most common.

For a while, aluminium was more valuable than gold; bars of aluminium were exhibited alongside the French crown jewels at the Exposition Universelle (1855). Napoleon is said to have used aluminum plates for his most honored guests at dinner. However, the price dropped continually and collapsed altogether when an easy extraction method, the Hall-Héroult process, was discovered in 1886.

The rarity of various metals may again be in for a shift, however. Meanwhile, silver is in a structural supply deficit, with 300 million troy ounces (9,000,000 kg) more being consumed each year than is mined—it may currently be more rare than gold [1].

Astronomy

In the specialised usage of astronomy and astrophysics, the term "metal" is often used to refer to any element other than hydrogen or helium, and includes substances as chemically non-metallic as neon, fluorine, and oxygen. Because the temperatures of stellar bodies allow practically no solid or liquid matter, and little normal chemistry can exist at temperatures that break down almost all chemical bonds, the term "metal" refers to materials that result from the triple-helium process which are much scarcer than hydrogen and helium in stars in the Main Sequence.

See metal-rich.

See also

External link

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