Metal

From New World Encyclopedia
Revision as of 00:27, 15 July 2006 by Dinshaw Dadachanji (talk | contribs) (editing)
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 (usually) has a shiny surface and is a relatively good conductor of heat and electricity. 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.

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.

Nonmetal elements are more abundant in nature than are metallic elements, but metals in fact constitute most of the periodic table. Some well-known metals are aluminum, copper, gold, iron, lead, silver, titanium, uranium, and zinc.


All metals, except mercury, are solids at room temperature. In addition, all metals, except copper and gold, are silvery gray in color.


88888888888 The key feature that distinguishes metals from non-metals is their bonding. Metallic materials have free electrons. In the case of pure metals, the outermost layer of electrons is not bound to any given atom, instead these electrons are free to roam from atom to atom. Thus, the structure of metallic material can be thought of a consisting of positive centers (or ions) sitting in a “gas” of free-electrons 88888888888


Metals are sometimes described as a lattice of positive ions (cations) surrounded by a cloud of delocalized electrons.

The allotropes of metals tend to be lustrous, ductile, malleable, and good conductors, while nonmetals generally speaking are brittle (for solid nonmetals), lack luster, and are insulators.

The properties of conductivity are mainly because each atom exerts only a loose hold on its outermost electrons (valence electrons); thus, the valence electrons form a sort of sea around the close-packed metal nuclei cations.

Most metals are chemically unstable, reacting with oxygen in the air to form oxides over varying timescales (for example iron rusts over years and potassium burns in seconds). The alkali metals react quickest followed by the alkaline earth metals, found in the leftmost two groups of the periodic table. The transition metals take much longer to oxidise (such as iron, copper, zinc, nickel). Others, like 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).

Painting or anodising metals are good ways to prevent their corrosion.

Physical properties

Traditionally, metals have certain characteristic physical properties: they are usually shiny (they have "luster"), have a high density, are ductile and malleable, usually have a high melting point, are usually hard, and conduct electricity and heat well. However this is mainly because the low density, soft, low melting point metals happen to be reactive, and we rarely encounter them in their elemental, metallic form. Metals conduct sound well, that is, they are sonorous.

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 engines, may contain more than ten elements.

Base metals

In chemistry, the term base metal is used informally to refer to a metal that oxidizes or 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.

In alchemy, a base metal was a common and inexpensive metal, as opposed to precious metals, mainly gold and silver. A longtime goal of the alchemists was the transmutation of base metals into 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.

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

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.