Metal
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In chemistry, a
metal (
Greek:
Metallon) is an
element that readily forms
ions (cations) and has
metallic bonds. Metals are sometimes described as a lattice of positive ions (cations) surrounded by a cloud of delocalized electrons. The metals are one of the three groups of elements as distinguished by their ionisation and bonding properties, along with the
metalloids and
nonmetals. 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
aluminium,
copper,
gold,
iron,
lead,
silver,
titanium,
uranium, and
zinc.
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), although this may be considered a rule of thumb, rather than an absolute condition.
Painting or
anodising metals are good ways to prevent their
corrosion.
Traditionally, metals have certain characteristic physical properties: they are usually shiny (they have "lustre"), have a high
density, are
ductile and
malleable, usually have ahigh
melting point, are usually hard, and conduct
electricity and
heatwell. 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.
An
alloy is a mixture of two or more elements in
solid solution in which the major component is a metal. Most pure metals are either too soft, brittle or chemically reactive. Combining different ratios of metals as alloys modify the properties of pure metals to produce desirable characteristics. The aim of making alloys is to make them less brittle, harder, resistant to corrosion, or have a more desirable color and lustre. 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.
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.
A
precious metal is a rare
metallic chemical element of high
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
14 July 2006, valued ($324
USD per
ounce) at a little under half the price of gold ($665 USD/ounce), and platinum ($1,247 USD/ounce) at around twice that of gold. Silver is substantially less expensive ($11.47 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.
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.
*
Base metal*
Transition metal*
Electric field screening*
Metalworking*
Metallic bond*
Precious metal*
Metallurgy*
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