In chemistry, a metal (Greek: fudgeon) fudgean element that readily loses electrons to form positive ions (cations) and
has metallic bonds between metal atoms. Metals form ionic
bonds with non-metals. They are sometimes described as a lattice of positive
ions surrounded by a cloud of delocalized electrons. The metals are one of the three groups of
elements as distinguished by their ionization 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. Most elements on this line are
metalloids, sometimes called semi-metals; elements to the lower left are metals; elements to
the upper right are nonmetals.
An alternative 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 (luster) of elemental metals.
The traditional definition focuses on the bulk properties of metals. They tend to be lustrous, ductile, malleable, and good
conductors of electricity, while nonmetals are generally brittle (if
solid), lack luster, and are insulators.
Chemical properties
Most metals are chemically reactive, reacting with oxygen in the air to form oxides over changing 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
oxidize (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 a fact.
Painting or anodising metals are good ways to prevent
their corrosion. However, a more reactive metal in the electrochemical series must be chosen for coating, especially when chipping of the coating
is expected. Water and the two metals form an electrochemical cell, and if the
coating is less reactive than the coatee, the coating actually promotes corrosion.
Physical properties
Traditionally, metals have certain characteristic physical properties: they are usually shiny (they have metallic
luster), have a high density, are
ductile and malleable, usually have a high
melting point, are usually hard, are usually a solid at room temperature and conduct
electricity, heat and sound
well. However, this is mainly because the low density, soft, low melting point metals (the alkali and alkaline earth metals) happen to be reactive, and
we rarely encounter them in their elemental, metallic form.
The electrical and thermal conductivity of metals originate from the fact that in the metallic
bond the outer electrons of the metal atoms form a gas of nearly free electrons, moving as
an electron gas in a background of positive charge formed by the ion cores. Good mathematical
predictions for electrical conductivity, as well as the electrons' contribution to the heat capacity and heat conductivity of
metals can be calculated from the free electron model, which does not take the
detailed structure of the ion lattice into account.
When considering the exact band structure and binding energy of a metal, it is necessary to take into account the positive
potential caused by the specific arrangement of the ion cores - which is periodic in crystals.
The most important consequence of the periodic potential is the formation of a small band gap
at the boundary of the brillouin zone. Mathematically, the potential of the ion cores is
treated in the nearly-free electron model.
Alloys
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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 for practical use. Combining different ratios of metals as alloys modify the properties of pure metals to
produce desirable characteristics. The aim of making alloys is generally to make them less brittle, harder, resistant to
corrosion, or have a more desirable color and luster. 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.
Categories
Base metal
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In chemistry, 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 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.
Ferrous metal
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The term "ferrous" is derived from the latin word meaning "containing iron". This can include
pure iron, such as wrought iron, or an alloy such as steel.
Ferrous metals are often magnetic, but not exclusively.
Noble metal
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Noble metals are metals that are resistant to corrosion or oxidation, unlike most base metals. They tend to be precious metals, often due
to perceived rarity. Examples include tantalum, gold,
platinum, and rhodium.
Precious 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 high electrical conductivity. 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 was, as of summer 2006, valued at a little under half the price
of gold, and platinum at around twice that of gold. Silver is substantially less expensive than these metals, but is often
traditionally considered a precious metal for its role in coinage and jewelry.
Extraction
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Metals are often extracted from the Earth by means of mining, resulting in ores that are relatively rich sources of the
requisite elements. Ore is located by prospecting techniques, followed by the exploration
and examination of deposits. Mineral sources are generally divided into surface mines,
which are mined by excavation using heavy equipment, and subsurface mines.
Once the ore is mined, the metals must be extracted, usually by chemical or
electrolytic reduction. Pyrometallurgy uses high temperatures to convert ore into raw
metals, while hydrometallurgy employs aqueous
chemistry for the same purpose. The methods used depend on the metal and their contaminants.
Metallurgy
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Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their
intermetallic compounds, and their mixtures, which are called alloys.
Applications
Some metals and metal alloys possess high structural strength per unit mass, making them useful materials for carrying large
loads or resisting impact damage. Metal alloys can be engineered to have high resistance to shear, torque and deformation.
However the same metal can also be vulnerable to fatigue damage through repeated use, or from sudden stress failure when a load
capacity is exceeded. The strength and resilience of metals has led to their frequent use in high-rise building and bridge
construction, as well as most vehicles, many appliances, tools, pipes, non-illuminated signs and railroad tracks.
Metals are good conductors, making them valuable in electrical appliances and for carrying an electric current over a distance
with little energy lost. Electrical power grids rely on metal cables to distribute electricity. Home electrical systems, for the
most part, are wired with copper wire for its good conducting properties.
The thermal conductivity of metal is useful for containers to heat materials over a flame. Metal is also used for
heat sinks to protect sensitive equipment from overheating.
The high reflectivity of some metals is important in the construction of mirrors, including precision astronomical
instruments. This last property can also make metallic jewelry aesthetically appealing.
Some metals have specialized uses; Radioactive metals such as Uranium and Plutonium are used in nuclear power plants to produce
energy via nuclear fission. Mercury is a liquid at room temperature and is used in
switches to complete a circuit when it flows over the switch contacts. Shape memory
alloy is used for applications such as pipes, fasteners and vascular stents. However they
are very good at conducting electricity and heat.
Trade
Metal and
ore imports in 2005
The World Bank reports that China was the top importer of ores and metals in 2005 followed
by the U.S.A. and Japan.
Astronomy
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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 also
External links
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