A
metal is a
chemical
element that is a good
conductor of
both
electricity and
heat and forms
cations and
ionic bonds with
non-metals. In
chemistry,
a
metal (
Ancient
Greek métallon, μέταλλον) is an
element,
compound, or
alloy
characterized by high
electrical
conductivity. In a metal, atoms readily lose
electrons to form positive
ions
(
cations). Those ions are surrounded by
delocalized electrons, which are responsible for the conductivity.
The solid thus produced is held by electrostatic interactions
between the ions and the electron cloud, which are called
metallic bonds.
Definition
Metals are sometimes described as an arrangement of positive ions
surrounded by a cloud of delocalized
electrons. They are one of the three groups of
elements as distinguished by their ionization and bonding
properties, along with the
metalloids and
non-metals.
Metals occupy the bulk of the periodic table, while non-metallic
elements can only be found on the right-hand-side of the
Periodic Table of the
Elements. 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
semiconductors. This
is due to the fact that these elements exhibit
electrical properties common to both conductors
and insulators. Elements to the lower left of this division line
are called
metals, while elements to the
upper right of the division line are called
non-metals.
An alternative definition of metal refers to the
band theory. If one fills the energy bands of a
material with available electrons and ends up with a top band
partly filled then the material is a metal. 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 gray reflectiveness (luster) of elemental metals.
Properties
Chemical
Metals are usually inclined to form
cations
through electron loss, reacting with oxygen in the air to form
oxides over changing timescales (iron
rusts over years, while
potassium burns in seconds).Examples:
- 4 Na + O2 → 2 Na2O (sodium oxide)
- 2 Ca + O2 → 2 CaO (calcium oxide)
- 4 Al + 3 O2 → 2 Al2O3
(aluminium oxide)
The
transition metals (such as
iron,
copper,
zinc, and
nickel) take much
longer to oxidize. 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 generally
basic, as opposed to those of nonmetals,
which are
acidic.
Painting,
anodizing or
plating 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
Metals in general have high
electrical conductivity,
thermal conductivity,
luster and
density, and the ability to be deformed under stress
without
cleaving. While there are
several metals that have low density, hardness, and melting points,
these (the
alkali and
alkaline earth metals) are extremely
reactive, and are rarely encountered in their elemental, metallic
form. Optically speaking, metals are opaque, shiny and
lustrous. This is due to the fact that visible
lightwaves are not readily transmitted through the bulk of their
microstructure. The large number of free electrons in any typical
metallic solid (element or alloy) is responsible for the fact that
they can never be categorized as
transparent materials.
The majority of metals have higher
densities
than the majority of nonmetals. Nonetheless, there is wide
variation in the densities of metals;
lithium is the least dense solid element and
osmium is the densest. The metals of groups I
A and II A are referred to as the
light
metals because they are exceptions to this generalization. The
high density of most metals is due to the tightly packed crystal
lattice of the metallic structure. The strength of metallic bonds
for different metals reaches a maximum around the center of the
transition series, as those elements have large amounts of
delocalized electrons in a metallic bond. However, other factors
(such as
atomic radius,
nuclear charge, number of bonding
orbitals, overlap of orbital energies, and
crystal form) are involved as well.
Electrical
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 can be treated by various models, the simplest
being the
nearly free
electron model.
Mechanical
Mechanical properties of metals include their
ductility, which is largely due to their inherent
capacity for
plastic
deformation. Thus,
elasticity in metals can be described by
Hooke's Law for restoring forces, where
the
stress is linearly
proportional to the
strain. Larger forces in
excess of the
elastic limit may cause
a permanent (irreversible) deformation of the object. This is what
is known in the literature as
plastic deformation -- or
plasticity. This irreversible change in
atomic arrangement may occur as a result of either (or both) of the
following factors:
In the former case, the applied force may be
tensile (pulling) force,
compressive (pushing) force,
shear,
bending or
torsion (twisting) forces. In the latter
case, the most significant factor which is determined by the
temperature is the mobility of the
structural defects such as
grain boundaries, point vacancies, line and
screw
dislocations, stacking faults and
twins in both
crystalline and
non-crystalline solids. The movement or
displacement of such mobile
defects is
thermally activated, and thus
limited by the rate of
atomic
diffusion.
Viscous flow near grain boundaries, for
example, can give rise to internal
slip,
creep and
fatigue in metals. It can also contribute
to significant changes in the microstructure like
grain growth and localized densification due to
the elimination of intergranular
porosity.
Screw
dislocations may
slip in the direction of any
lattice plane containing the dislocation,
while the principal driving force for "dislocation climb" is the
movement or
diffusion of vacancies through
a
crystal lattice.
In addition, the nondirectional nature of metallic bonding is also
thought to contribute significantly to the ductility of most
metallic solids. When the planes of an
ionic
bond slide past one another, the resultant change in location
shifts ions of the same charge into close proximity, resulting in
the
cleavage of the crystal. Such shift is
not observed in
covalently bonded
crystals where fracture and crystal
fragmentation occurs.
Alloys
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
modifies 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. Of all the metallic alloys in use
today, the alloys of
iron (
steel,
stainless steel,
cast iron,
tool
steel,
alloy steel) make up the
largest proportion both by quantity and commercial value. Iron
alloyed with various proportions of carbon gives low, mid and high
carbon steels, with increasing carbon levels reducing ductility and
toughness. The addition of
silicon will
produce cast irons, while the addition of
chromium,
nickel and
molybdenum to carbon steels (more than 10%)
results in stainless steels.
Other significant metallic alloys are those of
aluminium,
titanium,
copper and
magnesium. Copper alloys have been known since the
Bronze Age, and have many applications
today, most importantly in electrical wiring. while the alloys of
the other three metals have been developed relatively recently -
chemical reactivity of these metals, requires modern electrolytic
extraction processes. The alloys of aluminium, titanium and
magnesium are also known and valued for their high
strength-to-weight ratios and, in the case of magnesium, for the
ability to provide
electromagnetic shielding. These
materials are ideal for situations where high strength-to-weight
ratios are more important than bulk cost, such as in aerospace and
in certain automotive applications.
Alloys specially designed for highly demanding applications, such
as
jet engines, may contain more than ten
elements.
Categories
Base metal
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
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
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, silver and
rhodium.
Precious metal
A gold nugget
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
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.
When a metal ore is an ionic compound of that metal and a
non-metal, the ore must usually be
smelted
— heated with a reducing agent — to extract the pure metal. Many
common metals, such as iron, are smelted using
carbon as a reducing agent. Some metals, such as
aluminium and
sodium, have no commercially
practical reducing agent, and are extracted using
electrolysis instead.
Sulfide ores are not reduced directly to the metal but are roasted
in air to convert them to oxides.
Metallurgy
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.
The two most commonly used structural metals, iron and aluminium,
are also the most abundant metals in the
Earth's crust.
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.
Trade
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
In the specialized usage of
astronomy and
astrophysics, the term "metal" is often
used to refer to any element other than
hydrogen or
helium, including
substances as chemically non-metallic as
neon,
fluorine, and
oxygen.
Nearly all the hydrogen and helium in the
Universe was created in
Big Bang nucleosynthesis, whereas
all the "metals" were produced by
nucleosynthesis in
stars
or
supernovae. The
Sun and the
Milky
Way Galaxy are composed of roughly 74% hydrogen, 24% helium,
and 2% "metals" (the rest of the elements; atomic numbers 3-118) by
mass.
See also
References
- Ductility - strength of materials
- Frank Kreith and Yogi Goswami, eds. (2004). The CRC
Handbook of Mechanical Engineering, 2nd edition. Boca Raton. p.
12-2.
- Structure of merchandise imports
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