A world made of metal


What is metal

In chemistry, a metal (Greek: Metallon)two metals form an electrochemical cell,
is an element that readily formsand if the coating is less reactive than
positive ions (cations) and has metallicthe coatee, the coating actually
bonds. Metals are sometimes described aspromotes corrosion.
a lattice of positive ions surrounded byPhysical properties
a cloud of delocalized electrons. TheTraditionally, metals have certain
metals are one of the three groups ofcharacteristic physical properties: they
elements as distinguished by theirare usually shiny (they have "luster"),
ionization and bonding properties, alonghave a high density, are ductile and
with the metalloids and nonmetals. Onmalleable, usually have a high melting
the periodic table, a diagonal linepoint, are usually hard, and conduct
drawn from boron (B) to polonium (Po)electricity and heat well. However this
separates the metals from the nonmetals.is mainly because the low density, soft,
Most elements on this line arelow melting point metals happen to be
metalloids, sometimes calledreactive, and we rarely encounter them
semi-metals; elements to the lower leftin their elemental, metallic form.
are metals; elements to the upper rightMetals conduct sound well, that is, they
are nonmetals.are sonorous.
A modern definition of metals is thatThe electrical and thermal conductivity
they have overlapping conduction bandsof metals originate from the fact that
and valence bands in their electronicin the metallic bond the outer electrons
structure. This definition opens up theof the metal atoms form a gas of nearly
category for metallic polymers and otherfree electrons, moving as an electron
organic metals, which have been made bygas in a background of positive charge
researchers and employed in high-techformed by the ion cores. Good
devices. These synthetic materials oftenmathematical predictions for electrical
have the characteristic silvery-greyconductivity, as well as the electrons'
reflectiveness (luster) of elementalcontribution to the heat capacity and
metals.heat conductivity of metals can be
The traditional definition focuses oncalculated from the free electron model,
the bulk properties of metals. They tendwhich does not take the detailed
to be lustrous, ductile, malleable, andstructure of the ion lattice into
good conductors of electricity, whileaccount.
nonmetals are generally brittle (ifWhen considering the exact band
solid), lack luster, and are insulators.structure and binding energy of a metal,
Chemical propertiesit is necessary to take into account the
Most metals are chemically reactive,positive potential caused by the
reacting with oxygen in the air to formspecific arrangement of the ion cores -
oxides over varying timescales (forwhich is periodic in crystals. The most
example iron rusts over years andimportant consequence of the periodic
potassium burns in seconds). The alkalipotential is the formation of a small
metals react quickest followed by theband gap at the boundary of the
alkaline earth metals, found in thebrillouin zone. Mathematically, the
leftmost two groups of the periodicpotential of the ion cores is treated in
table. The transition metals take muchthe nearly-free electron model.
longer to oxidize (such as iron, copper,Alloys
zinc, nickel). Others, like palladium,An alloy is a mixture of two or more
platinum and gold, do not react with theelements in solid solution in which the
atmosphere at all. Some metals form amajor component is a metal. Most pure
barrier layer of oxide on their surfacemetals are either too soft, brittle or
which cannot be penetrated by furtherchemically reactive for practical use.
oxygen molecules and thus retain theirCombining different ratios of metals as
shiny appearance and good conductivityalloys modify the properties of pure
for many decades (like aluminium, somemetals to produce desirable
steels, and titanium). The oxides ofcharacteristics. The aim of making
metals are basic (as opposed to those ofalloys is generally to make them less
nonmetals, which are acidic), althoughbrittle, harder, resistant to corrosion,
this may be considered a rule of thumb,or have a more desirable color and
rather than a fact.luster. Examples of alloys are steel
Painting or anodising metals are good(iron and carbon), brass (copper and
ways to prevent their corrosion.zinc), bronze (copper and tin), and
However, a more reactive metal in theduralumin (aluminium and copper). Alloys
electrochemical series must be chosenspecially designed for highly demanding
for coating, especially when chipping ofapplications, such as jet engines, may
the coating is expected. Water and thecontain more than ten elements.



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