   #copyright

Metal

2007 Schools Wikipedia Selection. Related subjects: General Chemistry

   Hot metal work from a blacksmith
   Enlarge
   Hot metal work from a blacksmith

   In chemistry, a metal ( Greek: Metallon) is an element that readily
   forms positive ions (cations) and has metallic bonds. Metals 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 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 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.

   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 (for solid
   nonmetals), lack lustre, and are insulators.

Occurrence

   Nonmetal elements are more commonplace on Earth than metallic elements
   are, but metals in fact constitute most of the periodic table. Some
   popular metals are aluminium, copper, gold, iron, lead, silver,
   titanium, uranium and zinc.

Chemical properties

   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.
   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 "lustre"), 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.

   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 arangement 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

   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 to make them less brittle, harder, resistant to corrosion, or have a
   more desirable colour 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.

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.

Precious metal

   A gold nugget
   Enlarge
   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 lustre 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.

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 metallicity.

   Retrieved from " http://en.wikipedia.org/wiki/Metal"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
