| In chemistry, a metal (Greek: Metallon) is an | | | | than the coatee, the coating actually |
| element that readily forms positive ions | | | | promotes corrosion. |
| (cations) and has metallic bonds. Metals are | | | | |
| sometimes described as a lattice of positive | | | | Physical properties |
| ions surrounded by a cloud of delocalized | | | | |
| electrons. The metals are one of the three | | | | Traditionally, metals have certain |
| groups of elements as distinguished by their | | | | characteristic physical properties: they are |
| ionization and bonding properties, along with | | | | usually shiny (they have "luster"), have a |
| the metalloids and nonmetals. On the periodic | | | | high density, are ductile and malleable, |
| table, a diagonal line drawn from boron (B) | | | | usually have a high melting point, are |
| to polonium (Po) separates the metals from | | | | usually hard, and conduct electricity and |
| the nonmetals. Most elements on this line are | | | | heat well. However this is mainly because the |
| metalloids, sometimes called semi-metals; | | | | low density, soft, low melting point metals |
| elements to the lower left are metals; | | | | happen to be reactive, and we rarely |
| elements to the upper right are nonmetals. | | | | encounter them in their elemental, metallic |
| | | | form. Metals conduct sound well, that is, |
| A modern definition of metals is that they | | | | they are sonorous. |
| have overlapping conduction bands and valence | | | | |
| bands in their electronic structure. This | | | | The electrical and thermal conductivity of |
| definition opens up the category for metallic | | | | metals originate from the fact that in the |
| polymers and other organic metals, which have | | | | metallic bond the outer electrons of the |
| been made by researchers and employed in | | | | metal atoms form a gas of nearly free |
| high-tech devices. These synthetic materials | | | | electrons, moving as an electron gas in a |
| often have the characteristic silvery-grey | | | | background of positive charge formed by the |
| reflectiveness (luster) of elemental metals. | | | | ion cores. Good mathematical predictions for |
| | | | electrical conductivity, as well as the |
| The traditional definition focuses on the | | | | electrons' contribution to the heat capacity |
| bulk properties of metals. They tend to be | | | | and heat conductivity of metals can be |
| lustrous, ductile, malleable, and good | | | | calculated from the free electron model, |
| conductors of electricity, while nonmetals | | | | which does not take the detailed structure of |
| are generally brittle (if solid), lack | | | | the ion lattice into account. |
| luster, and are insulators. | | | | |
| | | | When considering the exact band structure and |
| Chemical properties | | | | binding energy of a metal, it is necessary to |
| | | | take into account the positive potential |
| Most metals are chemically reactive, reacting | | | | caused by the specific arrangement of the ion |
| with oxygen in the air to form oxides over | | | | cores - which is periodic in crystals. The |
| varying timescales (for example iron rusts | | | | most important consequence of the periodic |
| over years and potassium burns in seconds). | | | | potential is the formation of a small band |
| The alkali metals react quickest followed by | | | | gap at the boundary of the brillouin zone. |
| the alkaline earth metals, found in the | | | | Mathematically, the potential of the ion |
| leftmost two groups of the periodic table. | | | | cores is treated in the nearly-free electron |
| The transition metals take much longer to | | | | model. |
| oxidize (such as iron, copper, zinc, nickel). | | | | |
| Others, like palladium, platinum and gold, do | | | | Alloys |
| not react with the atmosphere at all. Some | | | | |
| metals form a barrier layer of oxide on their | | | | An alloy is a mixture of two or more elements |
| surface which cannot be penetrated by further | | | | in solid solution in which the major |
| oxygen molecules and thus retain their shiny | | | | component is a metal. Most pure metals are |
| appearance and good conductivity for many | | | | either too soft, brittle or chemically |
| decades (like aluminium, some steels, and | | | | reactive for practical use. Combining |
| titanium). The oxides of metals are basic (as | | | | different ratios of metals as alloys modify |
| opposed to those of nonmetals, which are | | | | the properties of pure metals to produce |
| acidic), although this may be considered a | | | | desirable characteristics. The aim of making |
| rule of thumb, rather than a fact. | | | | alloys is generally to make them less |
| | | | brittle, harder, resistant to corrosion, or |
| Painting or anodising metals are good ways to | | | | have a more desirable color and luster. |
| prevent their corrosion. However, a more | | | | Examples of alloys are steel (iron and |
| reactive metal in the electrochemical series | | | | carbon), brass (copper and zinc), bronze |
| must be chosen for coating, especially when | | | | (copper and tin), and duralumin (aluminium |
| chipping of the coating is expected. Water | | | | and copper). Alloys specially designed for |
| and the two metals form an electrochemical | | | | highly demanding applications, such as jet |
| cell, and if the coating is less reactive | | | | engines, may contain more than ten elements. |