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