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