Metals differ so widely in hardness, ductility (the potentiality of being drawn into wire), malleability, tensile strength, density, and melting point that a definite line of distinction between them and the nonmetals cannot be drawn. The hardest elemental metal is chromium; the softest, cesium. Copper, gold, platinum, and silver are especially ductile. Most metals are malleable; gold, silver, copper, tin, and aluminum are extremely so. Some metals exhibiting great tensile strength are copper, iron, and platinum. Three metals (lithium, potassium, and sodium) have densities of less than one gram per cubic centimeter at ordinary temperatures and are therefore lighter than water. Some heavy metals, beginning with the most dense, are osmium, iridium, platinum, gold, tungsten, uranium, tantalum, mercury, hafnium, lead, and silver.
For many industrial uses, the melting points of the metals are important. Tungsten fuses, or melts, only at extremely high temperatures (3,370°C;.), while cesium has a melting point of 28.5°C;. The best metallic conductor of electricity is silver. Copper, gold, and aluminum follow in the order named. All metals are relatively good conductors of heat; silver, copper, and aluminum are especially conductive. The radioactive metal uranium is used in reactor piles to generate steam and electric power. Plutonium, another radioactive element, is used in nuclear weapons and nuclear reactors as well as in pacemakers. Some of the radioactive metals not found in nature, e.g., fermium and seaborgium, are produced by nuclear bombardment.
Some elements, e.g., arsenic and antimony, exhibit both metallic and nonmetallic properties and are called metalloids. Furthermore, although all metals form crystals, this is also characteristic of certain nonmetals, e.g., carbon and sulfur.
Chemically, the metals differ from the nonmetals in that they form positive ions and basic oxides and hydroxides. Upon exposure to moist air, a great many undergo corrosion, i.e., enter into a chemical reaction; e.g., iron rusts when exposed to moist air, the oxygen of the atmosphere uniting with the metal to form the oxide of the metal. Aluminum and zinc do not appear to be affected, but in fact a thin coating of the oxide is formed almost at once, stopping further action and appearing unnoticeable because of its close resemblance to the metal. Tin, lead, and copper react slowly under ordinary conditions. Silver is affected by compounds such as sulfur dioxide and becomes tarnished when exposed to air containing them. The metals are combined with nonmetals in their salts, as in carbides, carbonates, chlorides, nitrates, phosphates, silicates, sulfides, and sulfates.The Electromotive Series
On the basis of their ability to be oxidized, i.e., lose electrons, metals can be arranged in a list called the electromotive series, or replacement series. Metals toward the beginning of the series, like cesium and lithium, are more readily oxidized than those toward the end, like silver and gold. In general, a metal will replace any other metal, or hydrogen, in a compound that it precedes in the series, and under ordinary circumstances it will be replaced by any metal, or hydrogen, that it follows.Metals in the Periodic Table
Metals fall into groups in the periodic table determined by similar arrangements of their orbital electrons and a consequent similarity in chemical properties. Groups of similar metals include the alkali metals (Group 1 in the periodic table), the alkaline-earth metals (Group 2 in the periodic table), and the rare-earth metals (the lanthanide and actinide series of Group 3). Most metals other than the alkali metals and the alkaline earth metals are called transition metals (see transition elements). The oxidation states, or valence, of the metal ions vary from +1 for the alkali metals to as much as +7 for some transition metals.
Although a few metals occur uncombined in nature, the great majority are found combined in their ores. The separation of metals from their ores is called extractive metallurgy. Metals are mixed with each other in definite amounts to form alloys; a mixture of mercury and another metal is called an amalgam. Bronze is an alloy of copper and tin, and brass contains copper and zinc. Steel is an alloy of iron and other metals with carbon added for hardness.
Since metals form positive ions readily, i.e., they donate their orbital electrons, they are used in chemistry as reducing agents (see oxidation and reduction). Finely divided metals or their oxides are often used as surface catalysts. Iron and iron oxides catalyze the conversion of hydrogen and nitrogen to ammonia in the Haber process. Finely divided catalytic platinum or nickel is used in the hydrogenation of unsaturated oils. Metal ions orient electron-rich groups called ligands around themselves, forming complex ions. Metal ions are important in many biological functions, including enzyme and coenzyme action, nucleic acid synthesis, and transport across membranes.
For the uses of specific metals, see separate articles.
In chemistry, a metal (Greek: Metallo, Μέταλλο) is defined as an element that readily loses electrons to form positive ions (cations) and forms metallic bonds between other metal atoms (forming ionic bonds with non-metals).
Metals are sometimes described as a lattice of positive ions surrounded by a cloud of delocalized electrons. They are one of the three groups of elements as distinguished by their ionization 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. Most elements on this line are metalloids, sometimes called semi-metals; elements to the lower left are metals; elements to the upper right are nonmetals (see the periodic table showing the metals).
An alternative 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 (luster) of elemental metals.
Painting, anodising or plating 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.
Metals in general have superior electric and thermal conductivity, high luster and density, and the ability to be deformed under stress without cleaving. While there are several metals that have low density, hardness, and melting points, these (the alkali and alkaline earth metals) are extremely reactive, and are rarely encountered in their elemental, metallic form.lithium is the least dense solid element and osmium is the densest. The metals of groups I A and II A are referred to as the light metals because they are exceptions to this generalization. The high density of most metals is due to the tightly-packed crystal lattice of the metallic structure. The strength of metallic bonds for different metals reaches a maximum around the center of the transition series, as those elements have large amounts of delocalized electrons in a metallic bond. However, other factors (such as atomic radius, nuclear charge, number of bonding orbitals, overlap of orbital energies, and crystal form) are involved as well.
When the planes of an ionic bond are slid past one another, the resultant change in location shifts ions of the same charge into close proximity, resulting in the cleavage of the crystal. Covalently bonded crystals can only be deformed by breaking the bonds between atoms, thereby resulting in fragmentation of the crystal.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. 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.
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.
Chemically, the precious metals are less reactive than most elements, have high luster 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.
Metals are often extracted from the Earth by means of mining, resulting in ores that are relatively rich sources of the requisite elements. Ore is located by prospecting techniques, followed by the exploration and examination of deposits. Mineral sources are generally divided into surface mines, which are mined by excavation using heavy equipment, and subsurface mines.
Once the ore is mined, the metals must be extracted, usually by chemical or electrolytic reduction. Pyrometallurgy uses high temperatures to convert ore into raw metals, while hydrometallurgy employs aqueous chemistry for the same purpose. The methods used depend on the metal and their contaminants.
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys.
Metals are good conductors, making them valuable in electrical appliances and for carrying an electric current over a distance with little energy lost. Electrical power grids rely on metal cables to distribute electricity. Home electrical systems, for the most part, are wired with copper wire for its good conducting properties.
The thermal conductivity of metal is useful for containers to heat materials over a flame. Metal is also used for heat sinks to protect sensitive equipment from overheating.
The high reflectivity of some metals is important in the construction of mirrors, including precision astronomical instruments. This last property can also make metallic jewelry aesthetically appealing.
Some metals have specialized uses; radioactive metals such as uranium and plutonium are used in nuclear power plants to produce energy via nuclear fission. Mercury is a liquid at room temperature and is used in switches to complete a circuit when it flows over the switch contacts. Shape memory alloy is used for applications such as pipes, fasteners and vascular stents.