Any of a class of chemical elements with specific physical and chemical characteristics. Metallic elements compose about 75% of the 112 elements in the periodic table of the elements and are found towards the left-hand side and the bottom of the table.
Physical properties include a sonorous tone when struck; good conduction of heat and electricity; high melting and boiling points; opacity but good reflection of light; malleability, which enables them to be cold-worked and rolled into sheets; and ductility, which permits them to be drawn into thin wires.
The majority of metals are found in nature in a combined form only, as compounds or mineral ores; about 16 of them also occur in the elemental form, as native metals. Their chemical properties are largely determined by the extent to which their atoms can lose one or more electrons and form positive ions (cations).
All metals except mercury are solid at ordinary temperatures, and all of them will crystallize under suitable conditions. The chief chemical properties of metals also include their strong affinity for certain non-metallic elements, for example sulphur and chlorine, with which they form sulphides and chlorides. Metals will, when fused, enter into the forming of alloys.
By comparing the reactions of metals with oxygen, water, acids, and other substances, the metals can be arranged in order of reactivity, known as the reactivity series of metals.
Metals replace the hydrogen in an acid to form a salt. For example, with magnesium and sulphuric acid the products are magnesium sulphate and hydrogen:
Mg + H2SO4 → MgSO4 + H2
Most metals form oxides. For example magnesium will react with oxygen to form magnesium oxide:
2Mg + O2 → 2MgO
Some metals displace hydrogen in water to form hydroxides or oxides. For example when sodium is added to water sodium hydroxide is formed and hydrogen is given off:
2Na + 2 H2O → 2NaOH + H2
Extraction of metals
The method of extraction of a metal from its ore depends on the reactivity of the metal. The more reactive the metal, the more difficult it is to decompose its compounds. Iron may be extracted from iron ore by heating with a reducing agent such as carbon or carbon monoxide; this is carried out in a blast furnace. Aluminium, a reactive metal, has to be extracted from its ore, bauxite, by electrolysis. The science and technology of producing metals is called metallurgy.
The following are widely used in commerce: precious metals - gold, silver, and platinum, used principally in jewellery, but platinum and platinum group metals are also very important as catalysts; heavy metals - iron, copper, zinc, tin, and lead, the common metals of engineering; rarer heavy metals - nickel, cadmium, chromium, tungsten, molybdenum, manganese, cobalt, vanadium, antimony, and bismuth, used principally for alloying with the heavy metals; light metals - aluminium and magnesium; alkali metals - sodium, potassium, and lithium; and alkaline-earth metals - calcium, barium, and strontium, used principally for chemical purposes.
Other metals have come to the fore because of special nuclear requirements - for example, technetium, produced in nuclear reactors, is corrosion-inhibiting; zirconium may replace aluminium and magnesium alloy in canning uranium in reactors.
Metals have been put to many uses since prehistoric times, both structural and decorative, and the Copper Age, Bronze Age, and Iron Age are named after the metal that formed the technological base for that stage of human evolution. For many centuries metals have also been used to make products. Their resistance to damage made them ideal for weapons, armour, cutting tools, and cooking pots.
Metals for making tend to be classed as either ferrous or non-ferrous. Ferrous metals are those that contain iron as the main part. Most ferrous metals are alloys of iron that are called steels. Steels differ in the amount of carbon that they contain. High carbon steel has around 1.5% carbon; this makes it very hard and it is used for saw blades, files, and lathe tools. Mild steel has around 0.35% carbon; this allows the steel to be bent and twisted into shapes. It is used to make sheet metal for domestic appliances such as washing machines and fridges. Stainless steel contains about 1% carbon. It also contains other metals such as chromium and nickel in order to stop it rusting. Cast iron contains 4% carbon; this makes the metal very strong but brittle. It is used to make grids. The purest form of iron is wrought iron which has less than 0.1% carbon. Wrought iron is used to make ornate gates and fences and other forge work.
Non-ferrous metals include copper and aluminium as well as other metal alloys such as brass and bronze. Copper is a good electrical and heat conductor, making it useful for electrical cabling and cooking utensils. Copper can be easily shaped. Aluminium is very light and bendy. It is usually alloyed with other metals to make it stronger. Aluminium alloy (for example, duralumin) is used for vehicle bodywork of aeroplanes, buses, and even warships. Other metals used in metalworking include tin, lead, and zinc.
Composite metals are commonly called alloys. Smart materials are often composite metal alloys. They are a new development in metal-based products which are being used more and more. The properties of metals are altered by alloying. They can also be changed by hammering which in metalwork is called work hardening. Case hardening is a process used to harden only the outside of mild steel. Annealing reduces the risk of a metal cracking as it is being work hardened. Normalizing is a special type of annealing. Quenching is used to cool a metal quickly. Tempering is a process that is used with steel.
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