Iron in three different forms.
The Iron Bridge at Coalbrookdale, Shropshire, England – the first major bridge in the world to be made of cast iron. It was built by Abraham Darby III in 1779
A contemporary photo of the Iron Bridge at Coalbrookdale.
Iron (Fe) is a silvery-white, lustrous, malleable, ductile, magnetic or magnetizable, metallic element. It lies in group VIII of the periodic table and is a transition element. Iron is the most abundant element in Earth's core and the fourth most abundant (by weight) in Earth's crust; it also occurs in meteorites. Iron nuclei are created mostly by Type Ia supernovae, with additional contributions from Type Ib, Ic, and II supernovae. Iron is, from the point of view of its nucleus, the most stable element.
|relative atomic mass||55.847|
|relative density||7.874 (at 20°C)|
|melting point||1,535°C (2,795°F)|
|boiling point||2,750°C (4,982°F)|
Combined iron is found most notably as hematite (Fe2O3), limonite (Fe2O3.H2O), magnetite (Fe3O4), and taconite, and also as siderite, goethite, chromite, and pyrite. It is extracted by smelting oxide ores in a blast furnace to produce pig iron which may be refined to produce cast iron or wrought iron, or converted to steel in the Bessemer process (now obsolete), open-hearth process, or Linz-Donawitz process.
Iron is nearly always used in the form of alloys, which are harder and stronger than the pure metal. Cast iron, wrought iron, and steel are the chief alloys made by adding carbon and small quantities of other elements to the iron. Except in special steels, such as stainless steel and tool steels, the percentage of iron in these alloys is very high – usually well over 95 percent. But it was only in the nineteenth century that any fairly cheap method of producing these alloys in large quantities was discovered.
Pure iron is very little used; it is chemically reactive, and oxidizes to rust in moist air. It has four allotropes (see allotropy). The stable oxidation states of iron are +2 (ferrous) and +3 (ferric), though +4 and +6 states are known. The ferrous ion (Fe2+) is pale green in aqueous solution; it is mild reducing agent, and does not readily form ligand complexes.
Pig iron is iron tapped from a blast furnace and cast into ingots known as pigs in preparation for conversion into steel, refined cast iron, or wrought iron. It has undergone no alloying or refinement.
Cast iron is a general term applied to various grades of iron, including gray iron and pig iron. It includes a wide range of iron-carbon-silicon alloys containing 1.7–4.5% carbon, with varying amounts of other elements, used for casting. Its properties depend largely on the composition and the annealing process used. Gray iron (so-called because its fracture looks grayish) is the most widely used for casting vehicle engines, machinery parts, and many other products.
Wrought iron is the other main commercial form of smelted iron. It contains less than 0.3% carbon with 1 or 2% slag mixed with it. Originally it was made from ore in a forge, and later in a 'puddling' furnace, where it never becomes molten. Wrought iron replaced bronze in Asia Minor (c. 2000 BC) at the beginning of the Iron Age. In the nineteeth century wrought iron began to be used in building construction, but was replaced by steel after the invention of the Bessemer process and open-hearth process. Although it has useful mechanical properties, little wrought iron is now manufactured.
Compounds of iron
Iron (II) sulfate (FeSO4.7H2O), also known as green vitriol or copperas, is a green crystalline solid, made by treating iron ore with sulfuric acid, used in tanning, in medicine to treat iron-deficiency anemia, and to make ink, fertilizers, pesticides, and other iron compounds. Melting point 64°C. Iron (III) sulfate is used in solution to stop bleeding, for example in pulpotomy of primary teeth.
Iron oxide is any of three compounds that exist in three different oxidation states: iron (II) oxide (ferrous oxide, FeO); iron (III) oxide (ferric oxide, Fe2O3); and ferrosoferric oxide (Fe3O4), which has iron in both oxidation states. Iron (III) oxide is a red-brown powder used as a pigment and as jewelers' rouge; it occurs naturally as hematite. Melting point 1,565°C.
Iron and life
Iron is required as a trace element by terrestrial living organisms. It is the most abundant metal in humans with healthy adults possessing some 3–4 grams. Most of this occurs in the oxygen-carrying pigment hemoglobin found in red blood cells. It is also a constituent as cytochromes.
Iron is contained in a variety of foods, such as liver, meat, cereals (especially whole-grain), fish, green leafy vegetables, nuts, and beans. During pregnancy, iron supplements may be necessary for the healthy development of the baby.
Iron as a construction material
The first cast-iron bridge, built over the River Severn at Coalbrookdale, England, in 1779, represented the beginnings of iron as a construction material. The key to cheap iron was the development of blast furnaces and a smelting process using coke instead of coal. This was introduced from 1709, also at Coalbrookdale, by the Englishman Abraham Darby.