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| Name, Symbol, Number | iron, Fe, 26 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Chemical series | transition metals | ||||||||||||||||||||||||||||||||||||||||||||||||
| Group, Period, Block | 8, 4, d | ||||||||||||||||||||||||||||||||||||||||||||||||
| Appearance | lustrous metallic with a grayish tinge  | ||||||||||||||||||||||||||||||||||||||||||||||||
| Standard atomic weight | 55.845(2) g·mol−1 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Electron configuration | [Ar] 4s2 3d6 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Electrons per shell | 2, 8, 14, 2 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Physical properties | |||||||||||||||||||||||||||||||||||||||||||||||||
| Phase | solid | ||||||||||||||||||||||||||||||||||||||||||||||||
| Density (near r.t.) | 7.86 g·cm−3 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Liquid density at m.p. | 6.98 g·cm−3 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Melting point | 1811 K (1538 °C, 2800 °F) | ||||||||||||||||||||||||||||||||||||||||||||||||
| Boiling point | 3134 K (2861 °C, 5182 °F) | ||||||||||||||||||||||||||||||||||||||||||||||||
| Heat of fusion | 13.81 kJ·mol−1 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Heat of vaporization | 340 kJ·mol−1 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Heat capacity | (25 °C) 25.10 J·mol−1·K−1 | ||||||||||||||||||||||||||||||||||||||||||||||||
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| Atomic properties | |||||||||||||||||||||||||||||||||||||||||||||||||
| Crystal structure | body-centered cubic a=286.65 pm; face-centered cubic between 1185–1667 K | ||||||||||||||||||||||||||||||||||||||||||||||||
| Oxidation states | 2, 3, 4, 6 (amphoteric oxide) | ||||||||||||||||||||||||||||||||||||||||||||||||
| Electronegativity | 1.83 (Pauling scale) | ||||||||||||||||||||||||||||||||||||||||||||||||
| Ionization energies (more) | 1st: 762.5 kJ·mol−1 | ||||||||||||||||||||||||||||||||||||||||||||||||
| 2nd: 1561.9 kJ·mol−1 | |||||||||||||||||||||||||||||||||||||||||||||||||
| 3rd: 2957 kJ·mol−1 | |||||||||||||||||||||||||||||||||||||||||||||||||
| Atomic radius | 140 pm | ||||||||||||||||||||||||||||||||||||||||||||||||
| Atomic radius (calc.) | 156 pm | ||||||||||||||||||||||||||||||||||||||||||||||||
| Covalent radius | 125 pm | ||||||||||||||||||||||||||||||||||||||||||||||||
| Miscellaneous | |||||||||||||||||||||||||||||||||||||||||||||||||
| Magnetic ordering | ferromagnetic | ||||||||||||||||||||||||||||||||||||||||||||||||
| 1043 K | |||||||||||||||||||||||||||||||||||||||||||||||||
| Electrical resistivity | (20 °C) 96.1 nΩ·m | ||||||||||||||||||||||||||||||||||||||||||||||||
| Thermal conductivity | (300 K) 80.4 W·m−1·K−1 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Thermal expansion | (25 °C) 11.8 µm·m−1·K−1 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Speed of sound (thin rod) | (r.t.) (electrolytic) 5120 m·s−1 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Young's modulus | 211 GPa | ||||||||||||||||||||||||||||||||||||||||||||||||
| Shear modulus | 82 GPa | ||||||||||||||||||||||||||||||||||||||||||||||||
| Bulk modulus | 170 GPa | ||||||||||||||||||||||||||||||||||||||||||||||||
| Poisson ratio | 0.29 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Mohs hardness | 4.0 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Vickers hardness | 608 MPa | ||||||||||||||||||||||||||||||||||||||||||||||||
| Brinell hardness | 490 MPa | ||||||||||||||||||||||||||||||||||||||||||||||||
| CAS registry number | 7439-89-6 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Selected isotopes | |||||||||||||||||||||||||||||||||||||||||||||||||
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| References | |||||||||||||||||||||||||||||||||||||||||||||||||
Iron (IPA: /ˈaɪə(ɹ)n/) is a chemical element with the symbol Fe (Latin: ferrum) and atomic number 26. Iron is a group 8 and period 4 metal. Iron is a lustrous, silvery soft metal. Iron and nickel are notable for being the final elements produced by stellar nucleosynthesis, and thus are the heaviest elements which do not require a supernova or similarly cataclysmic event for formation. Iron and nickel are therefore the most abundant metals in metallic meteorites and in the dense-metal cores of planets such as Earth.
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[edit] Characteristics
Iron is believed to be the sixth most abundant element in the universe, and the fourth most abundant on earth. The concentration of iron in the various layers in the structure of the Earth ranges from high (probably greater than 80%, perhaps even a nearly pure iron crystal) at the inner core, to only 5% in the outer crust. Iron is second in abundance to aluminium among the metals and fourth in abundance in the crust. Iron is the most abundant element by mass of our entire planet, making up 35% of the mass of the Earth as a whole.
Iron is a metal extracted from iron ore, and is almost never found in the free elemental state. In order to obtain elemental iron, the impurities must be removed by chemical reduction. Iron is the main component of steel, and it is used in the production of alloys or solid solutions of various metals, as well as some non-metals, particularly carbon. The many iron-carbon alloys, which have very different properties, are discussed in the article on steel.
Nuclei of iron have some of the highest binding energies per nucleon, surpassed only by the nickel isotope 62Ni. The universally most abundant of the highly stable nuclides is, however, 56Fe. This is formed by nuclear fusion in stars. Although a further tiny energy gain could be extracted by synthesizing 62Ni, conditions in stars are unsuitable for this process to be favoured, and iron abundance on Earth greatly favors iron over nickel, and also presumably in supernova element production.[citation needed] When a very large star contracts at the end of its life, internal pressure and temperature rise, allowing the star to produce progressively heavier elements, despite these being less stable than the elements around mass number 60, known as the "iron group". This leads to a supernova.
Iron (as Fe2+, ferrous ion) is a necessary trace element used by all known living organisms. Iron-containing enzymes, usually containing heme prosthetic groups, participate in catalysis of oxidation reactions in biology, and in transport of a number of soluble gases. See hemoglobin, cytochrome, and catalase.
[edit] Applications
Iron is the most used of all the metals, comprising 95% of all the metal tonnage produced worldwide. Its combination of low cost and high strength make it indispensable, especially in applications like automobiles, the hulls of large ships, and structural components for buildings. Steel is the best known alloy of iron, and some of the forms that iron can take include:
- Pig iron has 4% – 5% carbon and contains varying amounts of contaminants such as sulfur, silicon and phosphorus. Its only significance is that of an intermediate step on the way from iron ore to cast iron and steel.
- Cast iron contains 2% – 4.0% carbon , 1% – 6% silicon , and small amounts of manganese. Contaminants present in pig iron that negatively affect the material properties, such as sulfur and phosphorus, have been reduced to an acceptable level. It has a melting point in the range of 1420–1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together. Its mechanical properties vary greatly, dependent upon the form carbon takes in the alloy. 'White' cast irons contain their carbon in the form of cementite, or iron carbide. This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken carbide, a very pale, silvery, shiny material, hence the appellation. In grey iron the carbon exists free as fine flakes of graphite, and also renders the material brittle due to the stress-raising nature of the sharp edged flakes of graphite. A newer variant of grey iron, referred to as ductile iron is specially treated with trace amounts of magnesium to alter the shape of graphite to spheroids, or nodules, vastly increasing the toughness and strength of the material.
- Carbon steel contains between 0.4% and 1.5% carbon, with small amounts of manganese, sulfur, phosphorus, and silicon.
- Wrought iron contains less than 0.2% carbon. It is a tough, malleable product, not as fusible as pig iron. It has a very small amount of carbon, a few tenths of a percent. If honed to an edge, it loses it quickly. Wrought iron is characterised, especially in old samples, by the presence of fine 'stringers' or filaments of slag entrapped in the metal. Wrought iron does not rust particularly quickly when used outdoors. It has largely been replaced by mild steel for "wrought iron" gates and blacksmithing. Mild steel does not have the same corrosion resistance but is cheaper and more widely available.
- Alloy steels contain varying amounts of carbon as well as other metals, such as chromium, vanadium, molybdenum, nickel, tungsten, etc. They are used for structural purposes, as their alloy content raises their cost and necessitates justification of their use. Recent developments in ferrous metallurgy have produced a growing range of microalloyed steels, also termed 'HSLA' or high-strength, low alloy steels, containing tiny additions to produce high strengths and often spectacular toughness at minimal cost.
- Iron(III) oxides are used in the production of magnetic storage media in computers. They are often mixed with other compounds, and retain their magnetic properties in solution.
The main drawback to iron and steel is that pure iron, and most of its alloys, suffer badly from rust if not protected in some way. Painting, galvanization, plastic coating and bluing are some techniques used to protect iron from rust by excluding water and oxygen or by sacrificial protection.
Iron is believed to be the critical missing nutrient in the ocean that limits the growth of plankton. Experimental iron fertilization of areas of the ocean using iron(II) sulfate has proven successful in increasing plankton growth[1][2][3]. Larger scaled efforts are being attempted with the hope that iron seeding and ocean plankton growth can remove carbon dioxide from the atmosphere, thereby counteracting the greenhouse effect that causes global warming[4].
[edit] Iron compounds
- See also iron compounds.
- Iron(III) ammonium oxalate (Fe(NH4)3(C2O4)4) is used in blueprints.
- Iron(III) arsenate (FeAsO4) is used in insecticide.
- Iron(III) chloride (FeCl3) is used: in water purification and sewage treatment, in the dyeing of cloth, as a coloring agent in paints, as an additive in animal feed, and as an etching material for engravement, photography and printed circuits.
- Iron(III) hydroxide (Fe(OH)3) is used as a brown pigment for rubber and in water purification systems.
- Iron(III) phosphate (FePO4) is used in fertilizer and as an additive and human and animal food.
- Iron(II) acetate (Fe(C2H3O2)2 is used in the dyeing of fabrics and leather, and as a wood preservative.
- Iron(II) gluconate (Fe(C6H11O7)2) is used as a dietary supplement in iron pills.
- Iron(II) oxalate (FeC2O4) is used as yellow pigment for paints, plastics, glass and ceramics, and in photography.
- Iron(II) sulfate (FeSO4) is used in water purification and sewage treatment systems, as a catalyst in the production of ammonia, as an ingredient in fertilizer and herbicide, as an additive in animal feed, in wood preservative and as an additive to flour to increase iron levels.
[edit] History
The first iron used by mankind, far back in prehistory, came from meteors. The smelting of iron in bloomeries probably began in Anatolia or the Caucasus in the second millennium BC or the latter part of the preceding one. Cast iron was first produced in China about 550 BC, but not in Europe until the medieval period. During the medieval period, means were found in Europe of producing wrought iron from cast iron (in this context known as pig iron) using finery forges. For all these processes, charcoal was required as fuel.
Steel (with a smaller carbon content than pig iron but more than wrought iron) was first produced in antiquity. New methods of producing it by carburizing bars of iron in the cementation process were devised in the 17th century AD. In the Industrial Revolution, new methods of producing bar iron without charcoal were devised and these were later applied to produce steel. In the late 1850s, Henry Bessemer invented a new steelmaking process, involving blowing air through molten pig iron, to produce mild steel. This and other 19th century and later processes have led to wrought iron no longer being produced.
[edit] Occurrence
Iron is one of the most common elements on Earth, making up about 5% of the Earth's crust. Most of this iron is found in various iron oxides, such as the minerals hematite, magnetite, and taconite. The earth's core is believed to consist largely of a metallic iron-nickel alloy. About 5% of the meteorites similarly consist of iron-nickel alloy. Although rare, these are the major form of natural metallic iron on the earth's surface.
The reason for Mars's red colour is thought to be an iron-oxide-rich soil.
See also Iron minerals.
[edit] Production of iron from iron ore
Industrially, iron is produced starting from iron ores, principally haematite (nominally Fe2O3) and magnetite (Fe3O4) by a carbothermic reaction (reduction with carbon) in a blast furnace at temperatures of about 2000 °C. In a blast furnace, iron ore, carbon in the form of coke, and a flux such as limestone (which is used to remove impurities in the ore which would otherwise clog the furnace with solid material) are fed into the top of the furnace, while a blast of heated air is forced into the furnace at the bottom.
In the furnace, the coke reacts with oxygen in the air blast to produce carbon monoxide:
The carbon monoxide reduces the iron ore (in the chemical equation below, hematite) to molten iron, becoming carbon dioxide in the process:
The flux is present to melt impurities in the ore, principally silicon dioxide sand and other silicates. Common fluxes include limestone (principally calcium carbonate) and dolomite (calcium-magnesium carbonate). Other fluxes may be used depending on the impurities that need to be removed from the ore. In the heat of the furnace the limestone flux decomposes to calcium oxide (quicklime):
Then calcium oxide combines with silicon dioxide to form a slag.
The slag melts in the heat of the furnace, which silicon dioxide would not have. In the bottom of the furnace, the molten slag floats on top of the more dense molten iron, and apertures in the side of the furnace are opened to run off the iron and the slag separately. The iron once cooled, is called pig iron, while the slag can be used as a material in road construction or to improve mineral-poor soils for agriculture.
Pig iron is not pure iron, but has 4-5% carbon dissolved in it. This is subsequently reduced to steel or commercially pure iron, known as wrought iron, using other furnaces or converters.
In 2005, approximately 1,544Mt (million tons) of iron ore was produced worldwide. China was the top producer of iron ore with atleast one-fourth world share followed by Brazil, Australia and India, reports the British Geological Survey.
[edit] Isotopes
Naturally occurring iron consists of four isotopes: 5.845% of radioactive 54Fe (half-life: >3.1×1022 years), 91.754% of stable 56Fe, 2.119% of stable 57Fe and 0.282% of stable 58Fe. 60Fe is an extinct radionuclide of long half-life (1.5 million years).
Much of the past work on measuring the isotopic composition of Fe has centered on determining 60Fe variations due to processes accompanying nucleosynthesis (i.e., meteorite studies) and ore formation. In the last decade however, advances in mass spectrometry technology have allowed the detection and quantification of minute, naturally-occurring variations in the ratios of the stable isotopes of iron. Much of this work has been driven by the Earth and planetary science communities, although applications to biological and industrial systems are beginning to emerge.[5]
The isotope 56Fe is of particular interest to nuclear scientists. A common misconception is that this isotope represents the most stable nucleus possible, and that it thus would be impossible to perform fission or fusion on 56Fe and still liberate energy. This is not true, as both 62Ni and 58Fe are more stable. However, since 56Fe is much more easily produced from lighter nuclei in nuclear reactions, it is the endpoint of fusion chains inside extremely massive stars and is therefore common in the universe, relative to other metals.
In phases of the meteorites Semarkona and Chervony Kut a correlation between the concentration of 60Ni, the daughter product of 60Fe, and the abundance of the stable iron isotopes could be found which is evidence for the existence of 60Fe at the time of formation of the solar system. Possibly the energy released by the decay of 60Fe contributed, together with the energy released by decay of the radionuclide 26Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of 60Ni present in extraterrestrial material may also provide further insight into the origin of the solar system and its early history. Of the stable isotopes, only 57Fe has a nuclear spin (−1/2).
[edit] Iron in organic synthesis
The usage of iron metal filings in organic synthesis is mainly for the reduction of nitro compounds.[6] Additionally, iron has been used for desulfurizations[7], reduction of aldehydes[8], and the deoxygenation of amine oxides[9].
[edit] Iron in biology
Iron is essential to nearly all known organisms. In cells, iron is generally stored in the centre of metalloproteins, because "free" iron -- which binds non-specifically to many cellular components -- can catalyse production of toxic free radicals.
In animals, plants, and fungi, iron is often incorporated into the heme complex. Heme is an essential component of cytochrome proteins, which mediate redox reactions, and of oxygen carrier proteins such as hemoglobin, myoglobin, and leghemoglobin. Inorganic iron also contributes to redox reactions in the iron-sulfur clusters of many enzymes, such as nitrogenase (involved in the synthesis of ammonia from nitrogen and hydrogen) and hydrogenase. Non-heme iron proteins include the enzymes methane monooxygenase (oxidizes methane to methanol), ribonucleotide reductase (reduces ribose to deoxyribose; DNA biosynthesis), hemerythri
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