Iron


 * This article is about metallic iron.

From Wikipedia, the free encyclopedia.

General Name, Symbol, Number iron, Fe, 26 Chemical series                              transition metal Group, Period, Block 8 (VIIIB), 4, d Density, Hardness 7874 kg/m3, 4.0 Appearance lustrous metallic with a greyish tinge Atomic properties Atomic weight                                55.845 amu Atomic radius (calc.)                         140 (156) pm Covalent radius                       125 pm   van der Waals radius                          no information Electron configuration                        [ Ar ] 3d64s2 e- 's per energy level 2, 8, 14, 2 Oxidation states (Oxide)                 2,3,4,6 (amphoteric) Crystal structure                            cubic, body centered Physical properties State of matter                          solid (ferromagnetic) Melting point                                1808 K (2795 °F) Boiling point                                3023 K (4982 °F) Molar volume                                 7.09 &times;10-6 m3/mol Heat of vaporization                         349.6 kJ/mol Heat of fusion                               13.8 kJ/mol Vapor pressure                               7.05 Pa at 1808 K   Speed of sound                             4910 m/s at 293.15 K  Miscellaneous Electronegativity                            1.83 (Pauling scale) Specific heat capacity                       440 J/(kg*K) Electrical conductivity                      9.93 106/(m·ohm) Thermal conductivity                         80.2 W/(m*K) 1st ionization potential          762.5 kJ/mol 2nd ionization potential              1561.9 kJ/mol 3rd ionization potential              2957  kJ/mol 4th ionization potential              5290  kJ/mol Most stable isotopes SI units & STP are used except where noted. Iron is a chemical element in the periodic table that has the symbol Fe and atomic number 26. Iron is group 8 and period 4 metal.

Notable characteristics
A typical iron atom has 56 times the mass of a typical hydrogen atom. Iron is the most abundant metal, and is believed to be the tenth most abundant element, in the universe. Iron is also the most abundant (by mass, 34.6%) element making up the Earth; the concentration of iron in the various layers of the Earth ranges from high at the inner core to about 5% in the outer crust; it is possible the Earth's inner core consists of a single iron crystal although it is more likely to be a mixture of iron and nickel; the large amount of iron in the Earth is thought to contribute to its magnetic field. Its symbol Fe is an abbreviation of ferrum, the Latin word for iron.

Iron is a metal extracted from iron ore, and is hardly ever found in the free (elemental) state. In order to obtain elemental iron, the impurities must be removed by chemical reduction. Iron is used in the production of steel, which is not an element but an alloy, a solution of different metals (and some non-metals, particularly carbon).

The nucleus of iron has the highest binding energy per nucleon, so it is the heaviest element that is produced exothermically through fusion and the lightest through fission. When a star that has sufficient mass to produce iron does so, it can no longer produce energy in its core and a supernova will ensue.

Cosmological models with an open universe predict that there will be a phase where as a result of slow fusion and fission reactions, everything will become iron.

Applications
Iron is the most used of all the metals, comprising 95 percent 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 takes 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% – 3.5% carbon 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. It is extremely strong, hard and brittle. Working cast iron, even white hot cast iron, tends to break the object.
 * Carbon steel contains between 0.5% and 1.5% carbon, with small amounts of manganese, sulfur, phosphorus, and silicon.
 * Wrought iron contains less than 0.5% 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.
 * Alloy steels contain varying amounts of carbon as well as other metals, such as chromium, vanadium, molybdenum, nickel, tungsten, etc.
 * Iron (III) oxides are used in the production of magnetic storage in computers. They are often mixed with other compounds, and retain their magnetic properties in solution.

History
The first signs of use of iron come from the Sumerians and the Egyptians, where around 4000 BC, small items, such as the tips of spears and ornaments, were being fashioned from iron recovered from meteorites. Because meteorites fall from the sky some linguists have conjectured that the English word iron, which has cognates in many northern and western European languages, derives from the Etruscan aisar which means "the gods".

By 3000 BC to 2000 BC, increasing numbers of smelted iron objects (distinguishable from meteoric iron by the lack of nickel in the product) appear in Mesopotamia, Anatolia, and Egypt. However, their use appears to be ceremonial, and iron was an expensive metal, more expensive than gold. In The Illiad, weaponry is mosty bronze, but iron ingots are used for trade. Some resources (see the reference What Caused the Iron Age? below) suggest that iron was being created then as a by-product of copper refining, as sponge iron, and was not reproducible by the metallurgy of the time. By 1600 BC to 1200 BC, iron was used increasingly in the Middle East, but did not supplant the dominant use of bronze.



In the period from the 12th to 10th century BC, there was a rapid transition in the Middle East from bronze to iron tools and weapons. The critical factor in this transition does not appear to be the sudden onset of a superior ironworking technology, but instead the disruption of the supply of tin. This period of transition, which occurred at different times in different parts of the world, is the ushering in of an age of civilization called the Iron Age.

Concurrent with the transition from bronze to iron was the discovery of carburization, which was the process of adding carbon to the irons of the time. Iron was recovered as sponge iron, a mix of iron and slag with some carbon and/or carbide, which was then repeatedly hammered and folded over to free the mass of slag and oxidise out carbon content, so creating the product wrought iron. Wrought iron was very low in carbon content and was not easily hardened by quenching. The people of the Middle East found that a much harder product could be created by the long term heating of a wrought iron object in a bed of charcoal, which was then quenched in water or oil. The resulting product, which had a surface of steel, was harder and less brittle than the bronze it began to replace.

In China the first irons used were also meteoric iron, with archeological evidence for items made of wrought iron appearing in the northwest, near Xinjiang, in the 8th century BC. These items were made of wrought iron, created by the same processes used in the Middle East and Europe, and were thought to be imported by non-Chinese people.

In the later years of the Zhou Dynasty (ca 550 BC), a new iron manufacturing capability began because of a highly developed kiln technology. Producing blast furnaces capable of temperatures exceeding 1300 K, the Chinese developed the manufacture of cast, or pig iron.

If iron ores are heated with carbon to 1420–1470 K, a molten liquid is formed, an alloy of about 96.5% iron and 3.5% carbon. This product is strong, can be cast into intricate shapes, but is too brittle to be worked, unless the product is decarburized to remove most of the carbon. The vast majority of Chinese iron manufacture, from the Zhou dynasty onward, was of cast iron. Iron, however, remained a pedestrian product, used by farmers for hundreds of years, and did not really affect the nobility of China until the Qin dynasty (ca 221 BC).

Cast iron development lagged in Europe, as the smelters could only achieve temperatures of about 1000 K. Through a good portion of the Middle Ages, in Western Europe, iron was still being made by the working of sponge iron into wrought iron. Some of the earliest casting of iron in Europe occurred in Sweden, in two sites, Lapphyttan and Vinarhyttan, between 1150 and 1350 AD. There are suggestions by scholars that the practice may have followed the Mongols across Russia to these sites, but there is no clear proof of this hypothesis. In any event, by the late fourteenth century, a market for cast iron goods began to form, as a demand developed for cast iron cannonballs.

Early iron smelting (as the process is called) used charcoal as both the heat source and the reducing agent. In 18th century England, wood supplies ran down and coke, a fossil fuel, was used as an alternative. This innovation by Abraham Darby supplied the energy for the Industrial Revolution.

Occurrence
Iron is one of the more 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.

Industrially, iron is extracted from its ores, principally hematite (nominally Fe2O3) and magnetite (Fe3O4) by 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 are fed into the top of the furnace, while a blast of heated air is forced into the funace at the bottom.

In the furnace, the coke reacts with oxygen in the air blast to produce carbon monoxide:


 * 2 C + O2 &rarr; 2 CO

The carbon monoxide reduces the iron ore (in the chemical equation below, hematite) to molten iron, becoming carbon dioxide in the process:


 * 3 CO + Fe2O3 &rarr; 2 Fe + 3 CO2

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 (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):


 * CaCO3 &rarr; CaO + CO2

Then calcium oxide combines then with silicon dioxide to form a slag.


 * CaO + SiO2 &rarr; CaSiO3

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 liquid iron, and spouts in the side of the furnace may be opened to drain off either the iron or the slag. 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.

Approximately 1100Mt (million tons) of iron ore was produced in the world in 2000, with a gross market value of approximately 25 billion US dollars. While ore production occurs in 48 countries, the five largest producers were China, Brazil, Australia, Russia and India, accounting for 70% of world iron ore production. The 1100Mt of iron ore was used to produce approximately 572Mt of pig iron.

Compounds
Common oxidation states of iron include:
 * the Iron (II) state, Fe2+, previously ferrous
 * the Iron (III) state, Fe3+, previously ferric
 * the Iron (IV) state, Fe4+, previously ferryl, stabilized in some enzymes (e.g. peroxidases)
 * Iron (VI) is also known, if rare. In the form of potassium-ferrate (K2FeO4) it is a selective oxidizer for primary alcohols. The solid is only stable under vacuum and dark purple in both (caustic) solution and as a solid.
 * iron carbide Fe3C is known as cementite.
 * see also iron oxide

Biological Role
Iron is essential to all organisms, except for a few bacteria.

Animals incorporate iron into the heme complex, an essential component of proteins involved in redox reactions, including respiration. Inorganic iron is also found in the iron-sulfur clusters of many enzymes, such as nitrogenase (Synthesis of ammonia from nitrogen and hydrogen) and hydogenase. A class of non-heme-iron enzymes is responsible for a wide range of functions within several life forms, such as methane monooxygenase (oxidizes methane to methanol), ribonucleotide reductase (reduces ribose to desoxyribose; DNA biosynthesis), hemerythrins (oxygen transport and fixation in marine invertebrae) and purple acid phosphatase (hydrolysis of phosphate esters).

Iron distribution is heavily regulated in mammals. For example, when the body is fighting a bacterial infection, the body "hides" iron so it cannot be used by the bacteria (see transferrin).

Good sources of dietary iron include meat, fish, poultry, lentils, beans, spinach, tofu, chickpeas and black-eyed peas.

Iron provided by dietary supplements is often found as Iron (II) fumarate. The RDA for iron varies considerably based on the age, gender, and source of dietary iron (heme-based iron has higher bioavailability). Also note the section below on.

Isotopes
Iron has four naturally-occurring stable isotopes, 54Fe, 56Fe, 57Fe and 58Fe. The relative abundances of the Fe isotopes in nature are approximately 54Fe (5.8%), 56Fe (91.7%), 57Fe (2.2%) and 58Fe (0.3%). 60Fe is an extinct radionuclide which had a long half-life (1.5 Myr). 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. The isotope 56Fe is of particular interest to nuclear scientists as it represents the most stable nucleus possible. It is not possible to perform fission or fusion on 56Fe and still liberate energy. This does not hold true for any other element. In phases of the meteorites Semarkona und 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 time formation of 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 (&minus;1/2). For this reason, 57Fe has application as a spin isotope in chemistry and biochemistry.

Precautions
Excessive dietary iron is toxic, because excess ferrous iron reacts with peroxides in the body, producing free radicals. When iron is in normal quantity, the body's own antioxidant mechanisms can control this process. In excess, uncontrollable quantities of free radicals are produced.

The lethal dose of iron in a 2 year old is about 3 grams of iron. One gram can induce severe poisoning. There are reported cases of children being poisoned by consuming between 10-50 tablets of ferrous sulfate over a few hour period. Overconsumption of iron is the single highest cause of death in children by unintentional ingestion of pharmaceuticals. The DRI lists the Tolerable Upper Intake Level (UL) for adults as 45 mg/day. For children under 14 years old the UL is 40 mg/day.

If iron intake is excessive a number of iron overload disorders can result, such as hemochromatosis. For this reason, people shouldn't take iron supplements unless they suffer from iron deficiency and have consulted a doctor. Blood donors are at special risk of low iron levels and are often recommended to supplement their iron intake.