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Jai Ganesh

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Registered: 2005-06-28

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Iron

Iron

Gist

Iron is a chemical element; it has symbol Fe (from Latin ferrum 'iron') and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's outer and inner core.

Summary

A shiny, greyish metal that rusts in damp air.

Uses

Iron is an enigma – it rusts easily, yet it is the most important of all metals. 90% of all metal that is refined today is iron.

Most is used to manufacture steel, used in civil engineering (reinforced concrete, girders etc) and in manufacturing.

There are many different types of steel with different properties and uses. Ordinary carbon steel is an alloy of iron with carbon (from 0.1% for mild steel up to 2% for high carbon steels), with small amounts of other elements.

Alloy steels are carbon steels with other additives such as nickel, chromium, vanadium, tungsten and manganese. These are stronger and tougher than carbon steels and have a huge variety of applications including bridges, electricity pylons, bicycle chains, cutting tools and rifle barrels.

Stainless steel is very resistant to corrosion. It contains at least 10.5% chromium. Other metals such as nickel, molybdenum, titanium and copper are added to enhance its strength and workability. It is used in architecture, bearings, cutlery, surgical instruments and jewellery.

Cast iron contains 3–5% carbon. It is used for pipes, valves and pumps. It is not as tough as steel but it is cheaper. Magnets can be made of iron and its alloys and compounds.

Iron catalysts are used in the Haber process for producing ammonia, and in the Fischer–Tropsch process for converting syngas (hydrogen and carbon monoxide) into liquid fuels.

Biological role

Iron is an essential element for all forms of life and is non-toxic. The average human contains about 4 grams of iron. A lot of this is in haemoglobin, in the blood. Haemoglobin carries oxygen from our lungs to the cells, where it is needed for tissue respiration.

Humans need 10–18 milligrams of iron each day. A lack of iron will cause anaemia to develop. Foods such as liver, kidney, molasses, brewer’s yeast, cocoa and liquorice contain a lot of iron.

Natural abundance

Iron is the fourth most abundant element, by mass, in the Earth’s crust. The core of the Earth is thought to be largely composed of iron with nickel and sulfur.

The most common iron-containing ore is haematite, but iron is found widely distributed in other minerals such as magnetite and taconite.

Commercially, iron is produced in a blast furnace by heating haematite or magnetite with coke (carbon) and limestone (calcium carbonate). This forms pig iron, which contains about 3% carbon and other impurities, but is used to make steel. Around 1.3 billion tonnes of crude steel are produced worldwide each year.

Details

Iron (Fe), chemical element, metal of Group 8 (VIIIb) of the periodic table, the most-used and cheapest metal.

Element Properties

atomic number  :  26
atomic weight  :  55.847
melting point  :  1,538 °C (2,800 °F)
boiling point  :  3,000 °C (5,432 °F)
specific gravity  :  7.86 (20 °C)
oxidation states  :  +2, +3, +4, +6

Occurrence, uses, and properties

Iron makes up 5 percent of Earth’s crust and is second in abundance to aluminum among the metals and fourth in abundance behind oxygen, silicon, and aluminum among the elements. Iron, which is the chief constituent of Earth’s core, is the most abundant element in Earth as a whole (about 35 percent) and is relatively plentiful in the Sun and other stars. In the crust the free metal is rare, occurring as terrestrial iron (alloyed with 2–3 percent nickel) in basaltic rocks in Greenland and carbonaceous sediments in the United States (Missouri) and as a low-nickel meteoric iron (5–7 percent nickel), kamacite. Nickel-iron, a native alloy, occurs in terrestrial deposits (21–64 percent iron, 77–34 percent nickel) and in meteorites as taenite (62–75 percent iron, 37–24 percent nickel). Meteorites are classified as iron, iron-stone, or stony according to the relative proportion of their iron and silicate-mineral content. Iron is also found combined with other elements in hundreds of minerals; of greatest importance as iron ore are hematite (ferric oxide, Fe2O3), magnetite (triiron tetroxide, Fe3O4), limonite (hydrated ferric oxide hydroxide, FeO(OH)∙nH2O), and siderite (ferrous carbonate, FeCO3). Igneous rocks average about 5 percent iron content. The metal is extracted by smelting with carbon (coke) and limestone.

The average quantity of iron in the human body is about 4.5 grams (about 0.004 percent), of which approximately 65 percent is in the form of hemoglobin, which transports molecular oxygen from the lungs throughout the body; 1 percent in the various enzymes that control intracellular oxidation; and most of the rest stored in the body (liver, spleen, bone marrow) for future conversion to hemoglobin. Red meat, egg yolk, carrots, fruit, whole wheat, and green vegetables contribute most of the 10–20 milligrams of iron required each day by the average adult. For the treatment of hypochromic anemias (caused by iron deficiency), any of a large number of organic or inorganic iron (usually ferrous) compounds are used.

Iron, as commonly available, nearly always contains small amounts of carbon, which are picked up from the coke during smelting. These modify its properties, from hard and brittle cast irons containing up to 4 percent carbon to more malleable low-carbon steels containing less than 0.1 percent carbon.

Three true allotropes of iron in its pure form occur. Delta iron, characterized by a body-centred cubic crystal structure, is stable above a temperature of 1,390 °C (2,534 °F). Below this temperature there is a transition to gamma iron, which has a face-centred cubic (or cubic close-packed) structure and is paramagnetic (capable of being only weakly magnetized and only as long as the magnetizing field is present); its ability to form solid solutions with carbon is important in steelmaking. At 910 °C (1,670 °F) there is a transition to paramagnetic alpha iron, which is also body-centred cubic in structure. Below 773 °C (1,423 °F), alpha iron becomes ferromagnetic (i.e., capable of being permanently magnetized), indicating a change in electronic structure but no change in crystal structure. Above 773 °C (its Curie point), it loses its ferromagnetism altogether. Alpha iron is a soft, ductile, lustrous, gray-white metal of high tensile strength.

Pure iron is quite reactive. In a very finely divided state metallic iron is pyrophoric (i.e., it ignites spontaneously). It combines vigorously with chlorine on mild heating and also with a variety of other nonmetals, including all of the halogens, sulfur, phosphorus, boron, carbon, and silicon (the carbide and silicide phases play major roles in the technical metallurgy of iron). Metallic iron dissolves readily in dilute mineral acids. With nonoxidizing acids and in the absence of air, iron in the +2 oxidation state is obtained. With air present or when warm dilute nitric acid is used, some of the iron goes into solution as the Fe3+ ion. Very strongly oxidizing mediums—for example, concentrated nitric acid or acids containing dichromate—passivate iron (i.e., cause it to lose its normal chemical activity), however, much as they do chromium. Air-free water and dilute air-free hydroxides have little effect on the metal, but it is attacked by hot concentrated sodium hydroxide.

Iron compounds are amenable to study by taking advantage of a phenomenon known as the Mössbauer effect (the phenomenon of a gamma ray being absorbed and reradiated by a nucleus without recoil). Although the Mössbauer effect has been observed for about one-third of the elements, it is particularly for iron (and to a lesser extent tin) that the effect has been a major research tool for the chemist. In the case of iron the effect depends on the fact that the nucleus of iron-57 can be excited to a high energy state by the absorption of gamma radiation of very sharply defined frequency that is influenced by the oxidation state, electron configuration, and chemical environment of the iron atom and can thus be used as a probe of its chemical behaviour. The marked Mössbauer effect of iron-57 has been used in studying magnetism and hemoglobin derivatives and for making a very precise nuclear clock.

Additional Information

Iron is a chemical element; it has symbol Fe (from Latin ferrum 'iron') and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's outer and inner core. It is the fourth most abundant element in the Earth's crust, being mainly deposited by meteorites in its metallic state.

Extracting usable metal from iron ores requires kilns or furnaces capable of reaching 1,500 °C (2,730 °F), about 500 °C (900 °F) higher than that required to smelt copper. Humans started to master that process in Eurasia during the 2nd millennium BC and the use of iron tools and weapons began to displace copper alloys – in some regions, only around 1200 BC. That event is considered the transition from the Bronze Age to the Iron Age. In the modern world, iron alloys, such as steel, stainless steel, cast iron and special steels, are by far the most common industrial metals, due to their mechanical properties and low cost. The iron and steel industry is thus very important economically, and iron is the cheapest metal, with a price of a few dollars per kilogram or pound.

Pristine and smooth pure iron surfaces are a mirror-like silvery-gray. Iron reacts readily with oxygen and water to produce brown-to-black hydrated iron oxides, commonly known as rust. Unlike the oxides of some other metals that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing more fresh surfaces for corrosion. Chemically, the most common oxidation states of iron are iron(II) and iron(III). Iron shares many properties of other transition metals, including the other group 8 elements, ruthenium and osmium. Iron forms compounds in a wide range of oxidation states, −4 to +7. Iron also forms many coordination complexes; some of them, such as ferrocene, ferrioxalate, and Prussian blue have substantial industrial, medical, or research applications.

The body of an adult human contains about 4 grams (0.005% body weight) of iron, mostly in hemoglobin and myoglobin. These two proteins play essential roles in oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals.

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