Iron is a
chemical element with symbol
Fe (from
Latin:
ferrum) and
atomic number 26. It is a
metal in the
first transition series. It is by mass the most common element on
Earth, forming much of Earth's
outer and
inner core. It is the fourth most common
element in the Earth's crust. Its abundance in
rocky planets like Earth is due to its abundant production by
fusion in high-mass
stars, where it is the last element to be produced with release of energy before the violent collapse of a
supernova, which scatters the iron into space.
Like the other
group 8 elements,
ruthenium and
osmium, iron exists in a wide range of
oxidation states, −2 to +7, although +2 and +3 are the most common. Elemental iron occurs in
meteoroids and other low
oxygen environments, but is reactive to oxygen and
water. Fresh iron surfaces appear lustrous silvery-gray, but
oxidize in normal air to give
hydrated iron oxides, commonly known as
rust. Unlike the metals that form
passivating oxide layers, iron oxides occupy more volume than the metal and thus flake off, exposing fresh surfaces for corrosion.
Iron metal has been used since
ancient times, although
copper alloys, which have lower melting temperatures, were used even earlier in human history. Pure iron is relatively soft, but is unobtainable by
smelting because it is significantly hardened and strengthened by impurities, in particular
carbon, from the smelting process. A certain proportion of carbon (between 0.002% and 2.1%) produces
steel, which may be up to 1000 times harder than pure iron. Crude iron metal is produced in
blast furnaces, where ore is reduced by
coke to
pig iron, which has a high carbon content. Further refinement with oxygen reduces the carbon content to the correct proportion to make steel. Steels and iron
alloysformed with other metals (
alloy steels) are by far the most common industrial metals because they have a great range of desirable properties and iron-bearing rock is abundant.
Iron chemical compounds have many uses. Iron oxide mixed with aluminium powder can be ignited to create a
thermite reaction, used in welding and purifying ores. Iron forms binary compounds with the
halogens and the
chalcogens. Among its organometallic compounds is
ferrocene, the first
sandwich compound discovered.
Iron plays an important role in biology, forming complexes with molecular oxygen in
hemoglobin and
myoglobin; these two compounds are common
oxygen transport proteins in vertebrates. 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. In adult human males are some 3.8 grams of iron, and 2.3 grams in females, for whom iron is distributed in
hemoglobin and throughout the body. Iron is a critical element in the
metabolism of hundreds of
proteins and
enzymes involved in diverse body functions, such as
oxygen transport,
DNA synthesis, and
cell growth.
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