Iron

Iron, ₂₆Fe
General properties
Appearance	lustrous metallic with a grayish tinge
Standard atomic weight (Ar, standard)	55.845(2)
Iron in the periodic table
	manganese ← iron → cobalt
Atomic number (Z)	26
Group	group 8
Period	period 4
Element category	transition metal
Block	d-block
Electron configuration	[Ar] 3d6 4s2
Electrons per shell	2, 8, 14, 2
Physical properties
Phase at STP	solid
Melting point	1811 K (1538 °C, 2800 °F)
Boiling point	3134 K (2862 °C, 5182 °F)
Density (near r.t.)	7.874 g/cm3
when liquid (at m.p.)	6.98 g/cm3
Heat of fusion	13.81 kJ/mol
Heat of vaporization	340 kJ/mol
Molar heat capacity	25.10 J/(mol·K)
Atomic properties
Oxidation states	−4, −2, −1, +1,+2, +3, +4, +5,+6, +7(an amphoteric oxide)
Electronegativity	Pauling scale: 1.83
Ionization energies	1st: 762.5 kJ/mol; 2nd: 1561.9 kJ/mol; 3rd: 2957 kJ/mol; (more);
Atomic radius	empirical: 126 pm
Covalent radius	Low spin: 132±3 pm; High spin: 152±6 pm
	Spectral lines
	Spectral lines
Miscellanea
Crystal structure	body-centered cubic (bcc) ; a=286.65 pm
Crystal structure	face-centered cubic (fcc) ; between 1185–1667 K
Speed of sound thin rod	5120 m/s (at r.t.) (electrolytic)
Thermal expansion	11.8 µm/(m·K) (at 25 °C)
Thermal conductivity	80.4 W/(m·K)
Electrical resistivity	96.1 nΩ·m (at 20 °C)
Curie point	1043 K
Magnetic ordering	ferromagnetic
Young's modulus	211 GPa
Shear modulus	82 GPa
Bulk modulus	170 GPa
Poisson ratio	0.29
Mohs hardness	4
Vickers hardness	608 MPa
Brinell hardness	200–1180 MPa
CAS Number	7439-89-6
History
Discovery	before 5000 BC
Main isotopes of iron
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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 alloys formed 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.