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 common element in the Earth's crust, being mainly deposited by meteorites in its metallic state.

Iron, ₂₆Fe

Iron
Pronunciation	/ˈaɪərn/
Allotropes	see Allotropes of iron
Appearance	lustrous metallic with a grayish tinge
Standard atomic weight Ar°(Fe)
	55.845±0.002 55.845±0.002 (abridged)
Iron in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson – ↑ Fe ↓ Ru manganese ← iron → cobalt
Atomic number (Z)	26
Group	group 8
Period	period 4
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 ​(2861 °C, ​5182 °F)
Density (at 20° C)	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)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1728 1890 2091 2346 2679 3132
Atomic properties
Oxidation states	−4, −2, −1, 0, +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
Van der Waals radius	194 [1] pm
Spectral lines of iron
Other properties
Natural occurrence	primordial
Crystal structure	α-Fe: ​body-centered cubic (bcc) (cI2)
Lattice constant	a = 286.65 pm (at 20 °C)
Crystal structure	γ-Fe (912–1394 °C): ​face-centered cubic (fcc) (cF4)
Lattice constant	a = 364.68 pm (at 916°C)
Thermal expansion	12.07×10−6/K (at 20 °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
Speed of sound thin rod	5120 m/s (at r.t.) (electrolytic)
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
Symbol	"Fe": from Latin ferrum
Isotopes of iron v e
Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct 54Fe 5.85% stable 55Fe synth 2.73 y ε 55Mn 56Fe 91.8% stable 57Fe 2.12% stable 58Fe 0.28% stable 59Fe synth 44.6 d β− 59Co 60Fe trace 2.6×106 y β− 60Co
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Extracting usable metal from iron ores requires kilns or furnaces capable of reaching 1,500 °C (2,730 °F), about 500 °C (932 °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 compounds; 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.