Iron is a lustrous, silvery-gray transition metal essential for steels and biology (hemoglobin). It is ferromagnetic and rusts readily in moist air.
Iron (Fe) is a transition metal in Group 8, Period 4, and the d-block. A commonly cited ground-state configuration is [Ar] 3d6 4s2. The partially filled 3d subshell underlies many of iron’s magnetic and catalytic properties.
Rusting is an electrochemical corrosion process requiring oxygen and water. Iron oxidizes to hydrated iron(III) oxides/oxyhydroxides, commonly written as Fe2O3·xH2O. A simplified overall step is:
\(4\,\mathrm{Fe} + 3\,\mathrm{O_2} + 6\,\mathrm{H_2O} \;\to\; 4\,\mathrm{Fe(OH)_3}\;\xrightarrow{\text{dehydrate}}\; \mathrm{Fe_2O_3\cdot xH_2O}\)
Salts (electrolytes) accelerate rusting by enhancing ionic conduction.
The most common states are +2 (ferrous) and +3 (ferric). Iron can also reach +6 in ferrates (e.g., \(\mathrm{FeO_4^{2-}}\)). A simple redox couple is:
\(\mathrm{Fe^{2+}} \;\rightleftharpoons\; \mathrm{Fe^{3+}} + e^-\)
These redox changes are central to catalysis, bioinorganic chemistry, and corrosion.
Iron is produced by reduction of iron oxides (hematite/magnetite) using carbon monoxide formed from coke. Key simplified reactions are:
\(\mathrm{C} + \mathrm{O_2} \to \mathrm{CO_2};\quad \mathrm{CO_2} + \mathrm{C} \to 2\,\mathrm{CO}\)
\(\mathrm{Fe_2O_3} + 3\,\mathrm{CO} \to 2\,\mathrm{Fe} + 3\,\mathrm{CO_2}\)
The molten product (pig iron) is then refined to make steel.
Below its Curie temperature (about 770 °C), exchange interactions align unpaired 3d electrons into domains, giving ferromagnetism. Above the Curie point, thermal agitation destroys long-range order and iron becomes paramagnetic.
Steel is an alloy of iron with controlled amounts of carbon (typically 0.02–2.1%) and other elements (Mn, Cr, Ni, Mo, etc.). Alloying tailors strength, hardness, toughness, corrosion resistance, and other properties. For example, stainless steel contains ≥10.5% Cr forming a protective Cr2O3 layer.
In hemoglobin, iron is bound in a heme porphyrin and reversibly binds oxygen for transport in blood. A simplified binding step is:
\(\mathrm{Fe^{2+}\! -\! heme} + \mathrm{O_2} \;\rightleftharpoons\; \mathrm{Fe^{2+}\! -\! O_2\! -\! heme}\)
Iron also cycles between Fe(II)/Fe(III) in enzymes for electron transfer and catalysis.
Common strategies include:
Important oxides include:
Magnetite is ferrimagnetic and often used in magnetic recording and as a catalyst support.
With dilute acids, iron releases hydrogen:
\(\mathrm{Fe} + 2\,\mathrm{HCl} \;\to\; \mathrm{FeCl_2} + \mathrm{H_2}\uparrow\)
The thermite reaction (aluminothermy) reduces iron(III) oxide to molten iron:
\(\mathrm{Fe_2O_3} + 2\,\mathrm{Al} \;\to\; 2\,\mathrm{Fe} + \mathrm{Al_2O_3}\;\; (\Delta H \ll 0)\)
This is used for rail welding and metallurgical repairs.