The Chemistry of Iron

Physical Properties

Atomic Number:
Mass Number: 55.846
Electron Configuration: 1s22s22p63s23p64s23d6
Crystal Structure: Body-Centered Cubic
Atomic Radius: 117 pm
Ionic Radius: 76 pm (Fe2+), 64 pm (Fe3+)
Density: 7.87 g/cm3
Melting Point: 1535oC
Important Oxidation States: +2 (ferrous), +3 (ferric)


Iron is one of the most widely used metals. It is the fourth most abundant element in the Earth's crust and appears as several different minearals. These include hematite (Fe2O3), magnetite (Fe3O4), siderite (FeCO3), and iron pyrite (FeS2). Note that hematite is a compound of Fe(III) whereas magnetite contains both Fe(III) and Fe(III) and may better be formulated as [Fe(II)(Fe(III)2]O3. Iron pyrite is gold in color and has a distinctive lustor. For this reason it is often referred to as fool's gold. Since sulfur is in the same group as oxygen it is tempting to think of this compound as containing Fe(IV). However, this is not a stable oxidation state for iron. In pyrite, the sulfur is present as the disulfide ion (S22-); thereofre the iron is present as Fe(II). While an abundant mineral, pyrite is not used as as source of iron as the sulfur is not easily removed.

Properties and Reactions

Iron is fairly reactive and will react with acids. An exposed iron surface quickly develops a coating of rust, which is a hydrated form of iron(III) oxide. This process is acceleretd in a moist environment. Finely divided iron will burst into flame when heated in an atmosphere of pure oxygen.

Iron is a ferromagnetic material. In a ferromagnetic substance, the magnetic fields of individual atoms are aligned and reinforce each other to produce a large magnetic field. The phenomenon is most pronounced for iron but also occurs in cobalt and nickel to a lesser extent. Iron and alloys of iron are used to make such things as magnets and transfomer cores.

Steel is an important alloy of iron. All steel contains 0.50 - 1.50 percent carbon. Generally, the the higher the carbon content, the harder and more brittle the steel. Other metals are sometimes added depending upon the desired properties. For example, magnets are often made of steel containing nickel and cobalt. Stainless steel contains chromim and nickel and is resistant to corrosion. The addition of manganese produces a steel which is very hard and durable.

The +2 Oxidation State

Iron in the +2 oxidation state is known as the ferrous ion. This ion is pale green in color and is very easily oxidized to the ferric ion; even traces of dissolved oxygen in solution will accomplish this oxidation. For this reason, solutions of the ferrous ion are sometimes used as reducing agents.

Complexes of the ferrous ion usually have octahedral geometry; examples include [Fe(H2O)6]2+ and [Fe(CN)6]4-. The latter is a pale yellow solid commonlu known known as the ferrocyanide ion. Despite the toxicity of cyanide, the ferrocyanide ion is stable because the cyanide molecules are tightly bound. When added to a solution contianing the ferric ion, a precipitate known as prussian blue is formed (Figure 3). This compound is used in making blueprints and as a bluing agent in certain lundry detergents.

The ferrous ion has a particular affinity of amine ligands, and forms a characteristic red-orange complex ion {Fe(phen)3]2+ is formed when 1,10-phenanthroline is added (Figure 4). This serves as the basis of a chemical test for the ferrous ion.

The +3 Oxidation State

Iron in the +3 oxidation state is known as the ferric ion. The color of the hydrated ferous ion [Fe(H2O)6]2+ is actually violet. However, most solutions contining the ferric ion are usually yellow or yellow-brown due to the formtion of [Fe(H2O)5(OH)]2+. This ion is formed due to the high charge density of Fe3+ ion. Water molecules are coordinated so strongly that one one of the O-H bonds weakens, resulting in the formation of H3O+. As a result solutions of ferric ion are often acidic.

[Fe(H2O)6]3+ + H2O = [Fe(H2O)5(OH)]2+ + H3O+

The ferric ion forms an ion analgous to the ferrocyanide ion, known as the ferricyanide ion, [Fe(CN)6]3-. This is a red-orange solid that is considerably more toxic than the ferrocyanide ion, as the cyanide ions are not as tightly bound. Therefore, care should be taken not to confuse these two ions.

The ferric ion forms a characteristic blood-red complex ion with the thiocyanate (SCN-) ion; this serves as the basis for a chemical test for the ferric ion (Figure 5).

Figure 1. Magnetite, Fe3O4
Figure 2. Iron Pyrite, FeS2
Figure 3. Formation of Prussian Blue, Fe4[Fe(CN)6]3
Figure 4. Formation of [Fe(phen)3]2+
Figure 5. [Fe(H2O)5(OH)]2+ and [Fe(H2O)5SCN)]2+