IRON Iron is an important structural metal and the major component of steel. Unprotected iron objects have a tendency to corrode. The familiar coating of rust on many iron objects is hydrated iron(III) oxide.

Physical Properties

Atomic Number: 26
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: 1535^oC
Important Oxidation States: +2 (ferrous), +3 (ferric)

Occurence

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). Iron pyrite is gold in color and has a distinctive lustor. For this reason it is often referred to as fool's gold. While an abundant mineral, it is not used as as source of iron as the sulfur is not easily removed.

Magnetite. A sample of magnetite rock, Fe3O4. This ore contains iron in both the +2 and +3 oxidation states and is best formulated (Fe+3)2(Fe+2)O4.

Iron Pyrite. A Sample of iron pyrite, FeS2. It has a metallic lustor and is also known as fool's gold. This ion contains the disulfide ion (S22-) ion, which is analogous to the peroxide ion. Therefore pyrite contains iron in the +2 oxidation state.

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 including the hexaaquo and hexacyano. The latter is a common pale yellow solid 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. 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 when 1,10-phenanthroline is added.

Prussian Blue. Addition of ferric ion (Fe3+) to a solution of the ferrocyanide ion, [Fe(CN)6]4-, results in the formation of Prussian blue, which has the formula Fe4[Fe(CN)6]3. Prussian Blue is used in blueprints.

A Chemical Test for Iron(II). When 1,10-phenanthroline is added to even a dilute solution of iron(II), the highly colored complex ion [Fe(phen)3]2+ is produced.

Iron is of tremendous biological importance. The heme molecule, which is responsible for the uptake and distribution of molecular oxygen in the body, is a porphyrin molecule contining the ferrous ion. A diet insufficient in iron results in decreased ability of the body to transport oxygen, a condition known as anemia.

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 species such as [Fe(H₂O)₅(OH)]²⁺, and solutions of the ferric ion can be quite acidic due to the reaction with water. The ferric ion forms an ion analgous to the ferrocyanide ion, known as the ferricyanide ion. 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 very sensitive test for the ferric ion.

A Chemical Test for Fe(III). The ferric ion forms an intensely colored complex upon the addition of thiocyanate ion. Both test tubes contain a 0.10 M solution of ferric nitrate. A drop of KSCN was added to the test tube in the back. The color change is so dramatic that it can be used to test for the presence of iron even in very dilute solutions.