IRON

ron (pronounced /ˈаɪɚn/) is a chemical element with the symbol Fe (Latin: ferrum) and atomic number 26. Iron is a group 8 and period 4 element. Iron is lustrous and silvery in color. It is soft, about 80 Brinell,^[1] relative to steel, which is about 140 Brinell.^[2] It is one of the few ferromagnetic elements.

Iron-56 is the second heaviest stable isotope produced by the alpha process in stellar nucleosynthesis, the heaviest being nickel-62; heavier elements require a supernova for their formation. Iron is the most abundant element in the core of red giants, and is the most abundant metal in iron meteorites and in the dense metal cores of planets such as Earth. Iron and iron alloys are also the most common source of ferromagnetic materials in everyday use.

Occurrence

See also: Category:Iron minerals

The red appearance of this water is due to iron in the rocks.

Iron is the sixth most abundant element in the universe, formed as the final act of nucleosynthesis by carbon burning in massive stars. While it makes up about 5% of the Earth's crust, the earth's core is believed to consist largely of an iron-nickel alloy constituting 35% of the mass of the Earth as a whole. Iron is consequently the most abundant element on Earth, but only the fourth most abundant element in the Earth's crust.^[3] Most of the iron in the crust is found combined with oxygen as iron oxide minerals such as hematite and magnetite. About 1 in 20 meteorites consist of the unique iron-nickel minerals taenite (35–80% iron) and kamacite (90–95% iron). Although rare, meteorites are the major form of natural metallic iron on the Earth's surface.

The reason for Mars's red color is thought to be an iron-oxide-rich soil.

Characteristics

Iron is a metal extracted mainly from the iron ore hematite. It oxidises readily in air and water to form Fe₂O₃ and is rarely found as a free element. In order to obtain elemental iron, oxygen and other impurities must be removed by chemical reduction. The properties of iron can be modified by alloying it with various other metals and some non-metals, notably carbon and silicon to form steels.

Nuclei of iron atoms have some of the highest binding energies per nucleon, surpassed only by the nickel isotope ⁶²Ni. The universally most abundant of the highly stable nuclides is, however, ⁵⁶Fe. This is formed by nuclear fusion in stars. Although a further tiny energy gain could be extracted by synthesizing ⁶²Ni, conditions in stars are unsuitable for this process to be favoured, and iron abundance on Earth greatly favors iron over nickel, and also presumably in supernova element production.^[4]

Iron (as Fe²⁺, ferrous ion) is a necessary trace element used by almost all living organisms. The only exceptions are several organisms that live in iron-poor environments and have evolved to use different elements in their metabolic processes, such as manganese instead of iron for catalysis, or hemocyanin instead of hemoglobin. Iron-containing enzymes, usually containing heme prosthetic groups, participate in catalysis of oxidation reactions in biology, and in transport of a number of soluble gases. See hemoglobin, cytochrome, and catalase.

Allotropes

Main article: Allotropes of iron

Iron represents perhaps the best-known example of allotropy in a metal. There are three allotropic forms of iron, known as alpha, gamma, and delta.

As molten iron cools down it crystallises at 1538 °C into its delta allotrope, which has a body-centered cubic (BCC) crystal structure. As it cools further its crystal structure changes to face-centered cubic (FCC) at 1394 °C, when it is known as gamma-iron, or austenite. At 912 °C the crystal structure again becomes BCC as alpha-iron is formed, and at 770 °C (the Curie point, Tc) the iron becomes magnetic. As the iron passes through the Curie temperature there is no change in crystalline structure, but there is a change in "domain structure", where each domain contains iron atoms with a particular electronic spin. In unmagnetised iron, all the electronic spins of the atoms within one domain are in the same direction; however, in neighbouring domains they point in various directions and thus cancel out. In magnetised iron, the electronic spins of all the domains are all aligned, so that the magnetic effects of neighbouring domains reinforce each other. Although each domain contains billions of atoms, they are very small, about one thousandth of a centimetre across.

Iron is of most importance when mixed with certain other metals and with carbon to form steels. There are many types of steels, all with different properties; and an understanding of the properties of the allotropes of iron is key to the manufacture of good quality steels.

Alpha iron, also known as ferrite, is the most stable form of iron at normal temperatures. It is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910 °C).

Above 912 °C and up to 1401 °C alpha iron undergoes a phase transition from body-centred cubic to the face-centred cubic configuration of gamma iron, also called austenite. This is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04% by mass at 1146 °C). This form of iron is used in the type of stainless steel used for making cutlery, and hospital and food-service equipment.