this: the chemical element

Iron ( ) is a chemical element with the symbol Fe ( ) and atomic number 26. Iron is a group 8 and period 4 element. Iron is a lustrous, silvery soft metal. It is one of the few ferromagnetic elements.

Iron is notable for being the final elements produced by stellar nucleosynthesis, and are therefore the heaviest elements which do not require a red giant or supernova for formation. Iron and nickel are therefore the most abundant metals in metallic 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

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. 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' red colour 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 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.

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.