Iron 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 a lustrous, silvery soft metal. It is one of the few ferromagnetic elements.
Iron and nickel are notable for being the final elements produced by stellar nucleosynthesis, and thus are 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.
Iron is believed to be the sixth [3] most abundant element in the universe, formed as the final act of nucleosynthesis by carbon burning in massive stars. Iron is the most abundant element on Earth. While it makes up only about 5% of the Earth's crust, the earth's core is believed to consist largely of a metallic iron-nickel alloy comprising 35% of the mass of the Earth as a whole. Iron is the fourth most abundant element in the Earth's crust[4] and the second most abundant metal (after aluminium). Most of the iron in the crust is found combined with oxygen as iron oxide minerals such as hematite, magnetite, and taconite[citation needed]. About 5% of the meteorites similarly consist of iron-nickel alloy. Although rare, these 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.
Iron is a metal extracted mainly from the iron ore hematite. It oxidises readily in air and water and is rarely found as a free element. In order to obtain elemental iron, oxygen and other impurities must be removed by chemical reduction. Iron is the main constituent of steel, and it is used in the production of alloys or solid solutions of various metals, as well as some non-metals, particularly carbon. The many iron alloys, which have very different properties, are discussed in the article on steel.
Nuclei of iron 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 a few prokaryotic organisms which live in iron-poor conditions (such as the lactobacilli in iron-poor milk) which use manganese for catalysis instead as well as organisms which use 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.
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