Iron is a chemical element with the symbol Fe (from Latin: ferrum) and atomic number 26. It is a metal in the first transition series. [3] This is the most common element on earth based on mass, forming most parts of the inner and outer cores. Iron is the fourth largest element in the earth's crust. Its abundance in rocky planets like the earth is due to the abundance of production due to fusion reactions in large mass stars, where nickel-56 production (which decays into the most common iron isotope) is the last nuclear exothermic fusion reaction. As a result, radioactive nickel is the last element produced before the collapse of supernovae. The collapse scattered iron radionuclide precursors into space.
Like other Group 8 elements, iron is in the wide oxidation range, −2 to +6, although +2 and +3 are the most numerous. Iron elements are found in meteorites and other low-oxygen environments, but are reactive with oxygen and water. The fresh iron surface looks silvery gray, but oxidized in normal air produces iron oxide hydrate, known as rust. Unlike other metals that form the passive oxide layer, iron oxide occupies more space than the metal itself and then peels off, exposing the fresh surface to corrosion.
Iron metal has been used since ancient times, although copper alloys, which have lower melting points, were used earlier in human history. Pure iron is relatively soft, but cannot be obtained through smelting. This material hardens and is significantly strengthened by dirt, especially carbon, from the smelting process. With a certain proportion of carbon (between 0.002% and 2.1%) it produces steel, which is harder than pure iron, maybe up to 1000 times. Raw iron metal is produced in high kilns, where ore is reduced by coal to pig iron, which has a high carbon content. Further processing with oxygen reduces the carbon content so that it reaches the right proportion for steel making. Low-carbon steel and iron alloys along with other metals (alloy steel) are by far the most common metals used by industry, because of the wide range of properties obtained and the abundance of iron-containing rocks.
Iron chemical compounds have many benefits. Iron oxide mixed with aluminum powder can be lighted to make a thermite reaction, which is used in the welding and refining of ore. Iron forms binary compounds with halogen and calcogen. The organometallic compounds include ferocene, the first sandwich compound found.
Iron plays an important role in biology, forming complexes with molecular oxygen in hemoglobin and myoglobin; both of these compounds are oxygen carrying proteins in vertebrates. Iron is also a metal in the active part of most redox enzymes that play a role in cellular respiration as well as oxidation and reduction in plants and animals.