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The chemical element iron has the symbol Fe and the atomic number 26. It is a metal that belongs to the periodic table’s first transition series and group 8. It is the most abundant element on Earth by mass, just ahead of oxygen (32.1 percent and 30.1 percent, respectively), and it makes up much of the planet’s outer and inner core. In the Earth’s crust, it is the fourth most prevalent element.

Iron, in its metallic state, is extremely rare in the Earth’s crust, with meteorite deposition accounting for the majority of it. Iron ores, on the other hand, are abundant in the Earth’s crust, albeit extracting useable metal from them necessitates kilns or furnaces capable of temperatures of 1,500 °C (2,730 °F) or more, which is approximately 500 °C (900 °F) greater than that necessary to smelt copper. By roughly 2000 BCE, humans had mastered the process in Eurasia, and by 1200 BCE, the use of iron tools and weaponry had begun to supplant copper alloys in certain areas. The shift from the Bronze to the Iron Age is marked by this occurrence. Because of their mechanical qualities and low cost, iron alloys such as steel, stainless steel, cast iron, and special steels are by far the most prevalent industrial metals in today’s globe.

Mirror–like silvery-gray surfaces of pure iron are pristine and smooth. Iron, on the other hand, quickly interacts with oxygen and water to form hydrated iron oxides, which are generally known as rust. Unlike the oxides of certain other metals, which form passivating layers, rust takes up more space than the metal and hence flakes off, exposing new corrosive surfaces. Despite the fact that iron reacts readily, high purity iron, also known as electrolytic iron, has a higher corrosion resistance.

Iron makes up around 4 grams (0.005% of body weight) in an adult human’s body, largely in hemoglobin and myoglobin. These two proteins perform critical functions in vertebrate metabolism, transporting oxygen through the blood and storing oxygen in muscles, respectively. Human iron metabolism needs a minimum amount of iron in the diet to sustain the required levels. Iron is also the metal in the active site of several key redox enzymes in plants and animals that deal with cellular respiration, oxidation, and reduction.

Iron’s most prevalent oxidation states are iron(II) and iron(III) (III). Iron shares many features with other transition metals, such as ruthenium and osmium, which belong to Group 8. Iron has a wide variety of oxidation states, ranging from 2 to 7. Iron may also produce a variety of coordination compounds, some of which have important economic, medicinal, or scientific applications, such as ferrocene, ferrioxalate, and Prussian blue.

At normal pressures, the first three types are visible. Melted iron crystallizes into its allotrope, which has a body-centered cubic (bcc) crystal structure, when it cools past its freezing point of 1538 °C. It converts to its -iron allotrope, a face-centered cubic (fcc) crystal structure, or austenite, when it cools to 1394 °C. At temperatures below 912 °C, the crystal structure reverts to the bcc-iron allotrope.

Because of its importance to hypotheses regarding the Earth’s and other planets’ cores, the physical characteristics of iron at extremely high pressures and temperatures have also been intensively researched. Above 10 GPa and temperatures of a few hundred kelvin or less, -iron transforms into another hexagonal close-packed (hcp) structure known as -iron. The higher-temperature -phase transforms into -iron as well, but at a greater pressure.

At pressures more than 50 GPa and temperatures greater than 1500 K, some experimental evidence for a stable phase exists. Its structure is intended to be orthorhombic or double hcp. (Confusingly, the word “-iron” is also used to describe to -iron above its Curie point, when it transitions from ferromagnetic to paramagnetic, despite the fact that its crystal structure remains same.)

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