A short briefing of iron

Iron is a chemical element and atomic number 26, a metal belonging to group 8 of the periodic table.It is the most common element on Earth by mass, second only to oxygen (32.1% and 30.1%, respectively), and makes up most of Earth's outer and inner cores.It is the fourth most common element in the Earth's crust and is deposited mostly from meteorites in a metallic state, where its ores have also been found.Extracting usable metals from iron ore requires kilns or furnaces capable of reaching temperatures of 1,500 °C (2,730 °F) or higher, about 500 °C (932 °F) higher than that required to smelt copper.Humans began to master the process in Eurasia in the 2nd millennium BC, and the use of iron tools and weapons began to replace copper alloys-in some areas, only around 1200 BC.This event is considered to be the transition from the Bronze Age to the Iron Age.In the modern world, ferroalloys such as steel, stainless steel, cast iron and special steels are by far the most common industrial metals due to their mechanical properties and low cost.The steel industry is therefore economically very important, and iron is the cheapest metal, priced at a few dollars per kilogram or pound.The pure and smooth pure iron surface is mirror-like silver-gray.Iron reacts readily with oxygen and water to form brown to black hydrated iron oxides, commonly known as rust.Unlike the oxides of some other metals that form a passivating layer, rust occupies a larger volume than the metal and so flakes off, exposing more of the new surface to corrode.High-purity iron (such as electrolytic iron) is more resistant to corrosion.The adult human body contains about 4 grams (0.005% of body weight) of iron, mainly in hemoglobin and myoglobin.These two proteins play important roles in the metabolism of vertebrates, oxygen transport in the blood and oxygen storage in the muscles, respectively.To maintain the necessary levels, iron metabolism in the body requires a minimum amount of iron in the diet.Iron is also the metal in the active sites of many important oxidoreductases that handle cellular respiration and oxidation and reduction in plants and animals.Chemically,the most common oxidation states of iron are iron(II) and iron(III).Iron shares many of the properties of other transition metals, including other Group 8 elements, ruthenium and osmium. Iron forms compounds in various oxidation states,-2 to +7. Iron also forms many coordination compounds; some of these, such as ferrocene, ferric oxalate, and Prussian blue, have important industrial, medical, or research applications.

Features   

Allotrope:

At least four allotropes (different arrangements of atoms in solids) of iron are known, usually denoted α, γ, δ, and ε.The first three forms are observed at normal pressure.When molten iron is cooled above its freezing point of 1538 °C, it crystallizes into the delta allotrope with a body-centered cubic (bcc) crystal structure.When it is further cooled to 1394 °C, it becomes the gamma iron allotrope, the face centered cubic (fcc) crystal structure or austenite.At and below 912 °C, the crystal structure changes again to the bcc α-iron allotrope.The physical properties of iron at very high pressures and temperatures have also been extensively studied,as they are relevant to core theories of the Earth and other planets.At temperatures above about 10 GPa and a few hundred Kelvin or lower, α-iron transforms into another hexagonal close-packed (hcp) structure, also known as ε-iron.The higher temperature γ phase also transforms to ε iron, but at higher pressure.There is some controversial experimental evidence for the existence of a stable β phase at pressures above 50 GPa and temperatures of at least 1500 K. It should have an orthogonal or dual hcp structure.(Confusingly, the term "β-iron" is also sometimes used to refer to α-iron above the Curie point when it changes from ferromagnetic to paramagnetic, even though its crystal structure does not change.)It is generally believed that the inner core of the earth is composed of iron-nickel alloy with ε (or β) structure.

Melting point and boiling point

The melting and boiling points of iron and its enthalpy of atomization are lower than those of the earlier 3d elements from scandium to chromium, suggesting that the contribution of 3d electrons to metal bonding decreases as they are increasingly attracted to inertness However, they higher than the value of the previous element manganese, since this element has a half-filled 3d subshell, so its d electrons are less prone to delocalization.The same trend appears for ruthenium rather than osmium.The melting point of iron is well defined experimentally for pressures less than 50 GPa.For higher pressures, published data (as of 2007) still vary by tens of gigapascals and over a thousand Kelvin.

Magnetic Properties

Magnetization curves of nine ferromagnetic materials, showing saturation.

1.Steel plate, 2. Silicon steel, 3. Cast steel, 4. Tungsten steel, 5. Magnetic steel, 6. Cast iron, 7. Nickel, 8. Cobalt, 9. Magnetite Below the Curie point of 770 °C (1,420 °F; 1,040 K), α-iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom usually align with the spins of their neighbors,resulting in an overall magnetic field.This occurs because the orbitals of these two electrons (dz2 and dx2 −.y2) do not point to neighboring atoms in the crystal lattice and thus do not participate in metal bonding.In the absence of an external magnetic field source, the atoms spontaneously divide into magnetic domains approximately 10 micrometers wide,such that the atoms in each domain have parallel spins, but some domains have other orientations.Therefore, the total magnetic field of a macroscopic piece of iron is almost zero.Applying an external magnetic field causes domain magnetized in the same direction to grow at the expense of adjacent magnetic domains pointing in other directions, thereby enhancing the external field.This effect is used in devices such as transformers, magnetic recording heads, and electric motors that require a guided magnetic field to function as designed. Impurities, lattice defects or grain and particle boundaries can "fix" the magnetic domains in new positions, so the effect persists even after the external magnetic field is removed thus turning iron objects into (permanent) magnets.Some iron compounds exhibit similar behavior, such as ferrites, including the mineral magnetite, the crystalline form of the mixed iron(II,III) oxide Fe3O4 (though the atomic-scale mechanism, ferrimagnetism, is somewhat different). Magnetite (lodestone), with its natural permanent magnetization, provided the earliest nautical compasses. Magnetite particles were widely used in magnetic recording media such as magnetic core memory, magnetic tape, floppy and magnetic disks until they were replaced by cobalt-based materials.