Transition metal
Chromium is a chemical element; it has symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium is valued for its high corrosion resistance and hardness. A major development in steel production was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to form stainless steel. Stainless steel and chrome plating (electroplating with chromium) together comprise 85% of the commercial use. Chromium is also greatly valued as a metal that is able to be highly polished while resisting tarnishing. Polished chromium reflects almost 70% of the visible spectrum, and almost 90% of infrared light. The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color, because many chromium compounds are intensely colored.
Industrial production of chromium proceeds from chromite ore (mostly FeCr2O4) to produce ferrochromium, an iron-chromium alloy, by means of aluminothermic or silicothermic reactions. Ferrochromium is then used to produce alloys such as stainless steel. Pure chromium metal is produced by a different process: roasting and leaching of chromite to separate it from iron, followed by reduction with carbon and then aluminium. …
Chromium was not used anywhere until the experiments of French pharmacist and chemist Nicolas Louis Vauquelin (1763–1829) in the late 1790s.
From the 1970s, until 2019, it was widely believed that the technique of chromium plating to prevent metal corrosion had been invented in ancient China. This technique was thought to have been used, for example, to protect bronze artifacts such as arrowheads and sword blades buried as grave goods in the mausoleum of the First Emperor of Qin. However, a detailed scientific investigation in 2019 revealed that the chromium found on these artifacts originated naturally from the lacquer applied to them. Scientists now believe that the excellent preservation of the artifacts was not due to intentional chromium plating, but rather to the burial environment: the soil was fine-grained and alkaline, which limited aeration and the growth of organic matter, thereby creating optimal conditions for metal preservation.
Chromium minerals as pigments came to the attention of the west in the eighteenth century. On 26 July 1761, Johann Gottlob Lehmann found an orange-red mineral in the Beryozovskoye mines in the Ural Mountains which he named Siberian red lead. Though misidentified as a lead compound with selenium and iron components, the mineral was in fact crocoite with a formula of PbCrO4. In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that was discovered to possess useful properties as a pigment in paints. …
Gaseous chromium has a ground-state electron configuration of [Ar] 3d5 4s1. It is the first element in the periodic table whose configuration violates the Aufbau principle. Exceptions to the principle also occur later in the periodic table for elements such as copper, niobium and molybdenum.
Chromium is the first element in the 3d series where the 3d electrons start to sink into the core; they thus contribute less to metallic bonding, and hence the melting and boiling points and the enthalpy of atomisation of chromium are lower than those of the preceding element vanadium. Chromium(VI) is a strong oxidising agent in contrast to the molybdenum(VI) and tungsten(VI) oxides.
Chromium is the third hardest element after carbon (diamond) and boron. Its Mohs hardness is 8.5, which means that it can scratch samples of quartz and topaz, but can be scratched by corundum. Chromium is highly resistant to tarnishing, which makes it useful as a metal that preserves its outermost layer from corroding, unlike other metals such as copper, magnesium, and aluminium.
Chromium has a melting point of 1907 °C (3465 °F), which is relatively low compared to the majority of transition metals. However, it still has the second highest melting point out of all the period 4 elements, being topped by vanadium by 3 °C (5 °F) at 1910 °C (3470 °F). …
Chromium is the 21st most abundant element in Earth's crust with an average concentration of 100 ppm. Chromium compounds are found in the environment from the erosion of chromium-containing rocks, and can be redistributed by volcanic eruptions. Typical background concentrations of chromium in environmental media are: atmosphere <10 ng/m3; soil <500 mg/kg; vegetation <0.5 mg/kg; freshwater <10 μg/L; seawater <1 μg/L; sediment <80 mg/kg. Chromium is mined as chromite (FeCr2O4) ore.
About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, about a third in Kazakhstan, while India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa. Although rare, deposits of native chromium exist. The Udachnaya Pipe in Russia produces samples of the native metal. This mine is a kimberlite pipe, rich in diamonds, and the reducing environment helped produce both elemental chromium and diamonds.
The relation between Cr(III) and Cr(VI) strongly depends on pH and oxidative properties of the location. In most cases, Cr(III) is the dominating species, but in some areas, the ground water can contain up to 39 μg/L of total chromium, of which 30 μg/L is Cr(VI).
Chromium was not used anywhere until the experiments of French pharmacist and chemist Nicolas Louis Vauquelin (1763–1829) in the late 1790s.
From the 1970s, until 2019, it was widely believed that the technique of chromium plating to prevent metal corrosion had been invented in ancient China. This technique was thought to have been used, for example, to protect bronze artifacts such as arrowheads and sword blades buried as grave goods in the mausoleum of the First Emperor of Qin. However, a detailed scientific investigation in 2019 revealed that the chromium found on these artifacts originated naturally from the lacquer applied to them. Scientists now believe that the excellent preservation of the artifacts was not due to intentional chromium plating, but rather to the burial environment: the soil was fine-grained and alkaline, which limited aeration and the growth of organic matter, thereby creating optimal conditions for metal preservation.
Chromium minerals as pigments came to the attention of the west in the eighteenth century. On 26 July 1761, Johann Gottlob Lehmann found an orange-red mineral in the Beryozovskoye mines in the Ural Mountains which he named Siberian red lead. Though misidentified as a lead compound with selenium and iron components, the mineral was in fact crocoite with a formula of PbCrO4. In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that was discovered to possess useful properties as a pigment in paints. …
Naturally occurring chromium is composed of four stable isotopes; 50Cr, 52Cr, 53Cr and 54Cr, with 52Cr being the most abundant (83.789% natural abundance). Twenty-five radioisotopes have been characterized, ranging from 42Cr to 70Cr; the most stable radioisotope is 51Cr with a half-life of 27.70 days. All of the remaining radioactive isotopes have half-lives that are less than a day and the majority less than a minute. Chromium also has two metastable nuclear isomers.
The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.
53Cr is the radiogenic decay product of 53Mn (half-life 3.7 million years). Chromium and manganese are found together sufficiently for measurement of both to find application in isotope geology. Manganese-chromium isotope ratios reinforce the evidence from 26Al and 107Pd concerning the early history of the Solar System. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites indicate a non-zero initial 53Mn/55Mn ratio that suggests Cr isotopic composition variations must result from in-situ decay of 53Mn in differentiated planetary bodies. Hence 53Cr provides additional evidence for nucleosynthetic processes immediately before coalescence of the Solar System.
The ratio 53Cr/52Cr has also been posited as a proxy for atmospheric oxygen concentration.
The possible nutritional value of chromium(III) is unproven. Although chromium is regarded as a trace element and dietary mineral, its suspected roles in the action of insulin – a hormone that mediates the metabolism and storage of carbohydrate, fat, and protein – have not been adequately established. The mechanism of its actions in the body is undefined, leaving in doubt whether chromium has a biological role in healthy people.
In contrast, hexavalent chromium (Cr(VI) or Cr6+) is highly toxic and mutagenic. Ingestion of chromium(VI) in water has been linked to stomach tumors, and it may also cause allergic contact dermatitis.
"Chromium deficiency", involving a lack of Cr(III) in the body, or perhaps some complex of it, such as glucose tolerance factor, is not accepted as a medical condition, as it has no symptoms and healthy people do not require chromium supplementation. Some studies suggest that the biologically active form of chromium(III) is transported in the body via an oligopeptide called low-molecular-weight chromium-binding substance (chromodulin), which might play a role in the insulin signaling pathway.
The chromium content of common foods is generally low (1–13 micrograms per serving). The chromium content of food varies widely, due to differences in soil mineral content, growing season, plant cultivar, and contamination during processing. Chromium (and nickel) leach into food cooked in stainless steel, with the effect being largest when the cookware is new. …