Chromium

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 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 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.
Trivalent chromium occurs naturally in many foods and is sold as a dietary supplement, although there is insufficient evidence that dietary chromium provides nutritional benefit to people. In 2014, the European Food Safety Authority concluded that research on dietary chromium did not justify it to be recognized as an essential nutrient.
While chromium metal and Cr ions are considered non-toxic, chromate and its derivatives, often called "hexavalent chromium", is toxic and carcinogenic. According to the European Chemicals Agency, chromium trioxide that is used in industrial electroplating processes is a "substance of very high concern".

Physical properties

Atomic

Gaseous chromium has a ground-state electron configuration of 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 is a strong oxidising agent in contrast to the molybdenum and tungsten oxides.

Bulk

Chromium is the third hardest element after carbon 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, 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 at 1910 °C. The boiling point of 2671 °C, however, is comparatively lower, having the fourth lowest boiling point out of the Period 4 transition metals alone behind copper, manganese and zinc. The electrical resistivity of chromium at 20 °C is 125 nanoohm-meters.
Chromium has a high specular reflection in comparison to other transition metals. In infrared, at 425 μm, chromium has a maximum reflectance of about 72%, decreasing to a minimum of 62% at 750 μm before increasing again to 90% at 4000 μm. When chromium is used in stainless steel alloys and polished, the specular reflection decreases with the inclusion of additional metals, yet is still high in comparison with other alloys. Between 40% and 60% of the visible spectrum is reflected from polished stainless steel. Chromium's high reflectivity, especially the 90% in infrared, can be attributed to chromium's magnetic properties. Chromium has unique magnetic properties; it is the only elemental solid that shows antiferromagnetic ordering at room temperature and below. Above 38 °C, its magnetic ordering becomes paramagnetic. The antiferromagnetic properties, which cause the chromium atoms to temporarily ionize and bond with themselves, are present because the body-centric cubic's magnetic properties are disproportionate to the lattice periodicity. This is due to the magnetic moments at the cube's corners and the unequal, but antiparallel, cube centers. The frequency-dependent relative permittivity of chromium, deriving from Maxwell's equations and chromium's antiferromagnetism, results in increased reflectance at infrared and visible wavelengths.

Passivation

Chromium metal in air is passivated: it forms a thin, protective surface layer of chromium oxide with the corundum structure. Passivation can be enhanced by short contact with oxidizing acids like nitric acid. Passivated chromium is stable against acids. Passivation can be removed with a strong reducing agent that destroys the protective oxide layer on the metal. Chromium metal treated in this way readily dissolves in weak acids.
The surface chromia scale is adherent to the metal. In contrast, iron forms a more porous oxide which is weak and flakes easily and exposes fresh metal to the air, causing continued rusting. At room temperature, the chromia scale is a few atomic layers thick, growing in thickness by outward diffusion of metal ions across the scale. Above 950 °C volatile chromium trioxide forms from the chromia scale, limiting the scale thickness and oxidation protection.
Chromium, unlike iron and nickel, does not suffer from hydrogen embrittlement. However, it does suffer from nitrogen embrittlement, reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal parts.

Isotopes

Naturally occurring chromium is composed of four stable isotopes; ⁵⁰Cr, ⁵²Cr, ⁵³Cr and ⁵⁴Cr, with ⁵²Cr being the most abundant. 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. 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.

Chemistry and compounds

Chromium is a member of group 6, of the transition metals. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.

Common oxidation states

Chromium(0)

Many Cr complexes are known. Bischromium and chromium hexacarbonyl are highlights in organochromium chemistry.

Chromium(II)

Chromium compounds are uncommon, in part because they readily oxidize to chromium derivatives in air. Water-stable chromium chloride that can be made by reducing chromium chloride with zinc. The resulting bright blue solution created from dissolving chromium chloride is stable at neutral pH. Some other notable chromium compounds include chromium oxide, and chromium sulfate. Many chromium carboxylates are known. The red chromium acetate is somewhat famous. It features a Cr-Cr quadruple bond.

Chromium(III)

A large number of chromium compounds are known, such as chromium nitrate, chromium acetate, and chromium oxide. Chromium can be obtained by dissolving elemental chromium in acids like hydrochloric acid or sulfuric acid, but it can also be formed through the reduction of chromium by cytochrome c7. The ion has a similar radius to , and they can replace each other in some compounds, such as in chrome alum and alum.
Chromium tends to form octahedral complexes. Commercially available chromium chloride hydrate is the dark green complex Cl. Closely related compounds are the pale green Cl₂ and violet Cl₃. If anhydrous violet chromium chloride is dissolved in water, the violet solution turns green after some time as the chloride in the inner coordination sphere is replaced by water. This kind of reaction is also observed with solutions of chrome alum and other water-soluble chromium salts. A tetrahedral coordination of chromium has been reported for the Cr-centered Keggin anion ^5–.
Chromium hydroxide is amphoteric, dissolving in acidic solutions to form ³⁺, and in basic solutions to form. It is dehydrated by heating to form the green chromium oxide, a stable oxide with a crystal structure identical to that of corundum.

Chromium(VI)

are oxidants at low or neutral pH. Chromate anions and dichromate anions are the principal ions at this oxidation state. They exist at an equilibrium, determined by pH:
Chromium oxyhalides are known also and include chromyl fluoride and chromyl chloride. However, despite several erroneous claims, chromium hexafluoride remains unknown, as of 2020.
Sodium chromate is produced industrially by the oxidative roasting of chromite ore with sodium carbonate. The change in equilibrium is visible by a change from yellow to orange, such as when an acid is added to a neutral solution of potassium chromate. At yet lower pH values, further condensation to more complex oxyanions of chromium is possible.
Both the chromate and dichromate anions are strong oxidizing reagents at low pH:
They are, however, only moderately oxidizing at high pH:
Chromium compounds in solution can be detected by adding an acidic hydrogen peroxide solution. The unstable dark blue chromium peroxide is formed, which can be stabilized as an ether adduct.
Chromic acid has the hypothetical formula. It is a vaguely described chemical, despite many well-defined chromates and dichromates being known. The dark red chromium oxide, the acid anhydride of chromic acid, is sold industrially as "chromic acid". It can be produced by mixing sulfuric acid with dichromate and is a strong oxidizing agent.