Chromium

24 vanadium ← chromium → manganese - ↑ Cr ↓ Mo periodic table
General
Name, Symbol, Number	chromium, Cr, 24
Chemical series	transition metals
Group, Period, Block	6, 4, d
Appearance	silvery metallic
Atomic mass	51.9961(6) g/mol
Electron configuration	[Ar] 3d5 4s1
Electrons per shell	2, 8, 13, 1
Physical properties
Phase	solid
Density (near r.t.)	7.15 g/cm³
Liquid density at m.p.	6.3 g/cm³
Melting point	2180 K (1907 °C, 3465 °F)
Boiling point	2944 K (2671 °C, 4840 °F)
Heat of fusion	21.0 kJ/mol
Heat of vaporization	339.5 kJ/mol
Heat capacity	(25 °C) 23.35 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 1656 1807 1991 2223 2530 2942
Atomic properties
Crystal structure	cubic body centered
Oxidation states	6, 4, 3, 2 (strongly acidic oxide)
Electronegativity	1.66 (Pauling scale)
Ionization energies (more)	1st: 652.9 kJ/mol
	2nd: 1590.6 kJ/mol
	3rd: 2987 kJ/mol
Atomic radius	140 pm
Atomic radius (calc.)	166 pm
Covalent radius	127 pm
Miscellaneous
Magnetic ordering	AFM (rather: SDW)
Electrical resistivity	(20 °C) 125 nΩ·m
Thermal conductivity	(300 K) 93.9 W/(m·K)
Thermal expansion	(25 °C) 4.9 µm/(m·K)
Speed of sound (thin rod)	(20 °C) 5940 m/s
Speed of sound (thin rod)	(r.t.) 279 m/s
Shear modulus	115 GPa
Bulk modulus	160 GPa
Poisson ratio	0.21
Mohs hardness	8.5
Vickers hardness	1060 MPa
Brinell hardness	1120 MPa
CAS registry number	7440-47-3
Notable isotopes
Main article: Isotopes of chromium iso NA half-life DM DE (MeV) DP 50Cr syn > 1.8×1017y εε - 50Ti 51Cr syn 27.7025 d ε - 51V γ 0.320 - 52Cr 83.789% Cr is stable with 28 neutrons 53Cr 9.501% Cr is stable with 29 neutrons 54Cr 2.365% Cr is stable with 30 neutrons

Chromium (chemical symbol Cr, atomic number 24) is a steel-gray, shiny, hard metal that takes a high polish.

• a chemical element in the periodic table

Discovery and etymology

In 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains and named it "Siberian red lead." Though misidentified as a lead compound with selenium and iron components, the material was in fact lead chromate, with the chemical formula PbCrO₄. It is now known as the mineral crocoite.

In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red "lead" mineral that had very useful properties as a pigment in paints. The use of Siberian red lead as a paint pigment developed rapidly. In addition, a bright yellow made from crocoite became a fashionable color.

In 1797, Nicolas-Louis Vauquelin received samples of crocoite ore. By mixing crocoite with hydrochloric acid, he was able to produce chromium oxide, with the chemical formula CrO₃. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He was also able to detect traces of chromium in precious gemstones such as ruby and emerald.

During the 1800s, chromium was primarily used as a component of paints and in tanning salts. Now its primary use is for metal alloys, accounting for 85% of the use of chromium. The remainder is used in the chemical industry and refractory and foundry industries.

Chromium was named after the Greek word "chroma" meaning color, because of the many colorful compounds made from it.

Occurrence and isolation

Chromium is mined as chromite (FeCr₂O₄) ore. Roughly half this ore in the world is produced in South Africa. In addition, Kazakhstan, India, and Turkey are substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.

Deposits of native chromium metal are rare, but they have been discovered. The Udachnaya Mine 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 diamond.

To isolate the metal commercially, chromite ore is oxidized by reacting it with molten alkali (sodium hydroxide, NaOH). This produces sodium chromate (Na₂CrO₄), which is reduced with carbon to chromium(III) oxide (Cr₂O₃). The metal is obtained by heating the oxide in the presence of aluminum or silicon.

About 15 million tons of marketable chromite ore were produced in 2000 and converted into roughly 4 million tons of ferrochrome (consisting of 70% chromium alloyed with iron), with an approximate market value of 2.5 billion U.S. dollars.

Notable characteristics

Chromium is a transition metal in period 4 of the periodic table, situated between vanadium and manganese. It is placed in group 6 (former group 6B).

In the presence of oxygen, chromium rapidly produces a thin oxide layer that protects the metal from further reaction with oxygen.

Chromium has a high melting point.

Chromium forms compounds in which it has a variety of oxidation states. Its common oxidation states are +2, +3, and +6, with +3 being the most stable. In addition, the +1, +4, and +5 states have been observed in rare cases. Chromium compounds of oxidation state +6 are powerful oxidants.

Applications

Uses of chromium:

• In metallurgy, to impart corrosion resistance and a shiny finish:
  • as an alloy constituent, such as in stainless steel in cutlery
  • in chrome plating,
  • in anodized aluminium, literally turning the surface of aluminium into ruby.
• As dyes and paints.
  • Chromium(III) oxide is a metal polish known as green rouge.
  • Chromium salts color glass an emerald green.
  • Chromium is what makes a ruby red, and therefore is used in producing synthetic rubies.
• As a catalyst.
• Chromite is used to make molds for the firing of bricks.
• Chromium salts are used in the tanning of leather.
• Potassium dichromate is a chemical reagent, used in cleaning laboratory glassware and as a titrating agent. It is also used as a mordant (i.e., a fixing agent) for dyes in fabric.
• Chromium(VI) oxide (CrO₃) is used to manufacture magnetic tape, where its higher coercivity than iron oxide tapes gives better performance.
• In well drilling muds as an anti-corrosive.

Biological role

Trivalent chromium (Cr(III), or Cr³⁺) is required in trace amounts for sugar metabolism in humans, and its deficiency can cause chromium deficiency. In contrast, hexavalent chromium is very toxic.

Compounds

Potassium dichromate is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any possible organics. It is used as a saturated solution in concentrated sulphuric acid for washing the apparatus. For this purpose, however, sodium dichromate is sometimes used because of its higher solubility (20 g/100 ml and 5 g/100 ml). Chrome green is the green oxide of chromium, Cr₂O₃, used in enamel painting, and glass staining. Chrome yellow is a brilliant yellow pigment, PbCrO₄, used by painters.

Chromic acid has the hypothetical structure H₂CrO₄. Neither chromic nor dichromic acid is found in nature, but their anions are found in a variety of compounds. Chromium trioxide, CrO₃, the acid anhydride of chromic acid, is sold industrially as "chromic acid".

See also chromium compounds.

Chromium and the quintuple bond

Chromium is notable for its ability to form quintuple covalent bonds. Writing in Science, Tailuan Nguyen, a graduate student working with Philip Power of the University of California, Davis describes the synthesis of a compound of chromium(I) and a hydrocarbon radical which was shown via X-ray diffraction to contain a quintuple bond of length 183.51(4) pm joining the two central chromium atoms. This was accomplished through the use of an extremely bulky monodentate ligand which through its sheer size, is able to prevent further coordination. Chromium currently remains the only element for which quintuple bonds have been observed.

The compound had the Lewis structure

${\rm {Ar-Cr-Cr-Ar}}$

where ${\rm {Ar}}$ is the aryl group ${\rm {C_{6}H_{3}{\mbox{-}}2,6(C_{6}H_{3}{\mbox{-}}2,6{\mbox{-}}Pr_{2}^{i})_{2}}}$ (${\rm {Pr^{i}}}$ is isopropyl)

Isotopes

Naturally occurring chromium is composed of 3 stable isotopes; ⁵²Cr, ⁵³Cr, and ⁵⁴Cr with ⁵²Cr being the most abundant (83.789% natural abundance). 19 radioisotopes have been characterized with the most stable being ⁵⁰Cr with a half-life of (more than) 1.8x10¹⁷ years, and ⁵¹Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lifes that are less than 24 hours and the majority of these have half lifes that are less than 1 minute. This element also has 2 meta states.

⁵³Cr is the radiogenic decay product of ⁵³Mn. Chromium isotopic contents are typically combined with manganese isotopic contents and have found application in isotope geology. Mn-Cr isotope ratios reinforce the evidence from ²⁶Al and ¹⁰⁷Pd for the early history of the solar system. Variations in ⁵³Cr/⁵²Cr and Mn/Cr ratios from several meteorites indicate an initial ⁵³Mn/⁵⁵Mn ratio that suggests Mn-Cr isotope systematics 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 isotopes of chromium range in atomic weight from 43 amu (⁴³Cr) to 67 amu (⁶⁷Cr). The primary decay mode before the most abundant stable isotope, ⁵²Cr, is electron capture and the primary mode after is beta decay.

Precautions

Chromium metal and chromium(III) compounds are not usually considered health hazards, but hexavalent chromium (chromium VI) compounds can be toxic if orally ingested or inhaled. The lethal dose of poisonous chromium (VI) compounds is about one half teaspoon of material. Most chromium (VI) compounds are irritating to eyes, skin and mucous membranes. Chronic exposure to chromium (VI) compounds can cause permanent eye injury, unless properly treated. Chromium(VI) is an established human carcinogen. Exposure to the poisonous chemical hexavalent chromium in drinking water formed the plot of the motion picture Erin Brockovich.

World Health Organization recommended maximum allowable concentration in drinking water for chromium (VI) is 0.05 milligrams per liter.

As chromium compounds were used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at abandoned industrial site, now needing environmental cleanup and remediation per the treatment of brownfield land. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications.

See also

• Chromium compounds
• Chromium minerals
• Chromium VI

References

ISBN links support NWE through referral fees

General references

• Los Alamos National Laboratory - Chromium

External links

• Case Studies in Environmental Medicine: Chromium Toxicity
• IARC Monograph "Chromium and Chromium compounds"
• International Chromium Development Association
• It's Elemental – The Element Chromium
• National Pollutant Inventory - Chromium (III) compounds fact sheet
• The Merck Manual – Mineral Deficiency and Toxicity
• WebElements.com – Chromium

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