chromium

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(krō'mē-əm) pronunciation
n. (Symbol Cr)
A lustrous, hard, steel-gray metallic element, resistant to tarnish and corrosion and found primarily in chromite. It is used in the hardening of steel alloys and the production of stainless steels, in corrosion-resistant decorative platings, and as a pigment in glass. Atomic number 24; atomic weight 51.996; melting point 1,890°C; boiling point 2,482°C; specific gravity 7.18; valence 2, 3, 6.

[From French chrome. See chrome.]



Metallic chemical element, one of the transition elements, chemical symbol Cr, atomic number 24. A hard, steel-gray metal that takes a high polish, it is used in alloys (e.g., ferrochromium, steel, stainless steel) to increase strength and corrosion resistance. It usually has valence 2, 3, or 6 and always occurs combined with other elements, especially oxygen; chromite is its only commercial source. Various coloured gemstones (e.g., ruby, emerald, serpentine) owe their colour to chromium. Sodium chromate and dichromate are used in leather tanning, in metal surface treatment, and as catalysts. Chromium trioxide is used in chrome plating and as a colorant for ceramics. Chromium oxide, lead chromate, and various other chromium compounds are used as pigments. Chromium dioxide, strongly magnetic, is used in recording tapes and as a catalyst.

For more information on chromium, visit Britannica.com.

A chemical element, Cr, atomic number 24, and atomic weight 51.996, which is the weighted average for several isotopes weighing 50 (4.31%), 52 (83.76%), 53 (9.55%), and 54 (2.38%). The orbital arrangement of the electrons is 1s2, 2s2, 2p6, 3s2, 3p6, 3d5, 4s1. The stability of the half-filled d shell doubtless accounts for this rather unusual arrangement. In the crust of the Earth, chromium is the twenty-first element in abundance, which ranks it along with vanadium, zinc, nickel, and copper. Traces of chromium are present in the human body; in fact, it is essential to life. See also Periodic table.

The element was discovered in 1797 and isolated the following year by the French chemist L. N. Vauquelin. It was named chromium because of the many colors of its compounds. It occurs in nature largely as the mineral chromite (FeO · Cr2O3), which is a spinel, but the ore is usually contaminated with Al3+, Fe3+, Mn2+, and Mg2+. Smaller quantities are found as the yellow mineral crocoite (PbCrO4).

As a transition metal, chromium exists in all oxidation states from 2− to 6+. The chemistry of its aqueous solutions, at least in the 3+ (chromic) state, is complicated by the fact that the compounds exist in many isomeric forms, which have quite different chemical properties.

Pure chromium metal has a bluish-white color, reflects light well, and takes a high polish. When pure, it is ductile, but even small amounts of impurities render it brittle. The metal melts at about 1900°C (3452°F) and boils at 2642°C (4788°F). Chromium shows a wide range of oxidation states; the compounds in which the metal is in a low oxidation state are powerful reducing agents, whereas those in which it shows a high oxidation state are strong oxidizing agents.

The bright color and resistance to corrosion make chromium highly desirable for plating plumbing fixtures, automobile radiators and bumpers, and other decorative pieces. Unfortunately, chrome plating is difficult and expensive. It must be done by electrolytic reduction of dichromate in sulfuric acid solution. This requires the addition of six electrons per chromium ion. This reduction does not take place in one step, but through a series of steps, most of which are not clearly understood. The current efficiency is low (maybe 12%), and the chromium plate contains microscopic cracks and other flaws, and so it does not adequately protect the metal under it from corrosion. It is customary, therefore, to first plate the object with copper, then with nickel, and finally, with chromium.

In alloys with iron, nickel, and other metals, chromium has many desirable properties. Chrome steel is hard and strong and resists corrosion to a marked degree. Stainless steel contains roughly 18% chromium and 8% nickel. Some chrome steels can be hardened by heat treatment and find use in cutlery; still others are used in jet engines. Nichrome and chromel consist largely of nickel and chromium; they have low electrical conductivity and resist corrosion, even at red heat, so they are used for heating coils in space heaters, toasters, and similar devices. Other important alloys are Hastelloy C (Cr, Mo, W, Fe, Ni), used in chemical equipment which is in contact with HCl, oxidizing acids, and hypochlorite. Stellite [Co, Cr, Ni, C, W (or Mo)], noted for its hardness and abrasion resistance at high temperatures, is used for lathes and engine valves, and Inconel (Cr, Fe, Ni) is used in heat treating and in corrosion-resistant equipment in the chemical industry.

Several chromium compounds are used as paint pigments—chrome oxide green (Cr2O3), chrome yellow (PbCrO4), chrome orange (PbCrO4 · PbO), molybdate orange (a solution of PbSO4, PbCrO4, and PbMoO4), chrome green (a mixture of PbCrO4 and Prussian blue), and zinc yellow (potassium zinc chromate). Several of these, particularly zinc yellow, are used to inhibit corrosion. The gems ruby, emerald, and alexandrite owe their colors to traces of chromium compounds.See also Paint.

Dichromates are widely used as oxidizing agents, as rust inhibitors on steel, and as wood preservatives. In the last application, they kill fungi, termites, and boring insects. The wood can still be painted and glued, and retains its strength. Other chromium compounds find use as catalysts, as drilling muds, and in photochemical reactions. The last are important in the printing industry. A metal plate is coated with a colloidal material (for example, glue, shellac, or casein) containing a dichromate. On exposure to strong light under a negative image, the dichromate is reduced to Cr3+, which reacts with the colloid, hardening it and making it resistant to removal by washing. The unexposed material is washed off, and the metal plate is etched with acid to give a printing plate.

Chromium is essential to life. A deficiency (in rats and monkeys) has been shown to impair glucose tolerance, decrease glycogen reserve, and inhibit the utilization of amino acids. It has also been found that inclusion of chromium in the diet of humans sometimes, but not always, improves glucose tolerance. Certain chromium(III) compounds enhance the action of insulin.

On the other hand, chromates and dichromates are severe irritants to the skin and mucous membranes, so workers who handle large amounts of these materials must be protected against dusts and mists. Continued breathing of the dusts finally leads to ulceration and perforation of the nasal septum. Contact of cuts or abrasions with chromate may lead to serious ulceration. Even on normal skin, dermatitis frequently results. Cases of lung cancer have been observed in plants where chromates are manufactured.


A metallic element that is a dietary essential. It forms an organic complex with nicotinic acid, known as the glucose tolerance factor, which facilitates the interaction of insulin with receptors on cell surfaces. Deficiency results in impaired glucose tolerance.

There is little evidence on which to base estimates of requirements; deficiency has been observed at intakes below 6 μg (0.12 μmol)/day; the US/Canadian adequate intake is 35 μg for men and 25 μg for women. High intakes of inorganic chromium salts (in excess of 1-2 mg/day) are associated with kidney and liver damage.

A metallic element essential in the diet for efficient carbohydrate metabolism. It improves the ability of insulin to convert glucose to glycogen (the main energy store in muscles).

Some body-builders use a salt of chromium (chromium picolinate) as an anabolic agent. They claim it burns fat and increases lean body mass by accelerating protein metabolism. They also claim that it may reduce the risk of cardiovascular diseases by increasing the proportion of high density lipoproteins in the blood. Studies in the USA have recently disproved these claims. Obesity experts state that chromium supplements do not speed up metabolism or burn fat. Nutritionists generally advise that you should get enough chromium from a well-balanced diet. Good sources of chromium include liver, meat, cheese, wholegrains, brewer's yeast, and wine.

Drug Info:

Chromium

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Brand names: Chroma-Pak®, CRM®

Chemical formula:



Chromium tablets or capsules

What is chromium?

CHROMIUM (Chroma-Burn®, Chromax®, Chroma Slim®, Chroma Trim®, chromium picolinate, chromium nicotinate) is an essential nutrient. People require very small amounts of chromium in our diet daily to help our bodies use and process dietary sugars properly. Chromium appears to help the body use and process fats and proteins. Very few people develop true chromium deficiency. Most people should obtain their needed amount of chromium by eating a balanced, healthy diet. There is some information available that chromium supplements, when used under a prescriber's advice and monitoring, may be useful to help regulate blood sugar in adults with Type 2 diabetes or diabetes due to the use of corticosteroids. Chromium supplements do not appear to increase athletic performance or muscle mass in athletes nor do they decrease body fat in dieters. Chromium supplements are not officially endorsed by the FDA for these uses. Generic chromium picolinate and chromium nicotinate are available.

What should I tell my health care provider before I take this medicine?

It is important for you to tell your prescriber or health care professional or other health care professional that you are using chromium.

Talk to your health care provider BEFORE taking chromium supplements if you have any of the following conditions:
• dependent on intravenous nutrition (TPN) or liquid nutrition products for food
• diabetes mellitus or high blood sugar levels
• kidney disease
• an unusual or allergic reaction to chromium or other medicines, foods, dyes, or preservatives
• pregnant or trying to get pregnant
• breast-feeding

How should I use this medicine?

Chromium tablets and capsules should be taken orally (i.e., swallowed). It is recommended that these products be taken just prior to food or meals with a sip of water or other liquid. Follow the directions on the product label, or consult your health care professional for advice on how to take this supplement.

Contact your pediatrician or health care professional regarding the use of this medicine in children. Special care may be needed. Chromium should not be used in children without a prescription.

What if I miss a dose?

Missing a dose is probably not harmful. If you miss a dose, simply resume taking it on your previous schedule. Do not take double doses to catch up, however.

What drug(s) may interact with chromium?

• medications used to treat diabetes or to lower blood sugar

For many nutritional supplements, interactions with other medications are unknown. That is why you should always be careful when mixing chromium with traditional medications. If you take any other medications, consult with your health care professional prior to taking chromium.

Tell your prescriber or health care professional about all other medicines you are taking, including non-prescription medicines, nutritional supplements, or herbal products. Also tell your prescriber or health care professional if you are a frequent user of drinks with caffeine or alcohol, if you smoke, or if you use illegal drugs. These may affect the way your medicine works. Check with your health care professional before stopping or starting any of your medicines.

What should I watch for while taking chromium?

If you notice any changes in your physical or mental health while taking chromium, you should contact your health care provider.

Different brands of chromium might contain different amounts of active ingredient so be careful to use the same brand. It is recommended that you use a brand from a reliable manufacturer. Your health care professional or pharmacist can assist you in finding a chromium product.

Certain forms of chromium are not for human use. Do not take chromium products that are not labeled for use as nutritional supplements or medicine.

If you have diabetes and are taking chromium on the advice of your prescriber, visit your prescriber frequently for regular checks on your progress. Learn how to monitor blood sugar and urine ketones regularly. Check with your prescriber or health care professional if your blood sugar is high or low. Do not change your diabetic medicine doses without talking to your health care prescriber. Do not skip meals. If you are exercising much more than usual you may need extra snacks to avoid side effects caused by low blood sugar. If you have mild symptoms of low blood sugar, eat or drink something containing sugar at once and contact your prescriber or health care professional. It is wise to check your blood sugar to confirm that it is low. It is important to recognize your own symptoms of low blood sugar so that you can treat them quickly. Make sure family members know that you can choke if you eat or drink when you have serious symptoms of low blood sugar, such as seizures or unconsciousness. They must get medical help at once.

What side effects may I notice from using chromium?

Side effects that you should report to your prescriber or health care professional as soon as possible:
Symptoms of low blood sugar, for example:
• chills
• cold sweats
• confusion
• headache
• hunger
• irregular heartbeat or palpitations
• nausea or vomiting
• nervousness or anxious
• seizures (convulsions) or unusual movements
• shakiness
• unusual tiredness
• trembling
• vision changes

Other side effects that should be reported to your prescriber or health care professional as soon as possible:
• blurred vision
• drowsiness
• changes in emotions or mood
• change in urine amount, color, or frequency
• fever or chills
• muscle aches, cramps, or pains
• retaining fluid in your legs or abdomen
• skin rash or unusual skin reaction
• unusual weakness or muscle fatigue

Side effects that usually do not require medical attention (report to your prescriber or health care professional if they continue or are bothersome):
• mild stomach upset

Where can I keep my medicine?

Keep out of the reach of children.

Store at room temperature between 8 and 25 degrees C (46 and 77 degrees F); do not freeze. Throw away any unused medicine after the expiration date.

GENERAL INFORMATION REGARDING DIETARY SUPPLEMENTS:
Dietary supplements include amino acids, vitamins, minerals, herbs, and other plant-derived substances, and extracts of these substances. Products are easy to identify as they must state "Dietary Supplement" on the label. A "Supplement Facts" panel is provided on the label for most products. Supplements are not drugs and are not regulated like pharmaceuticals. You should note that rigid quality control standards are not required for dietary supplements. Differences in the potency and purity of these products can occur. Scientific data to support the use of a dietary supplement for a certain condition may not be available. This product is not intended to diagnose, treat, cure or prevent any disease.

The Food and Drug Administration suggests the following to help consumers protect themselves:
• Always read product labels and follow directions.
• Look for products containing ingredients with the "USP" notation. This indicates the manufacturer followed the standards of the US Pharmacopoeia.
• "Natural" doesn't mean a product is safe for humans to consume.
• Supplements produced or distributed by a nationally known food or drug company are more likely to be made under tight controls as these companies have standards in place for their other products. You can write to the company or manufacturer for more information about the conditions under which the products are made.

Last updated: 7/1/2002

Important Disclaimer: The drug information provided here is for educational purposes only. It is intended to supplement, not substitute for, the diagnosis, treatment and advice of a medical professional. This drug information does not cover all possible uses, precautions, side effects and interactions. It should not be construed to indicate that this or any drug is safe for you. Consult your medical professional for guidance before using any prescription or over the counter drugs.

A hard, brittle metal resistant to corrosion, workable when annealed, gray-white in color; used in alloys, esp. steel, and in plating.


A metallic element essential for efficient glucose metabolism. Chromium is readily available in a variety of foods, including liver, meat, cheese, wholegrains, brewer's yeast, and wine. Because chromium deficiency is rare, there is no Reference Nutrient Intake in the UK, but a safe daily intake is set at 25 μg.


    Description
    Precautions
    Interactions
    Complications
    Resources

What is Chromium?

Chromium is a mineral that is essential to humans It is found naturally in a variety of foods, and supplements are available in capsules or tablets. Supplements are prepared using a number of formulas, including chromium (III), chromium aspartate, chromium chloride, chromium citrate, chromium nicotinate,

chromium picolinate, GTF chromium, and trivalent chromium.

What is the Purpose of Chromium?

Chromium supports the normal function of insulin, which is a hormone secreted by the pancreas. Insulin helps transport glucose from the bloodstream into liver, muscle, and fat cells. Once it is inside these cells, the sugar is metabolized into a source of energy. Insulin is also involved in regulating protein, fat, and catalytic enzyme processes. People with diabetes do not produce insulin (or produce very little) or their bodies cannot properly use the insulin that is produced. As a result, sugar builds up in the bloodstream, causing serious health effects. Numerous scientific studies have shown that chromium is useful in treating insulin resistance (metabolic syndrome) and diabetes. Diabetic peripheral neuropathy, a form of nerve damage that is a direct result of diabetes, is indirectly related to a lack of sufficient chromium.

Chromium

AgeAdequate Intake (mcg/day)
Children 0–6 mos0.2
Children 7–12 mos5.5
Children 1–3 yrs11
Children 4–8 yrs15
Boys 9–13 yrs25
Girls 9–13 yrs21
Boys 14–18 yrs35
Girls 14–18 yrs24
Men 19–50 yrs35
Women 19–50 yrs25
Men 50< yrs30
Women 50< yrs20
Pregnant women 18≤ yrs29
Pregnant women 19≥ yrs30
Breastfeeding women 18≤ yrs44
Breastfeeding women 19≥ yrs45
FoodChromium (mcg)
Broccoli, ½ cup11
Grape juice, 1 cup8
English muffin, whole wheat, 14
Garlic, dried, 1 tsp3
Potatoes, mashed, 1 cup3
Basil, dried, 1 tbsp2
Beef cubes, 3 oz2
Orange juice, 1 cup2
Turkey breast, 3 oz2
Whole wheat bread, 2 slices2
Red wine, 5 oz1–13
Apple, unpeeled, 1 med1
Banana, 1 med1
Green beans, ½ cup1
mcg = microgram 
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chromium (krō'mēəm) [Gr.,=color], metallic chemical element; symbol Cr; at. no. 24; at. wt. 51.9961; m.p. about 1,857°C; b.p. 2,672°C; sp. gr. about 7.2 at 20°C; valence +2, +3, +6. Chromium is a silver-gray, lustrous, brittle, hard metal that can be highly polished. It is found in Group 6 of the periodic table. It does not tarnish in air, but burns when heated, forming the green chromic oxide. When combined with oxygen, besides yielding chromic oxide, which is used as a pigment, it forms chromic anhydride (the red trioxide and anhydride of chromic acid). With other metallic elements, e.g., lead and potassium, together with oxygen, it forms the chromates and dichromates. These compounds are salts of chromic acid and are used as pigments in paints, in dyeing, and in the tanning of leather. Chrome yellow, a pigment, consists largely of lead chromate. Other chrome colors are black, red, orange, and green. In the chrome process for tanning leather, a dichromate is used, and chromium hydroxide, a basic compound of chromium, hydrogen, and oxygen, is precipitated and held in the leather. The hydroxide is used also as a mordant in dyeing cloth. A mixture of potassium dichromate and sulfuric acid is used as a powerful agent for cleaning laboratory glassware. Chromium is a comparatively rare element, never occurring by itself in nature but always in compounds. Its chief source is the mineral chromite, which is composed of iron, chromium, and oxygen and is found principally in the nations of the former Soviet Union, South Africa, Zimbabwe, Turkey, and the Philippines. The element, in the form of chromic oxide, gives the greenish tint to the emerald and the aquamarine. Metallic chromium is prepared by reduction of the oxide by aluminum or by carbon. It is used in plating other metals because of its hardness and nontarnishing properties. In alloys with other metals it contributes hardness, strength, and heat resistance. Its most important use is in the steel industry, where it is a constituent of several alloy steels, e.g., chromium steel or chrome steel. Stainless steel contains from 11% to 18% chromium. An alloy of nickel and chromium, often called Nichrome, is widely used as a heating element in electric toasters, coffeepots, and other appliances. Stellite is an extremely hard alloy of cobalt, chromium, and tungsten, with small amounts of iron, silicon, and carbon; it is used in metal cutting tools and for wear-resistant surfaces. A similar alloy, with molybdenum instead of tungsten, is used in surgical tools since it does not react with body fluids. Chromium was discovered in 1797 by L. N. Vauquelin.



symbol: Cr; the element of proton number 24. It is a paramagnetic, metallic transition element of group 6 of the IUPAC periodic table, with oxidation states of II, III, and IV. Natural chromium has a relative atomic mass of 51.966 and is a mixture of four stable nuclides with nucleon numbers of 50, 52, 53, and 54, the predominant one being chromium-52 (relative abundance 83.789 atom percent). A number of artificial radioactive nuclides are known, of which chromium-51 is the longest lived. Chromium has an abundance of about 0.02% of the igneous rocks of the earth and it occurs in nature exclusively in the combined form, the mixed oxide chromite Fe2Cr2O4 being the chief ore. It is an essential dietary trace metal, but functions in the body primarily as a component of glucose-tolerance factor, the chief symptom of experimental chromium deficiency in animals being impaired glucose tolerance.

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A chemical element, atomic number 24, atomic weight 51.996, symbol Cr.

  • c.-51 — a radioisotope of chromium having a half-life of 27.8 days; used to label red blood cells to determine red cell volume and red cell survival time. Symbol 51Cr. See also cr51 edta.
  • c. nutritional deficiency — possibly causally related to the onset of diabetes mellitus in primates.
  • c. trioxide — possibly carcinogenic in humans. See also chromate.

n
Cr

A hard, brittle, metallic element with an atomic number of 24 and an atomic weight of 51.996. Chromium strongly resists corrosion and is used extensively to plate other metals and as an alloy to harden steel. Stainless steels are more than 10% chromium.

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Random House Word Menu by Stephen Glazier
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  See crossword solutions for the clue Chromium.
vanadiumchromiummanganese
-

Cr

Mo
Appearance
silvery metallic
General properties
Name, symbol, number chromium, Cr, 24
Pronunciation /ˈkrmiəm/ KROH-mee-əm
Element category transition metal
Group, period, block 64, d
Standard atomic weight 51.9961(6)
Electron configuration [Ar] 4s13d5
Electrons per shell 2, 8, 13, 1 (Image)
Physical properties
Phase solid
Density (near r.t.) 7.19 g·cm−3
Liquid density at m.p. 6.3 g·cm−3
Melting point 2180 K, 1907 °C, 3465 °F
Boiling point 2944 K, 2671 °C, 4840 °F
Heat of fusion 21.0 kJ·mol−1
Heat of vaporization 339.5 kJ·mol−1
Molar heat capacity 23.35 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 1656 1807 1991 2223 2530 2942
Atomic properties
Oxidation states 6, 5, 4, 3, 2, 1, -1, -2
(strongly acidic oxide)
Electronegativity 1.66 (Pauling scale)
Ionization energies
(more)
1st: 652.9 kJ·mol−1
2nd: 1590.6 kJ·mol−1
3rd: 2987 kJ·mol−1
Atomic radius 128 pm
Covalent radius 139±5 pm
Miscellanea
Crystal structure body-centered cubic
Magnetic ordering AFM (rather: SDW[1])
Electrical resistivity (20 °C) 125 nΩ·m
Thermal conductivity 93.9 W·m−1·K−1
Thermal expansion (25 °C) 4.9 µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 5940 m·s−1
Young's modulus 279 GPa
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
Most stable isotopes
Main article: Isotopes of chromium
iso NA half-life DM DE (MeV) DP
50Cr 4.345% > 1.8×1017y εε - 50Ti
51Cr syn 27.7025 d ε - 51V
γ 0.320 -
52Cr 83.789% 52Cr is stable with 28 neutrons
53Cr 9.501% 53Cr is stable with 29 neutrons
54Cr 2.365% 54Cr is stable with 30 neutrons
· r

Chromium (play /ˈkrmiəm/ KROH-mee-əm) is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable. The name of the element is derived from the Greek word "chrōma" (χρώμα), meaning colour,[2] because many of its compounds are intensely coloured. Chromium oxide was used by the Chinese in the Qin dynasty over 2,000 years ago to coat weapons such as bronze crossbow bolts and steel swords found at the Terracotta Army. It later came to the attention of the West when it was discovered by Louis Nicolas Vauquelin in the mineral crocoite (lead(II) chromate) in 1797. Crocoite was used as a pigment, and after the discovery that the mineral chromite also contains chromium, this mineral was used to produce pigments as well.

Chromium was regarded with great interest because of its high corrosion resistance and hardness. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding chromium to form stainless steel. This application, along with chrome plating (electroplating with chromium) are currently the highest-volume uses of the metal. Chromium and ferrochromium are produced from the single commercially viable ore, chromite, by silicothermic or aluminothermic reaction or by roasting and leaching processes.

Although trivalent chromium (Cr(III)) is required in trace amounts for sugar and lipid metabolism, few cases have been reported where its complete removal from the diet has caused chromium deficiency. In larger amounts and in different forms, chromium can be toxic and carcinogenic. The most prominent example of toxic chromium is hexavalent chromium (Cr(VI)). Abandoned chromium production sites often require environmental cleanup.

Contents

Characteristics

Physical

Chromium is remarkable for its magnetic properties: it is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, it transforms into a paramagnetic state.[1]

Passivation

Chromium metal left standing in air is passivated by oxygen, forming a thin protective oxide surface layer. This layer is a spinel structure only a few atoms thick. It is very dense, and prevents the diffusion of oxygen into the underlying material. This barrier is in contrast to iron or plain carbon steels, where the oxygen migrates into the underlying material and causes rusting.[3] The passivation can be enhanced by short contact with oxidizing acids like nitric acid. Passivated chromium is stable against acids. The opposite effect can be achieved by treatment 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.[4]

Chromium, unlike metals such as 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.[5]

Occurrence

Crocoite (PbCrO4)

Chromium is the 21st most abundant element in Earth's crust with an average concentration of 100 ppm.[6] Chromium compounds are found in the environment, due to erosion of chromium-containing rocks and can be distributed by volcanic eruptions. The concentrations range in soil is between 1 and 3000 mg/kg, in sea water 5 to 800 µg/liter, and in rivers and lakes 26 µg/liter to 5.2 mg/liter.[7] Chromium is mined as chromite (FeCr2O4) ore.[8] About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, while Kazakhstan, India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.[9]

Although rare, deposits of native chromium exist.[10][11] 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 diamond.[12]

The relation between Cr(III) and Cr(VI) strongly depends on pH and oxidative properties of the location, but in most cases, the Cr(III) is the dominating species,[7] although in some areas the ground water can contain up to 39 µg/liter of total chromium of which 30 µg/liter is present as Cr(VI).[13]

Isotopes

Naturally occurring chromium is composed of three 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) 1.8×1017 years, and 51Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minute. This element also has 2 meta states.[14]

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 isotopic composition 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.[15]

The isotopes of chromium range in atomic mass from 43 u (43Cr) to 67 u (67Cr). The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.[14] 53Cr has been posited as a proxy for atmospheric oxygen concentration.[16]

Compounds

Oxidation
states[note 1][17]
−2 Na2[Cr(CO)5]
−1 Na2[Cr2(CO)10]
0 Cr(C6H6)2
+1 K3[Cr(CN)5NO]
+2 CrCl2
+3 CrCl3
+4 K2CrF6
+5 K3CrO8
+6 K2CrO4

Chromium is a member of the transition metals, in group 6. Chromium(0) has an electronic configuration of 4s13d5, owing to the lower energy of the high spin configuration. Chromium exhibits a wide range of possible oxidation states, where the +3 state is most stable energetically; the +3 and +6 states are most commonly observed in chromium compounds, whereas the +1, +4 and +5 states are rare.[17]

The following is the Pourbaix diagram for chromium in pure water, perchloric acid or sodium hydroxide:[7][18] Chromium in water pourbiax diagram.png

Chromium(III)

Chromium(III) chloride hexahydrate ([CrCl2(H2O)4]Cl·2H2O)
Anhydrous chromium(III) chloride (CrCl3)

A large number of chromium(III) compounds are known. Chromium(III) can be obtained by dissolving elemental chromium in acids like hydrochloric acid or sulfuric acid. The Cr3+ ion has a similar radius (63 pm) to the Al3+ ion (radius 50 pm), so they can replace each other in some compounds, such as in chrome alum and alum. When a trace amount of Cr3+ replaces Al3+ in corundum (aluminium oxide, Al2O3), the red-colored ruby is formed.

Chromium(III) ions tend to form octahedral complexes. The colors of these complexes is determined by the ligands attached to the Cr centre. The commercially available chromium(III) chloride hydrate is the dark green complex [CrCl2(H2O)4]Cl. Closely related compounds have different colours: pale green [CrCl(H2O)5]Cl2 and the violet [Cr(H2O)6]Cl3. If water-free green chromium(III) chloride is dissolved in water then the green solution turns violet after some time, due to the substitution of water by chloride in the inner coordination sphere. This kind of reaction is also observed with solutions of chrome alum and other water-soluble chromium(III) salts.

Chromium(III) hydroxide (Cr(OH)3) is amphoteric, dissolving in acidic solutions to form [Cr(H2O)6]3+, and in basic solutions to form [Cr(OH)6]3−. It is dehydrated by heating to form the green chromium(III) oxide (Cr2O3), which is the stable oxide with a crystal structure identical to that of corundum.[4]

Chromium(VI)

Chromium(VI) oxide

Chromium(VI) compounds are powerful oxidants at low or neutral pH. Most important are chromate anion (CrO2−
4
) and dichromate (Cr2O72-) anions, which exist in equilibrium:

2 [CrO4]2- + 2 H+ \overrightarrow{\leftarrow} [Cr2O7]2- + 2 H2O

Chromium(VI) halides are known also and include hexafluoride CrF6 and chromyl chloride (CrO2Cl2).[4]

Sodium chromate is produced industrially by the oxidative roasting of chromite ore with calcium or sodium carbonate. The dominant species is therefore, by the law of mass action, determined by the pH of the solution. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), 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:[4]

Sodium chromate (Na2CrO4)
Cr2O2−
7
+ 14 H3O+ + 6 e → 2 Cr3+ + 21 H2O0 = 1.33 V)

They are, however, only moderately oxidizing at high pH:[4]

CrO2−
4
+ 4 H2O + 3 eCr(OH)3 + 5 OH0 = −0.13 V)

Chromium(VI) compounds in solution can be detected by adding an acidic hydrogen peroxide solution. The unstable dark blue chromium(VI) peroxide (CrO5) is formed, which can be stabilized as an ether adduct CrO5·OR2.[4]

Chromic acid has the hypothetical formula H2CrO4. It is a vaguely described chemical, despite many well-defined chromates and dichromates are known. The dark red chromium(VI) oxide CrO3, the acid anhydride of chromic acid, is sold industrially as "chromic acid".[4] It can be produced by mixing sulfuric acid with dichromate, and is a strong oxidizing agent.

Chromium(V) and chromium(IV)

The oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate. The only binary compound is the volatile chromium(V) fluoride (CrF5). This red solid has a melting point of 30 °C and a boiling point of 117 °C. It can be synthesized by treating chromium metal with fluorine at 400 °C and 200 bar pressure. The peroxochromate(V) is another example of the +5 oxidation state. Potassium peroxochromate (K3[Cr(O2)4]) is made by reacting potassium chromate with hydrogen peroxide at low temperatures. This red brown compound is stable at room temperature but decomposes spontaneously at 150–170 °C.[19]

Compounds of chromium(IV) (in the +4 oxidation state) are slightly more common than those of chromium(V). The tetrahalides, CrF4, CrCl4, and CrBr4, can be produced by treating the trihalides (CrX3) with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water.

Chromium(II) and chromium(I)

Many chromium(II) compounds are known, including the water-stable chromium(II) chloride, CrCl2, which can be made by reduction of chromium(III) chloride with zinc. The resulting bright blue solution is only stable at neutral pH.[4] Many chromous carboxylates are also known, most famously, the red chromous acetate (Cr2(O2CCH3)4), which features a quadruple bond.

Chromium(I)

Most Cr(I) compounds are obtained by oxidation of electron-rich, octahedral Cr(0) complexes. Other Cr(I) complexes contain cyclopentadienyl ligands. As verified by X-ray diffraction, a Cr-Cr quintuple bond (length 183.51(4)  pm) has also been described.[20] Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactions.

Chromium compound determined experimentally to contain a Cr-Cr quintuple bond

Chromium(0) and chromium(I)

Many chromium(0) compounds are known. Most are derivatives of chromium hexacarbonyl or bis(benzene)chromium.

History

Weapons found in burial pits dating from the late 3rd century BC Qin Dynasty of the Terracotta Army near Xi'an, China have been analyzed by archaeologists. Although buried more than 2,000 years ago, the ancient bronze tips of crossbow bolts and swords found at the site showed no sign of corrosion, because the bronze was coated with chromium.[21]

Chromium later came to the attention of westerners in the 18th 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 Crocoite (lead chromate) with a formula of PbCrO4.[22]

In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that had useful properties as a pigment in paints. The use of Siberian red lead as a paint pigment developed rapidly. A bright yellow pigment made from crocoite also became fashionable.[22]

The red colour of rubies is from a small amount of chromium(III).

In 1797, Louis Nicolas Vauquelin received samples of crocoite ore. He produced chromium trioxide (CrO3) by mixing crocoite with hydrochloric acid. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven.[23] He was also able to detect traces of chromium in precious gemstones, such as ruby or emerald.[22][24]

During the 1800s, chromium was primarily used as a component of paints and in tanning salts. At first, crocoite from Russia was the main source, but in 1827, a larger chromite deposit was discovered near Baltimore, United States. This made the United States the largest producer of chromium products till 1848 when large deposits of chromite where found near Bursa, Turkey.[8]

Chromium is also known for its luster when polished. It is used as a protective and decorative coating on car parts, plumbing fixtures, furniture parts and many other items, usually applied by electroplating. Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924.[25]

Metal alloys now account for 85% of the use of chromium. The remainder is used in the chemical industry and refractory and foundry industries.

Production

Piece of chromium produced with aluminothermic reaction
World production trend of chromium
chromium, remelted in a horizontal arc zone-refiner, showing large visible crystal grains

Approximately 4.4 million metric tons of marketable chromite ore were produced in 2000, and converted into ~3.3 million tons of ferro-chrome with an approximate market value of 2.5 billion United States dollars.[26] The largest producers of chromium ore have been South Africa (44%) India (18%), Kazakhstan (16%) Zimbabwe (5%), Finland (4%) Iran (4%) and Brazil (2%) with several other countries producing the rest of less than 10% of the world production.[26]

The two main products of chromium ore refining are ferrochromium and metallic chromium. For those products the ore smelter process differs considerably. For the production of ferrochromium, the chromite ore (FeCr2O4) is reduced in large scale in electric arc furnace or in smaller smelters with either aluminium or silicon in an aluminothermic reaction.[27]

Chromium ore output in 2002[26]

For the production of pure chromium, the iron has to be separated from the chromium in a two step roasting and leaching process. The chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms the stable Fe2O3. The subsequent leaching at higher elevated temperatures dissolves the chromates and leaves the insoluble iron oxide. The chromate is converted by sulfuric acid into the dichromate.[27]

4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2
2 Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O

The dichromate is converted to the chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium.[27]

Na2Cr2O7 + 2 C → Cr2O3 + Na2CO3 + CO
Cr2O3 + 2 Al → Al2O3 + 2 Cr

Applications

Metallurgy

Decorative chrome plating on a motorcycle.

The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. The high speed tool steels contain between 3 and 5% chromium. Stainless steel, the main corrosion-proof metal alloy, is formed when chromium is added to iron in sufficient concentrations, usually above 11%. For its formation, ferrochromium is added to the molten iron. Also nickel-based alloys increase in strength due to the formation of discrete, stable metal carbide particles at the grain boundaries. For example, Inconel 718 contains 18.6% chromium. Because of the excellent high temperature properties of these nickel superalloys, they are used in jet engines and gas turbines in lieu of common structural materials.[28]

The relative high hardness and corrosion resistance of unalloyed chromium makes it a good surface coating, being still the most "popular" metal coating with unparalleled combined durability. A thin layer of chromium is deposited on pretreated metallic surfaces by electroplating techniques. There are two deposition methods: Thin, below 1 µm thickness, layers are deposited by chrome plating, and are used for decorative surfaces. If wear-resistant surfaces are needed then thicker chromium layers are deposited. Both methods normally use acidic chromate or dichromate solutions. To prevent the energy consuming change in oxidation state, the use of Chromium(III) sulfate is under development, but for most applications, the established process is used.[25]

In the chromate conversion coating process, the strong oxidative properties of chromates are used to deposit a protective oxide layer on metals like aluminium, zinc and cadmium. This passivation and the self healing properties by the chromate stored in the chromate conversion coating, which is able to migrate to local defects, are the benefits of this coating method.[29] Because of environmental and health regulations on chromates, alternative coating method are under development.[30]

Anodizing of aluminium is another electrochemical process, which does not lead to the deposition of chromium, but uses chromic acid as electrolyte in the solution. During anodization, an oxide layer is formed on the aluminium. The use of chromic acid, instead of the normally used sulfuric acid, leads to a slight difference of these oxide layers.[31] The high toxicity of Cr(VI) compounds, used in the established chromium electroplating process, and the strengthening of safety and environmental regulations demand a search for substitutes for chromium or at least a change to less toxic chromium(III) compounds.[25]

Dye and pigment

School bus painted in chrome yellow[32]

The mineral crocoite (lead chromate PbCrO4) was used as a yellow pigment shortly after its discovery. After a synthesis method became available starting from the more abundant chromite, chrome yellow was, together with cadmium yellow, one of the most used yellow pigments. The pigment does not photo degrade but it tends to darken due to the formation of chromium(III) oxide. It has a strong color, and was used for school buses in the US and for Postal Service (for example Deutsche Post) in Europe. The use of chrome yellow declined due to environmental and safety concerns and was replaced by organic pigments or alternatives free from lead and chromium. Other pigments based on chromium are, for example, the bright red pigment chrome red, which is a basic lead chromate (PbCrO4·Pb(OH)2). A very important chromate pigment, which was used widely in metal primer formulations was zinc chromate, now replaced by zinc phosphate. A wash primer was formulated to replace the dangerous practice of pretreating aluminium aircraft bodies with a phosphoric acid solution. This used zinc tetroxychromate dispersed in a solution of polyvinyl butyral. An 8% solution of phosphoric acid in solvent was added just before application. It was found that an easily oxidized alcohol was an essential ingredient. A thin layer of about 10–15 microns was applied, which turned from yellow to dark green when it was cured. There is still a question as to the correct mechanism. Chrome green is a mixture of Prussian blue and chrome yellow, while the chrome oxide green is Chromium(III) oxide.[33]

A red color is achieved by doping chromium(III) into the crystals of corundum, which are then called ruby. Therefore, chromium is used in producing synthetic rubies.[34]

Chromium oxides are also used as a green color in glassmaking and as a glaze in ceramics.[35] Green chromium oxide is extremely light-fast and as such is used in cladding coatings. It is also the main ingredient in IR reflecting paints, used by the armed forces, to paint vehicles, to give them the same IR reflectance as green leaves.[36]

Wood preservative

Because of their toxicity, chromium(VI) salts are used for the preservation of wood. For example, chromated copper arsenate (CCA) is used in timber treatment to protect wood from decay fungi, wood attacking insects, including termites, and marine borers.[37] The formulations contain chromium based on the oxide CrO3 between 35.3% and 65.5%. In the United States, 65,300 metric tons of CCA solution have been used in 1996.[37]

Tanning

Chromium(III) salts, especially chrome alum and chromium(III) sulfate, are used in the tanning of leather. The chromium(III) stabilizes the leather by cross linking the collagen fibers.[38] Chromium tanned leather can contain between 4 and 5% of chromium, which is tightly bound to the proteins.[8] Although the form of chromium used for tanning is not the toxic hexavalent variety, there remains interest in management of chromium in the tanning industry such as recovery and reuse, direct/indirect recycling,[39] use of less chromium or "chrome-less" tanning are practiced to better manage chromium in tanning.

Refractory material

The high heat resistivity and high melting point makes chromite and chromium(III) oxide a material for high temperature refractory applications, like blast furnaces, cement kilns, molds for the firing of bricks and as foundry sands for the casting of metals. In these applications, the refractory materials are made from mixtures of chromite and magnesite. The use is declining because of the environmental regulations due to the possibility of the formation of chromium(VI).[27]

Catalysts

Several chromium compounds are used as catalysts for processing hydrocarbons. For example the Phillips catalysts for the production of polyethylene are mixtures of chromium and silicon dioxide or mixtures of chromium and titanium and aluminium oxide.[40] Fe-Cr mixed oxides are employed as high-temperature catalysts for the water gas shift reaction.[41][42] Copper chromite is a useful hydrogenation catalyst.[43]

Other use

  • Chromium(IV) oxide (CrO2) is a magnetic compound. Its ideal shape anisotropy, which imparts high coercivity and remnant magnetization, made it a compound superior to the γ-Fe2O3. Chromium(IV) oxide is used to manufacture magnetic tape used in high-performance audio tape and standard audio cassettes.[44] Chromates can prevent corrosion of steel under wet conditions, and therefore chromates are added to drilling muds.[45]
  • Chromium(III) oxide is a metal polish known as green rouge.
  • Chromic acid is a powerful oxidizing agent and is a useful compound for cleaning laboratory glassware of any trace of organic compounds. It is prepared in situ by dissolving potassium dichromate in concentrated sulfuric acid, which is then used to wash the apparatus. Sodium dichromate is sometimes used because of its higher solubility (50 g/L versus 200 g/L respectively). 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.

Biological role

Chromium has no verified biological role and has been classified as not essential for mammals.[46] (Cr(III) or Cr3+) occurs in trace amounts and appears to be benign.[47] Chromium deficiency is controversial or is at least extremely rare. It has been attributed to only three people on parenteral nutrition, which is when a patient is fed a liquid diet through intravenous drips.[48] In contrast, hexavalent chromium (Cr(VI) or Cr6+) is very toxic and mutagenic when inhaled. Cr(VI) has not been established as a carcinogen when in solution, although it may cause allergic contact dermatitis (ACD).[49] Although no biological role for chromium has ever been demonstrated, dietary supplements for chromium include chromium(III) picolinate, chromium(III) polynicotinate, and related materials. The benefit of those supplements is questioned by some studies.[50]

The use of chromium-containing dietary supplements is controversial owing to the absence of any verified biological role, the expense of these supplements, and the complex effects of their use.[51] The popular dietary supplement chromium picolinate complex generates chromosome damage in hamster cells (due to the picolinate ligand).[52] In the United States the dietary guidelines for daily chromium uptake were lowered from 50–200 µg for an adult to 35 µg (adult male) and to 25 µg (adult female).[53]

Precautions

Water insoluble chromium(III) compounds and chromium metal are not considered a health hazard, while the toxicity and carcinogenic properties of chromium(VI) have been known for a long time.[54] Because of the specific transport mechanisms, only limited amounts of chromium(III) enter the cells. Several in vitro studies indicated that high concentrations of chromium(III) in the cell can lead to DNA damage.[55] Acute oral toxicity ranges between 1.5 and 3.3 mg/kg.[56] The proposed beneficial effects of chromium(III) and the use as dietary supplements yielded some controversial results, but recent reviews suggest that moderate uptake of chromium(III) through dietary supplements poses no risk.[55]

Cr(VI)

The acute oral toxicity for chromium(VI) ranges between 50 and 150 µg/kg.[56] In the body, chromium(VI) is reduced by several mechanisms to chromium(III) already in the blood before it enters the cells. The chromium(III) is excreted from the body, whereas the chromate ion is transferred into the cell by a transport mechanism, by which also sulfate and phosphate ions enter the cell. The acute toxicity of chromium(VI) is due to its strong oxidational properties. After it reaches the blood stream, it damages the kidneys, the liver and blood cells through oxidation reactions. Hemolysis, renal and liver failure are the results of these damages. Aggressive dialysis can improve the situation.[57]

The carcinogenity of chromate dust is known for a long time, and in 1890 the first publication described the elevated cancer risk of workers in a chromate dye company.[58][59] Three mechanisms have been proposed to describe the genotoxicity of chromium(VI). The first mechanism includes highly reactive hydroxyl radicals and other reactive radicals which are by products of the reduction of chromium(VI) to chromium(III). The second process includes the direct binding of chromium(V), produced by reduction in the cell, and chromium(IV) compounds to the DNA. The last mechanism attributed the genotoxicity to the binding to the DNA of the end product of the chromium(III) reduction.[60]

Chromium salts (chromates) are also the cause of allergic reactions in some people. Chromates are often used to manufacture, amongst other things, leather products, paints, cement, mortar and anti-corrosives. Contact with products containing chromates can lead to allergic contact dermatitis and irritant dermatitis, resulting in ulceration of the skin, sometimes referred to as "chrome ulcers". This condition is often found in workers that have been exposed to strong chromate solutions in electroplating, tanning and chrome-producing manufacturers.[61][62]

Environmental issues

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 sites, 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.[63]

In 2010, the Environmental Working Group studied the drinking water in 35 American cities. The study was the first nationwide analysis measuring the presence of the chemical in U.S. water systems. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of list; 25 cities had levels that exceeded California's proposed limit.[64] Note: Concentrations of Cr VI in US municipal drinking water supplies reported by EWG are within likely, natural background levels for the areas tested and not necessarily indicative of industrial pollution (CalEPA Fact Sheet), as asserted by EWG. This factor was not taken into consideration in their report.

See also

Notes

  1. ^ Most common oxidation states of chromium are in bold. The right column lists a representative compound for each oxidation state.

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External links


Translations:

Chromium

Top

Dansk (Danish)
n. - krom

Nederlands (Dutch)
chromium

Français (French)
n. - chrome

Deutsch (German)
n. - Chrom

Ελληνική (Greek)
n. - (χημ.) χρώμιο

Italiano (Italian)
cromo

Português (Portuguese)
n. - cromo (m) (Quím.)

Русский (Russian)
хром

Español (Spanish)
n. - cromo

Svenska (Swedish)
n. - krom

中文(简体)(Chinese (Simplified))

中文(繁體)(Chinese (Traditional))
n. - 鉻

한국어 (Korean)
n. - 원소기호 크로뮴

日本語 (Japanese)
n. - クロミウム

العربيه (Arabic)
‏(الاسم) معدن الكروميوم‏

עברית (Hebrew)
n. - ‮כרום‬


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Cr
chromaffin (biology)
GTF
ferrochromium (metallurgy)