Chromium is a chemical element with an atomic number of 24 in the periodic table of elements. It’s one of the most abundant naturally occurring elements in Earth’s crust. This element, also known as the hardest metal on Earth, is a member of the transition metals family of periodic table elements and has six valence electron configuration that participates in numerous chemical bonds with the other chemical elements.
Chemical and Physical Properties of Chromium
The symbol in the periodic table of elements: (Cr)
Atomic number: 24
Atomic weight (mass): 51.996 g.mol -1
Group number: 6 (Transition metals)
Color: A lustrous silvery-gray metal with a bluish hue
Physical state: Solid at room temperature
Half-life: 27.8 days
Electronegativity according to Pauling: 1.6
Density: 7.19 g.cm-3 at 20°C
Melting point: 1907 °C
Boiling point: 2672 °C
Van der Waals radius: 0.127 nm
Ionic radius: N/A
Most characteristic isotope: 52Cr, 53Cr, and 54Cr
Electronic shell: [Ar] 3d5 4s1
The energy of the first ionization: 651.1 kJ.mol-1
The energy of the second ionization: 1590.1 kJ.mol-1
Discovery date: In 1797 by N. L. Vauquelin
Chromium is a transition metal assigned with the periodic table symbol Cr, atomic number 24, has an atomic mass of 51.996 g.mol-1, and electron configuration [Ar] 3d5 4s1. It reaches its boiling point at 2672 °C, while the melting point is achieved at 1907 °C.
This lustrous, brittle, and corrosion-resistant hard metal can be polished to a highly reflective surface. Apart from being resistant to heat, wear, and corrosion, chromium is also classified as a refractory metal, along with the elements molybdenum (Mo) and tungsten (W). After the elements carbon and boron, chromium is the third hardest chemical element in the periodic table. Also, it’s the only element that does not display any magnetic properties in its solid form.
Highly reactive with the other chemical elements, chromium displays an electronegativity of 1.6 according to Pauling, whereas its atomic radius according to van der Waals is 0.127 nm. When exposed to high temperatures, the pure elemental form of chromium burns and forms the green chromium oxide compound.
Passivation of Chromium Metal
When the metal form of chromium comes into contact with air, the oxygen forms a thin layer of chromium oxide on the metal’s surface. This so-called passivation, i.e. lack of chemical reactivity, protects chromium against corrosion. The German-Swiss chemist Christian Friedrich Schönbein (1799-1868) is the inventor of this passivation effect.
Nowadays, passivation is widely used with stainless steel to protect the reaction of the metal with oxygen. It acts as an anti-corrosive barrier, prolongs the use of the metal products treated with this chemical procedure, and reduces the maintenance requirements of these products.
How Does Passivation Work?
The combination of chromium, iron, and nickel as primary elements results in the production of stainless steel. Also, molybdenum, silicon, manganese, aluminum, and carbon may be added to this alloy. The strong anti-corrosive properties of chromium come forth when this chemical element is exposed to oxygen molecules as a part of the stainless steel alloy.
The passivation process starts by applying acid to the stainless steel compound in order to remove the free iron molecules from the alloy’s surface, while chromium remains unaffected. After the acid exposure, the alloy is exposed to oxygen in order to trigger the formation of the protective anti-corrosive barrier. When there are no free iron molecules on the surface of the alloy, there is no corrosive reaction because the thin chromium oxide film, formed upon contact of Cr with O2, protects the other metal components of the stainless steel alloy from corrosion.
The process of passivation additionally reinforces the anti-corrosive property of the chromium since the formation of a chemically non-reactive surface on the alloy protects the stainless steel against rust.
How Was Chromium Discovered?
Before discovering beryllium in 1798, French chemist Nicolas Louis Vauquelin (1763 – 1829) had conducted multiple other chemical trials that resulted in new scientific discoveries. It was 1780 when Vauquelin’s interest was piqued by a mineral sample of lead chromate (also known as crocoite) taken from the mines in the Ural Mountains, Siberia.
By attempting to prove that the mineral is not a lead ore as it was believed, the French chemist went on to boil the crushed mineral with potassium carbonate. The reaction triggered by this experimental attempt further increased his genuine belief that he was on the brink of a new discovery.
Namely, by conducting more and more experiments, Vauquelin tried to confirm his findings and prove that his work had produced a new chemical element. Finally, in 1781, the scientist succeeded in isolating the pure elemental form of chromium. First, he managed to remove lead from the mineral by exposing it to hydrochloric acid vapor.
The evaporation produced by the reaction gave Vauquelin the oxide which he heated in a charcoal oven, and – the result was monumental! In this way, the French chemist confirmed his 1798 findings and proved the existence of a new chemical element – chromium.
After the isolation of the pure chromium, Vauqelin also recognized traces of this element in the red and green precious stones – ruby and emerald.
The Story About the Origin of New York’s “Yellow Cab”
Nowadays, chrome yellow is one of the most popular colors. It’s become the most recognizable trademark of the school buses and cabs in the United States. The story about the popularization of the chromium yellow color begins in 1822.
In this year, the German chemist Andreas Kurtz (1782 – 1846), who was also Vauquelin’s student, attempted to produce larger quantities of potassium dichromate(VI), its chemical formula being K2Cr2O7. This inorganic chemical reagent with a bright, red-orange color, attracted the attention of the local manufacturers employed in the textile industry, so Kurtz started making profits out of the sales.
However, the competition made this business unprofitable very soon, but this didn’t discourage Vauquelin’s student. Motivated by his first commercial success, Kurtz began producing other chrome pigments. Among them, his ‘chrome yellow’ pigment got in the newspaper headlines when the British princess Charlotte painted all her carriages in vivid yellow paint, made from lead(II) chromate – the yellow form of chromium.
Throughout the times, the yellow color of the Princess’s carriage remained a characteristic feature of public transport. Thus, today the school busses in the U.S. and especially the iconic cabs of New York are all colored in ‘chromium yellow’, reminiscent of the princess’s practice and Kurtz’s achievement.
The Story About the Chrome King of the World
In the 1820s, Isaac Tyson, Jr. (1792–1861) was considered to be the biggest chromium magnate and successful industrialists. His story of success had begun in France, where he studied chemistry and mineralogy. This educational experience later proved to be his greatest asset.
Upon his return to his hometown of Baltimore, Maryland, U.S., Tyson began to exploit the chromite accumulations on his farm. One day, he had noticed an unusual piece of mineral supporting a barrel at the local market. The knowledge he gained in chemistry school helped him make a logical connection between chromite and serpentine barrens rich in metal ores. Tyson recognized the serpentine mineral as the rare green rock that is used in the ‘chrome yellow’ pigment production.
This acknowledgment spurred him to invest in all available serpentine barrens in the United States, assuming that there are large deposits of chromite in the surrounding areas. In a blink, he had bought all the major chromite mining sites in Maryland, Pennsylvania, and Virginia. Before too long, he held a monopoly over the largest chromite deposits in the world.
Following his business instincts, this American chemist and magnate commenced the production of chromium pigments in Philadelphia, United States, and the manufacture of dyes in Liverpool, England. By branching out his production, Tyson earned the title of “the Chrome King of The World”!
In 1996, this pioneer of the American chrome industry was inducted in the National Mining Hall of Fame in Leadville, Colorado, as a result of his successful initiatives and entrepreneurship.
How Did Chromium Get Its Name?
Because most chromium compounds always produce beautiful and vivid colors, this chemical element was labeled with the Greek word ‘χρῶμα’, i.e chrōma, meaning ‘color’.
Where Can You Find Chromium?
Rarely found in its free elemental form, chromium is one of the most widely distributed chemical elements in Earth’s crust that can be found as a part of the massive ultramafic and serpentine rocks, volcanic eruptions, some plants, animals, gasses, and in soil. Uvarovite, volkonskoite, kämmererite, and fuchsite are minerals also containing traces of chromium.
Chromite (FeCr2O4) is the most frequent form of ore mined for chromium production. Chromium(0), chromium(III) oxide, and chromium(VI) are right behind chromite as the most frequently occurring forms of chromium in nature. South Africa, India, Kazakhstan, and Zimbabwe are the locations where the largest accumulations of chromite ore are found.
The commercial form of chromium is obtained by oxidation of chromite ore to chromium(III) oxide (Cr2O3). After this process, the oxide is exposed to a high temperature in the presence of aluminum, carbon, or silicon. This reaction produces the pure form of chromium.
Chromium in Everyday Life
Due to its properties, chromium is widely used in many industries. It has a wide application in metallurgy, medicine, the chemical industry, the car industry, transport, the military, as well as in our everyday life:
- As a result of the strong anti-corrosive properties, the utensils and cooking pots we use every day are made of stainless steel and chromium;
- The hexavalent chromium is used in the industrial production of dyes, pigments, and plastic;
- Since chromium gives rubies their red color and the rich-green color to emeralds, it’s used for glass coloring;
- The bright and vivid yellow color produced by chromium pigments is used as a trademark color of the school buses and cabs in the United States, as well as for the European postal service vehicles and mailboxes;
- Chromium-based wood preservatives are used as protection against fungi and insects;
- Due to the hardness and the non-tarnishing properties of chromium, it’s used in the plating of other metals;
- As an added substance to an alloy, chromium provides strength, resistance to high temperatures and pressure, as well as hardness to the alloy (ex. Chromium steel and stainless steel);
- The home appliances that produce heat (such as coffee makers, heaters, electric toasters, microwaves) use the nichrome alloy (a nickel and chromium compound) as a heating element;
- Chromium (III) oxide is the main substance of the green rouge – a metal polish that adds high luster to the metal surfaces;
- The alloy made up of cobalt, chromium, and tungsten, with an addition of silicon, iron, and carbon, is applied in the production of wear-resistant surfaces and metal cutting tools;
- The aforementioned alloy, using molybdenum instead of tungsten, is used for the manufacturing of surgical tools;
- The high hardness of this metal produces less friction, which makes chromium the substance of choice for manufacturing of all kinds of tools, weapons, transport vehicles, machines, frequently used metal items;
- The textile industry uses salts of chromic acid for making chromium pigments. These chromium salts and pigments are further used for mordants in the process of fabric dyeing, as well as for tanning of leather.
Chromium in Modern Art
Art Deco (or: style moderne) in architecture, painting, sculpture, ceramics, fashion, and jewelry is a 1920s art movement. The artistic expression of Art Deco involves modernistically designed luxurious items created as an accent on the sophistication and wealth of the art-consumers.
The highly ornamented style of Art Deco features clear geometric lines, and excessive use of highly polished chromium and vividly colored plastic (mostly bakelite) as additional ornamentation of the expensive materials, such as ivory, jade, crystal, obsidian, etc.
Health Benefits of Chromium
As a trace element in the human body, chromium plays an important biological role in the metabolism of carbohydrates and lipids. This micronutrient also supports the formation of insulin and enhances the activity of the peptide hormone, regulates and balances the good and bad cholesterol levels in the blood of patients with cardiovascular diseases, and enables the storage of carbohydrates, proteins, and fats in the body.
Namely, the insulin hormone plays a vital role in supplying cells with the glucose required for their function, and it also helps them store glucose as a reserve source of energy.
Trivalent Chromium [Cr(III)]
The trivalent form of chromium, or chromium(III), acts as an essential nutrient that supports the protein metabolism of the cells and lowers the glucose levels in the blood. By supporting the relay of cellular signals that trigger the increase of glucose absorption in the blood, chromium helps with the regulation of the normal blood glucose concentration. In case of chromium(III) deficiency in the body, Type 2 diabetes may occur.
Chromium as a Trigger of Insulin Activity
The protein labeled as transferrin relays the absorbed chromium via the bloodstream to the insulin-insensitive cells. Afterward, this protein releases chromium in the designated cells where it binds to a low molecular weight chromium binding substance (LMWCr). Chromium’s main role is to deactivate the enzyme that prevents the insulin hormone to bind to the cell’s receptors. In this way, this chemical element becomes a part of the biological reaction that supports the function of the insulin hormone.
What Causes Chromium Deficiency?
Chromium deficiency is a metabolic disorder that occurs when the levels of this trace element drop below the required quantity due to various physical traumas, injuries, surgery, lack of chromium in nutrition, use of corticosteroids and antacids, etc. This may lead to a blockage of the arteries that supply oxygenated blood rich in nutrients and glucose to the bodily tissues.
What Are the Signs and Symptoms of Chromium Deficiency?
Increased anxiety, confusion, muscle weakness, weight loss, and changeable mood, are only a few of the signs that alert us of a sharp drop in chromium levels in the body.
Chromium and Impaired Glucose Tolerance
Impaired glucose tolerance (IGT) is a medical condition that occurs as a forewarning of type 2 diabetes mellitus. The list of risk factors that lead to dysfunctional glucose metabolism in the human body includes:
- Being overweight;
- A genetic predisposition for obesity;
- Sedentary lifestyle;
- Cardiovascular problems;
- High blood pressure;
- High cholesterol levels in the blood.
If these risk factors are not alleviated, the impaired glucose intolerance may progress into a more serious and complex condition, i.e. type 2 diabetes.
Chromium and Type 2 Diabetes
Type 2 diabetes (diabetes mellitus) is a metabolic disorder resulting from insulin resistance or the body’s inability to produce sufficient levels of this hormone. When impaired glucose intolerance progresses into type 2 diabetes, it’s a clear signal that the cells are unable to metabolize the glucose they use as energy, i.e. the sugar received via the bloodstream.
Due to this, the glucose levels in the blood accumulate over time, which may damage the tissues and organs. The abnormally high blood sugar levels are referred to as hyperglycemia.
What Are the Signs and Symptoms of Type 2 Diabetes (Diabetes Mellitus)?
Since the symptoms of type 2 diabetes take longer to develop, they are sometimes overlooked or hard to recognize. The most frequently occurring symptoms of Type 2 diabetes encompass:
- Extensive thirst of the affected individual;
- Increased hunger;
- Loss of muscular mass;
- Frequent urination.
Chromium-Rich Food Sources
Trivalent chromium can be also found in food. Chromium-based food is especially recommended to people diagnosed with early stages of diabetes or undergo HIV treatments. The list of foods considered as good sources of chromium includes the following items:
- Green beans;
- Nutritional yeast;
- Brown rice;
- Grape juice;
- Whole grains;
There are also some herbs with high chromium content:
- Wild yam;
- Red clover;
Chromium-rich food intake may prevent age-related memory loss. It also supports weight loss, relieves symptoms of depression, improves bodily composition, and prevents cardiovascular diseases and diabetes.
First identified in 1957 by the research scientist Walter Mertz, chromium supplements are typically administered as a preventive against diabetes. As a researcher for the U.S. Department of Agriculture, National Institutes of Health and the Walter Reed Army Institute of Research, and a director of the Human Nutrition Research Center of the Agricultural Research Service, Mertz was among the first nutrition researchers that made a connection between chromium in human nutrition and diabetes.
The main scientific achievement of this advisor to the World Health Organization and chairman of the committee that issued recommendations for trace element nutrition was the key role of chromium in the metabolism of carbohydrates via the formation of a specific compound that he labeled as the Glucose Tolerance Factor (GTF).
Chromium Intake Regulations
According to the United States Department of Health & Human Services, NIH – National Institutes of Health (Office of Dietary Supplements), the tolerable upper intake level recommendations for chromium are provided in the Dietary Reference Intakes (DRIs) developed by an expert committee of the FNB at the National Academies of Sciences, Engineering, and Medicine.
|Birth to 6 months||0.2 mcg||0.2 mcg|
|7–12 months||5.5 mcg||5.5 mcg|
|1–3 years||11 mcg||11 mcg|
|4–8 years||15 mcg||15 mcg|
|9–13 years||25 mcg||21 mcg|
|14–18 years||35 mcg||24 mcg||29 mcg||44 mcg|
|19–50 years||35 mcg||25 mcg||30 mcg||45 mcg|
|51+ years||30 mcg||20 mcg|
In 2001, the US Institute of Medicine had determined the adequate intake (AI) of chromium at 20-35 μg/day for adults. It’s important to note that all supplements must be prescribed by a licensed healthcare practitioner, since large doses of chromium (as well as all other nutritional supplements) may cause unwanted side effects, such as vertigo, watery stool, skin rash, headaches, etc.
In addition, the chromium supplementation may interact negatively if taken simultaneously with medicines, such as:
- H2 blockers;
- proton pump inhibitors;
- nicotinic acid;
- NSAID painkillers
- non-steroidal anti-inflammatory drugs (NSAIDs);
- prostaglandin inhibitors.
Chromium picolinate is often used by bodybuilders and athletes that strive to improve their performance. Apart from having beneficial effects on muscle growth, chromium picolinate also may contribute to the loss of body fat and the maintenance of overall good health.
Since chromium is able to break up the fat molecules in the body, this supplement supports the reduction of lipids in the blood which is highly beneficial for the treatment of diabetes.
How Dangerous Is Chromium?
There are two broad forms of chromium – the trivalent and the hexavalent chromium isotopes. While the trivalent chromium poses no significant human health hazard due to its inability to permeate the cell membranes of the bodily tissues and organs, the hexavalent chromium is considered a highly toxic form of this chemical substance.
Also, Cr(VI) is classified as a powerful mutagen due to its ability to trigger mutation of the cells, which leads to stomach cancer upon ingestion, or allergic contact dermatitis when it comes into skin contact.
The International Agency for Research on Cancer (IARC), OSHA (29 CFR 1910 Subpart Z), and ACGIH, chromium metal and Cr(III) have classified both chromium metal and its trivalent compounds as A4 category, meaning these forms of chromium are not classifiable as a human carcinogen. However, the hexavalent chromium, Cr(VI), is both a toxic and carcinogenic substance, i.e. it imposes a significant carcinogenic risk.
According to a study, some of the soluble hexavalent salts are labeled as mutagens and carcinogens. By being inhaled or ingested, they cause cancer of the gastrointestinal or respiratory tract, respectively.
Environmental Effects of Chromium
The high presence of chromium in the industries makes this metal an environmental pollutant. The isotopes of chromium may contaminate the air, soil, and waters as a waste material released from the chromate production plants or mines, cement-producing plants, leather tanneries, etc.
Chromium-based wood preservatives are also highly toxic to the environment. For instance, acid copper chromate (ACC) is a wood preservative that contains the most dangerous form of this chemical element – the hexavalent chromium [Cr(VI)]. Respiratory organs suffer the most upon exposure to chromium.
Isotopes of Chromium
A naturally-occurring elemental form of chromium is a combination of several isotopes of this chemical element – 50Cr (4.3%), 52Cr (83.8%), 53Cr (9.5%), and 54Cr (2.4%).
Chromium-51 is a radioactive isotope with a half-life of 27.7 days. It’s widely used in transfusion medicine as a medium for red blood cell labeling, a radioactive tracer in radiological examinations, and diagnostic support of hematological analysis.
|Z||N||Isotopic mass (Da)
[n 2][n 3]
[n 7][n 4]
|Natural abundance (mole fraction)|
|Excitation energy[n 4]||Normal proportion||Range of variation|
|43Cr||24||19||42.99771(24)#||21.6(7) ms||β+ (71%)||43V||(3/2+)|
|β+, p (23%)||42Ti|
|β+, 2p (6%)||41Sc|
|β+, α (<.1%)||39Sc|
|β+, p (7%)||43Ti|
|45Cr||24||21||44.97964(54)||50(6) ms||β+ (73%)||45V||7/2−#|
|β+, p (27%)||44Ti|
|45mCr||50(100)# keV||1# ms||IT||45Cr||3/2+#|
|50Cr||24||26||49.9460442(11)||Observationally Stable[n 8]||0+||0.04345(13)||0.04294–0.04345|
|59mCr||503.0(17) keV||96(20) µs||(9/2+)|
|61Cr||24||37||60.95472(27)||261(15) ms||β− (>99.9%)||61Mn||5/2−#|
|β−, n (<.1%)||60Mn|
|62Cr||24||38||61.95661(36)||199(9) ms||β− (>99.9%)||62Mn||0+|
List of Chromium Compounds
Chromium occurs in several oxidation states, mainly as metallic (Cr0), trivalent (+3), and hexavalent (+6) form of chromium. The trivalent chromium (Cr3+) is the most stable oxidation state of chromium. Chrome oxides are typically used as pigments, due to the empirical fact that the +2 oxidation state of chromium produces blue color, the +3 oxidation state gives a green color, while orange color is observed in a +6 oxidation state.
The extensive list of chromium compounds consists of the following minerals and chemicals:
- Chrome chalcedony
- Chromium hexacarbonyl
- Chromium(II) acetylacetonate
- Chromium(III) acetylacetonate
- Potassium hexacyanochromate(III)
- Potassium tetraperoxochromate(V)
- Chromium acetate
- Chromium oxide
- Chromium(I) hydride
- Chromium(II) carbide
- Hexavalent chromium
- Chromium hexafluoride
- Chromium pentafluoride
- Chromium(IV) chloride
- Chromium(IV) fluoride
- Chromyl chloride
- Chromyl fluoride
- Chromium acetate hydroxide
- Chromium(II) acetylacetonate
- Chromium(II) oxalate
- Chromium(II) oxide
- Chromium(II) sulfate
- Chromium(III) hydroxide
- Chromium(IV) oxide
- Copper chromite
- Iron(II) chromite
- Chromium(II) acetate
- Chromium(II) bromide
- Chromium(II) chloride
- Chromium(II) fluoride
- Chromium(II) hydride
- Chromium(II) oxalate
- Chromium(II) oxide
- Chromium(II) selenide
- Chromium(II) silicide
- Chrome alum
- Chromite (compound)
- Chromium acetate hydroxide
- Chromium nitride
- Chromium(III) 2-ethylhexanoate
- Chromium(III) acetate
- Chromium(III) boride
- Chromium(III) bromide
- Chromium(III) chloride
- Chromium(III) fluoride
- Chromium(III) hydroxide
- Chromium(III) iodide
- Chromium(III) nicotinate
- Chromium(III) nitrate
- Chromium(III) oxide
- Chromium(III) phosphate
- Chromium(III) picolinate
- Chromium(III) sulfate
- Chromium(III) sulfide
- Chromium(III) telluride
- Copper chromite
- Iron(II) chromite
Chromium and Ruby
The aluminum oxide crystal (Al2O3) is a form of the mineral corundum with an enchantingly brilliant red hue. Its characteristic color comes from the chromium isotopes in the molecular structure of the ruby. This trace element also contributes to the luster of the red gemstone by shedding a very brief fluorescence upon exposure to light.
Color is the most significant aspect of the precious stone’s value. The most valuable rubies occur in a vibrant red color with a deep purple hue. Gem Mountain, Spruce Pine, North Carolina, offers high prospects of finding rubies on U.S. territory.
Chromium and Emerald
The mineral beryl is the main source of emeralds. This deep-green crystal form of beryl is formed only in the presence of chromium, vanadium, and iron. The intense green color of this gemstone again comes from the chemical elements chromium and vanadium.
Emerald Hollow Mine, Hiddenite, North Carolina, is the location where emeralds can be found in the United States.
5 Interesting Facts And Explanations
- The term ‘refractory metal’ refers to a group of metals included in the periodic system of elements that are characterized by extremely high melting points and that show resistance to corrosion, wear, and deformation.
- Tungsten (W), Molybdenum (Mo), Tantalum (Ta), Niobium (Nb), Rhenium (Re), and Chromium (Cr) are commonly considered as refractory metals.
- Chromium comes 22nd in the line of most abundant chemical elements that occur naturally in Earth’s crust.
- In comparison to platinum, chromium has much higher vapor pressure.
- Ultramafic rocks are igneous rocks containing silica and potassium in traces but have large quantities of both magnesium and iron in their composition.