# Chemistry of Chromium

Chromium is the 24th element in the periodic table and it is found in about 0.0122% of the Earth's crust. It is named after the Greek word "chroma," meaning color. This element produces many beautifully colored compounds, as well as a wide array of colored solutions. Chromium is also a very useful industrial metal.

### Introduction

Discovered in 1797 by Louis-Nicholas Vauquelin and isolated a year later, chromium is a very hard steel-gray metal which takes its name from the Greek word chroma, for "color", suggesting the wide variety of colors which characterize its compounds, many of which are used as oil paint pigments. the pure metal is also a valuable alloying agent for spring steels and along with nickel, in stainless steels. Chromium(III) oxide (a popular green pigment) is the ninth most abundant chemical in the earth's crust.

• 3rdC BC - According to archaeologists, Qin Dynasty and the Terracotta Army weapons were coated with Chromium.
• 1761 - Johann Gottlob Lehmann found an orange-red metal in the Ural Mountains which was later found to be Lead chromate.
• 1770 - Peter Simon Pallas came to the site where Lehmann found the metal and Pallas found the metal to be useful in paint.
• 1797 - Louis Nicolas Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. This lead to the discovery of chromium metal.
• 1800s - Chromium was commonly used in paints and tanning salts.
• Today - Chromium is now commonly used as metal alloys and in industrial factories.

### Properties of Chromium

##### General Properties

 Name Chromium (Cr) Element Group Group 6 of the Transition Metals Atomic Number 24 Molecular Weight 51.9961 g mol-1 Electron Configuration [Ar] 3d5 4s1 Common Oxidation States +2, +3, +6 Other Oxidation States +1, +4, +5
##### Physical Properties
 Natural State Solid Density 7.19 g cm-3 Melting Point 2180 K Boiling Point 2944 K Specific Heat Capacity 23.35 J mol-1 K-1
##### Atomic Properties
 Electronegativity 3.72 eV Electron Affinity 64.3 kJ mol-1 Radius 124.9 pm Ionization Energy 652.7 kJ mol-1 Common Ions Cr2+, Cr3+, Cr6+
##### Metallic Properties

Chromium is a blue- gray metal that can be polished to achieve a high shine. It is extremely lustrous and while it is relatively hard, it is also very brittle. Chromium is a fairly active metal. While it does not react with water, it does react with most acids. It reacts with oxygen at room temperature to form chromium (III) oxide. One of chromium's most important properties is its self passivation. While it is stable in air, chromium will oxidize to form a thin layer that acts as a protective coating to prevent further corrosion

##### Magnetic Properties

Chromium is in elemental form, it displays paramagnetic properties. Recently, it has been discovered that chromium can display different magnetic properties depending on its heating and cooling, which affects the electrons' spinning alignments. Compounds of chromium, such as chromium dioxide, are considered to be ferromagnetic. The ferromagnetic properties of these compounds allow them to be used in data tape, a way to store information. Chromium can be added to other compounds while maintaining its magnetic properties, and this depends on the quantity of other elements in the compound. For example, some stainless steel compounds are magnetic depending on the amount of chromium they contain.

### Isotopes of Chromium

Chromium forms over 20 different isotopes. Three of them are considered stable while nineteen are considered to be radioactive, and many have half lives shorter than 24 hours. The ones listed below are the most stable and the most commonly found.

 Isotopes Half-life 48Cr 21.56 hours 49Cr 42.3 mins 50Cr 1.8 x 1017 years 51Cr 27.7025 days 52Cr Stable 53Cr Stable 54Cr Stable

### Occurrences of Chromium

Chromium is almost never found in its elemental state in nature, with the exception of a diamond deposit in Russia. However, this diamond deposit has a reducing environment, which helps produce small quantities of elemental chromium. Commonly it is found in numerous ores, usually in the mineral chromite. Chromite is the complex of $$FeCr_2O_3$$, or iron chromium oxide. Chromium is also found in other minerals, including magnesiochromite ($$MgCr_2O_4$$). Chromite is found in earth's mantle and is mined most abundantly in South Africa, India, Kazakhstan and Zimbabwe. Through the refining processes, chromite produces both ferrochromium and metallic chromium.

Similar in characteristics and practical uses to vanadium and titanium, pure chromium has a blue-white color. It is very brittle and corrosion resistant. It has three common oxidation states: +2, +3, and +6. The metal ore (principally chromite, $$FeCr_2O_4$$), is refined by heating in the presence of Si or Al. The resulting ferrosilicon or ferroaluminum compound is separated from the chromium by electrolysis.

 Oxidation State Compounds -2 $$Na_2[Cr(CO)_5]$$ -1 $$Na_2[Cr(CO)_{10}]$$ 0 $$Cr(CO)_6$$ +1 +2 $$CrO$$, $$CrF_2$$, $$CrCl_2$$, $$CrS$$, $$Cr_2(SO_4)_3$$ +3 $$Cr_2O_3$$, $$CrF_3$$, $$CrCl_3$$, $$[Cr(H_2O)_6]^{3+}$$ +4 $$CrO_2$$, $$CrF_4$$ +5 $$CrF_5$$ +6 $$CrO_3$$, $$Na_2Cr_2O_7$$, $$CrO_4^{2-}$$, $$CrOF_4$$

### Chromium Oxides and Hydroxides

Chromium will form many different oxides that exhibit general acid-base behavior as well as displaying a range of different colors.

##### Chromium (II) oxide

Chromium (II) oxide, CrO, is basic. It is found in the form of an insoluble black powder.

##### Chromium (III) oxide

Chromium (III) oxide, Cr2O3 is the main oxide of chromium. It is amphoteric and while it is insoluble in water, it will dissolve in acid. It is found in nature in the form of a rare mineral, eskolaite. It is used as a pigment, producing a dark green color.

##### Chromium dioxide

Chromium dioxide or chromium (IV) oxide, CrO2, in its natural state looks like black crystals. It exhibits ferromagnetic properties and was once widely used as a synthetic magnet in magnetic data tape such as audio cassette tapes. It was considered to be one of the most perfect magnetic pigments for recording tapes because of its thin, long, glass rod like crystals. This amorphous solid can be formed through the thermal decomposition of dichromate complexes.

##### Chromium trioxide

Chromium trioxide or chromium (VI) oxide, CrO3 is acidic oxide, or acidic anhydride of chromic acid. It will react with water to form chromic acid and will react with a base to form a chromium salt. In solid form, it is a dark red-orange granular complex. It is used in chrome-plating as a strong oxidizer, however, it is extremely toxic.

##### Chromate and Dichromate

Chromate, CrO42-, is a salt of chromic acid. This salt is associated with a yellow color in basic conditions, for example potassium chromate. Dichromate, Cr2O72-, is a salt of dichromic acid. This salt is associated with a strong orange color in acidic conditions, for example potassium dichromate. However, compounds of chromate or dichromate with heavy metals usually display a red color. Dichromate is a strong oxidizing agent but it is a bad precipitating agent. Chromate on the other hand is used as a precipitating agent but it is a poor oxidizing agent. Chemical equilibrium is displayed when either anion is in an aqueous solution.

$2CrO^{2-}_{4 (s)} + 2H^+_{ (aq)} \rightleftharpoons H_2O_{ (l)} + Cr_2O^{2-}_{7 (aq)} \;\;\; K_c = 3.2 \times 10^{14}$

In acidic solution, the forward reaction is favored. In basic solution, the reverse reaction is favored.

### Passivation of Chromium

Chromium is one of the few metals that has the property of passivation. Chromium will spontaneously react to form a thin layer of oxide that protects the metal against further corrosion. This surface is hard and nonreactive. This makes chromium ideal for electroplating other metals to protect them from oxidizing, and because of it's hardness, it is used to harden the surface of many objects, such as metal tools.

### Quintuple Bond

First discovered in 2005, Chromium, Molybdenum, Tungsten, and Rhenium, all have a special ability to form a quintuple bond; also known as a five fold bond. In Chromium cases, di-chromium was discovered to have a quintuple bond, which means that 10 electrons are participating in the bond with metal - metal. Chromium uses terphenyl ligand to perform this action, thus making it relatively weak, but stable up to 200°C. A quintuple bond is only possible if chromium atom has only one other ligand in order to keep the bond stable. Although the discovery is fairly recent, it has already provided great insight as to how transition metals are able to bond with each other.

### Production of Chromium

Chromium is produced from mined minerals such as chromite. The processes used to extract chromium are similar to those of other metals. Chromium oxides can be heated with other substances such as charcoal or aluminum. Through the process of heating, the carbon or aluminum form oxides, leaving chromium in pure metal form. Pure chromium can also be produced by running an electric current through some of its compounds. These are just two ways of extracting chromium from its compounds. There are other processes to extract chromium such the Thermite Process:

$Fe_2O_3 + 2 Al \rightarrow 2 Fe + Al_2O_3$

Chromite and Ferrochromium can also be used directly to add chromium to other substances such as steel.

### Applications of Chromium

Chromium has many applications. It has been used in dyes to act as a mordant, which will permanently fix dyes to different fabrics. Chromium has also been used in paints as pigments. This is because chromium exhibits many different colors including; black, gray, green, blue, violet, orange, yellow and red, depending on the compound. Chromium is used as an additive to stainless steel, giving the steel its "stainless" property. Because chromium is self-passivating, it will add a protective layer to the steel. It also acts to harden the steel. Stainless steel containing chromium is a very useful alloy and is used in safes, ball bearings, and surgical tools.

Chrome plating is another application of chromium's self passivation. Metals and other substances are chrome plated to add a protective and attractive outer coat. Chromium metal can be polished to a high shine which will make the metal more attractive. The magnetic properties of chromium make it perfect for magnetic tapes. Chromium dioxide is used in recording tapes. Potassium dichromate has been used in the tanning of leather. Small trace quantities of chromium have been found in semiprecious stones including: rubies, sapphires, emeralds, serpentine and jade. Chromium is also used to line ovens and molds because of its high melting point. This element is even found in the human body. Chromium is common in small quantities in the body and has been connected to the body's use of sugar. It is commonly found in foods such as romaine lettuce, onions and tomatoes. A deficiency of chromium leads to symptoms that are commonly seen in diabetics. Although chromium is good in small quantities, larger quantities of chromium can be extremely harmful to humans.

### Problems

1. Write the reaction for chromium (VI) equilibrium in aqueous solutions.
2. List different oxidation states of chromium and examples of compounds they form.
3. Who first made chromium in its metallic form?
4. List different oxides of chromium.
5. What is chromium's symbol, atomic number and weight?
6. What are some common uses of chromium today?
7. What is the magnetic property of chromium?
8. Write out the electron configuration of: Cr, Cr6+, Cr2+.
9. Why is passivation an important property of chromium?
10. What are some of the uses of chromium today?

### References

1. Kalbus, L. H.; Petrucci, R. H.; Forman, J. E.; Kalbus, G. E. "Titration of chromate-dichromate mixtures." J. Chem. Educ. 1991, 68, 677.

2. Lepora, Nathan. Chromium.New York. Marshall Caverdis Benchmark, 2006.
3. L F Bates and A Baqi 1936 Proc. Phys. Soc. 48 781 doi:
4. Petrucci, Ralph H. Genereal Chemistry: Principles and Modern Applications.9th ed. New Jersey: Pearson Education Inc. 2007.
5. Timberlake, Karen C. Chemistry : An Introduction to General, Organic, and Biological Chemistry. 10th ed. Prentice Hall Higher Education, 2008.
6. Whisnant, David. Chromium/Dichromate. The Division of Chemical Education Inc., of the American Chemical Society, 2007.
7. Images from:
1. iStockPhoto
2. everyStockPhoto

### Contributors

• Kaitlyn Kortright (UCD)

Stephen R. Marsden (ChemTopics)