Elements

The Colorful Element Chromium

The Element Chromium

The element chromium (Cr) can be found in group 6 on the periodic table. It is silvery, lustrous, and hard. Moreover, it has a rich chemistry and is well known for creating colorful compounds and is often used to both protect and polish other metals through a process called electroplating. Find out more about this radiant transition metal down below!

Interesting Facts about Chromium!

  1. Chromium is named after the Greek word ‘chroma’ or ‘chromos’ which translates to ‘color’ and references the element’s ability to compose colorful compounds.
  2. 75% of chromium produced today is used in alloys such as stainless steel to protect and shine base metals such as copper or zinc.
  3. Southern Africa and Zimbabwe supply 99% of the world’s chromite which is composed of iron(II) oxide and chromium(III) oxide.
  4. Trivalent chromium is a vital micronutrient and aids the efficiency of insulin in individuals who struggle with glucose intolerance.
  5. Hexavalent chromium or chromium is extremely toxic as it can cause cancer, destroy red blood cells, and give rise to genetic mutations.

Chromium in the Periodic Table

Chromium is a hard, silvery transition metal with atomic symbol Cr and atomic number 24. It lies to the left of manganese and the right of vanadium on the periodic table. It is the first element of group 6, along with molybdenum and tungsten. Chromium chemistry has some things in common with vanadium, manganese and molybdenum.

Chromium’s electron configuration is [Ar] 3d54s1 . It is the first element in the periodic table to violate the Aufbau principle. Naturally occurring chromium is a mix of four isotopes.

Electroplating with Chromium

Electroplating, also known as electrodeposition, is the process of plating one metal onto another to prevent corrosion or enhance the appearance. Chromium is a great choice for plating as it is lustrous, hard, and naturally resistant to tarnishing and corrosion. It is accomplished by passing an electric current between two electrodes immersed in an electrolyte bath with chromic acid. The result is a thin layer of chromium being left on top of either a metal or alloy substrate. Common household items that are manufactured with chrome plating are doorknobs, bathroom taps, and lighting fixtures. Chrome plating is also commonly used in automotive parts.

Chromium in Today’s World

Human Health and Chromium

Chromium metal alone exists at low toxicity. However, as the oxidation state increases, so does the health risks associated with it. For example, trivalent chromium, which has a +3 oxidation state, is an essential nutrient for humans. Lack of exposure to it can cause disturbances in biological processes such as metabolism and lead to health problems such as diabetes. However, hexavalent chromium or chromium (VI) is extremely toxic and causes a wide variety of adverse health effects. These include skin rashes, weakened immune systems, altered genetic material, lung cancer, and even death.

As previously stated, the toxicity comes from the high oxidizing power. For example, earlier in this article, it was mentioned that hexavalent chromium can destroy red blood cells. The way this happens is chromium(VI) enters the bloodstream and begins to create a series of oxidization reactions. Ultimately, this leads to in vitro hemolysis, which is the rupturing of red blood cells. One major consequence of this is that it may lead to kidney and liver failure.

Chromium in the Environment

Like with humans, chromium is also toxic for microorganisms, plants, and animals. The two stable forms found in the environment are trivalent and hexavalent chromium. As previously mentioned, trivalent chromium is less toxic than hexavalent. It is also insoluble whereas hexavalent chromium is soluble. Most of the chromium pollution in the environment is the result of waste from human activities such as mining and leather tanning. When disposed of in landfills, toxic chromium seeps from the soil into the water. One industry that contributes a large amount of chromium pollution is the ferrochrome industry. Unlike mercury, biomagnification is not a normal occurrence with chromium. However, when high concentrations of chromium waste pollute bodies of water, it causes damage to the gills of nearby fish. To solve the issue of environmental pollution, researchers are currently studying new bioremediation techniques to reduce the toxicity of chromium(VI).

History of Chromium

Who discovered the element chromium?

Chromium was discovered within the mineral Siberian red lead in 1797 by French chemist Louis-Nicolas Vauquelin. The mineral had been discovered two decades prior and took on a form and color that was unlike any other mineral. Vauquelin was convinced the uniqueness of this mineral was due to an undiscovered metal. Ultimately, he was correct and he successfully isolated the element chromium by heating charcoal with chromium trioxide one year later. This left behind tiny needles of pure chromium metal. He then conducted many experiments in which he combined chromium with various solutions, producing a multitude of vibrant compounds. Chromiums tendency to create vivid and colorful compounds led to its name, as “chroma” in Greek translates to “color”.

Chromium – compounds, reactions, isolation, and oxidation states

Chromium Reactions

At room temperature, Chromium metal does not react with water or air. It will react with the halogens fluorine, chlorine, bromine, and iodine to form colorful compounds. It can also dissolve with hydrochloric acid or sulphuric acid to create solutions containing the Cr(II) ion with H2.

Chromium Compounds

Chromium chemistry is very rich – chromium compounds are well-known for being vividly colorful. As previously stated, it can form compounds with halogens. Chromium(III) fluoride, chromium(III) iodide, and chromium(III) bromide all form green-colored compounds. Meanwhile, chromium(III) chloride forms a red-violet color. Moreover, compounds containing chromium(III) or chromium(VI) are used for chrome plating, pigment manufacturing, leather tanning, and more. Chromium (VI) compounds are highly toxic and are strong oxidizing agents.

Chromate and Dichromate Ions

The chromate ion, formula CrO42- and the dichromate ion, formula Cr2O72- are important cations in chemistry. They are strong oxidizing agents, quite toxic, and they form very colorful compounds. They are usually sold as their potassium or sodium salts.

In solution, the chromate and dichromate ions are in equilibrium, with orange dichromate being favored in acidic conditions, and yellow chromate dominant in basic conditions. This can be used to demonstrate Le Chatelier’s Principle. Sodium chromate and dichromate are used in leather tanning, metal surface treatment, and as catalysts in different industrial processes.

2 CrO42-(aq) + 2 H+ (aq) -> Cr2O72-(aq) + H2O (l)  

Some chromate compounds have interesting chemistries. Barium chromate is insoluble, but will dissolve in acidic solution to form either soluble barium chromate, or chromium (III) complexes. Lead chromate is an intense yellow to yellow-orange color, but lead can also form a basic chromate that is a more distinct orange. Both are used as pigments – called chrome yellow and chrome orange respectively.

Isolation of Chromium

One method to obtain elemental chromium is to heat chromium oxide (Cr2O3) with either charcoal or aluminum. The reason for using these two substances is that they will steal the oxygen from the compound and leave behind pure chromium metal. This is a method very similar to what Vauquelin did to isolate the element for the first time about 200 years ago. Another way to do it is by passing an electric current through its compounds. Lastly, chromium steel can be achieved by adding ferrochrome, which is an alloy-chromite, into hot liquid steel. The chromium then dissolves into the hot steel, so when it cools and hardens, it is now chromium steel.

Chromium Oxidation States

Chromium exists in a wide range of oxidation states. Most common are the oxidation states: +2, +3, and +6. Chromium (II) is not stable and is a strong reducing agent. It should be noted, the most stable oxidation state for chromium compounds is +3. Moreover, it can also exist in the oxidation states +1, +4, and +5. However, these are much rarer.

Chromium Physical Properties

  • Symbol: Cr
  • Melting point: 1907 °C
  • Boiling point: 2672 °C
  • Density: 7.19 g.cm-3 at 20°C
  • Atomic weight: 51.996 g.mol -1
  • Atomic number: 24
  • Electronegativity: 1.6
  • Classification: Group 6 transition metal
  • Crustal abundance: 102 parts per million by weight
  • Electron shell configuration: 51.996 g.mol -1
  • Isotopes – Chromium has 21 total isotopes, 4 are stable
  • Found naturally in the minerals chromite
  • Toxicity: Chromium toxicity rises with an increase in oxidation state.

Where can I buy chromium?

Chromium mineral supplements are available on Amazon and in many stores. However, for laboratory purposes, the element chromium is available in specialty shops, it is not expensive and costs about ten dollars per 100 grams.

Further Reading

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