Chromium (Cr)

• Element Symbol: Cr
• Atomic Number: 24
• Atomic Mass: 51.9961
• Group # in Periodic Table: 6
• Group Name: Transition metals
• Period in Periodic Table: 4
• Block of Periodic Table: d-block
• Discovered by: Louis-Nicolas Vauquelin (1797)

Chromium is a hard, silvery transition metal in the periodic table. The transition metals are the elements in groups 3–12 and include many commonly known metals, such as iron, nickel, copper, zinc, silver, platinum, and gold. Many of the transition metals are resistant to corrosion. Chromium has this quality. Its surface remains mirrorlike even when exposed to oxygen over long periods of time.

Chromium was discovered by French chemist Louis-Nicolas Vauquelin in 1797. Vauquelin was working with a mineral then known Siberian red lead, now called crocoite, that had been discovered in 1766. He dissolved the mineral in acid and removed the lead from the sample. In the liquid that remained, he identified a new element with the atomic number 24. Vauquelin called the element chromium, due to its ability to create a variety of colors in solution. The name comes from the Greek word chroma, which means "color."

Physical Properties

Chromium is a shiny, silvery element that is solid in its standard state at 298 kelvins (K). It is a hard and brittle metal, with a Mohs hardness of 8.5. Chromium has a density of 7.14 grams per cubic centimeter (g/cm³) at standard state. Its melting point is 1907 degrees Celsius (°C), and its boiling point is 2671 °C. The specific heat of chromium is 448 joules per kilogram-kelvin (J/kg·K). Chromium is a good conductor of both heat and electricity. Its thermal conductivity is 94 watts per meter-kelvin (W/m·K), and its electrical conductivity is 7.9 × 10⁶ siemens per meter (S/m). Its resistivity is 1.3 × 10⁻⁷ meter-ohms (m·Ω). Chromium is antiferromagnetic at 38 °C and below. It is paramagnetic above 38 °C.

Chemical Properties

Chromium has a body-centered cubic crystal structure. This element is nonreactive with air and water. It dissolves slowly in dilute sulfuric acid and hydrochloric acid.

The electron affinity of chromium is 64.3 kilojoules per mole (kJ/mol). Chromium has six valence electrons. The element has twenty-one ionization energies, with 652.9 kJ/mol being the lowest and 157,700 kJ/mol being the highest. It has three common oxidation states: +2, +3, and +6. Less common oxidation states include +1, +4, and +5.

Chromium has four naturally occurring stable isotopes. The most common of these is chromium-52, which represents around 84 percent of the chromium on Earth. The others are chromium-53, chromium-50, and chromium-54. Chromium has an electron configuration of [Ar]3d⁵4s¹.

Applications

Chromium is an abundant element that is most commonly found within the mineral chromite. Chromite exists in natural deposits within Earth’s crust. Large deposits of chromite can be found in South Africa, India, Kazakhstan, Russia, and Turkey. In 2010 South Africa produced 8,500 metric tons of chromium from chromite ore.

Chromium can be extracted from chromite through reduction with carbon at high temperatures. This procedure is usually done within an industrial furnace. The result is ferrochromium, which is an alloy containing both iron and chromium. Pure chromium metal can be produced through a series of chemical processes. These processes lead to the extraction of chromium from chromite and then a reduction of a variety of intermediate chromium compounds.

Chromium is an important component of many alloys. Its presence allows an alloy to resist oxidation and corrosion from air, water, and other substances.

Stainless steel is an alloy that contains at least 10.5 percent chromium. Stainless steel is often used to make utensils such as forks, spoons, and knives, as well as kitchen sinks and cooking pots and pans. It is also used in surgical implants and surgical instruments such as scalpels. It is likewise used in a wide variety of industries in which containers and piping that resist corrosion and oxidation are needed.

Chrome-vanadium steel is another alloy containing chromium. Like stainless steel, this alloy is resistant to oxidation and corrosion. It is also very strong. Chrome-vanadium steel is used in many tools, such as wrenches and pliers.

A thin layer of chromium can be electroplated onto a layer of nickel to create chrome, which can then be applied to the surfaces of metallic or plastic objects to give them a mirrorlike finish. Chrome does not need to be polished, and it is relatively resistant to corrosion. Chrome is commonly applied to bathroom and kitchen faucets and pipes. It is used to add shine to many objects, including jewelry. Chrome was widely used in the mid-twentieth century on the bumpers and trim of automobiles.

Leather must be tanned to make it soft and flexible. Most leather tanning involves the use of chromium salts. These salts initially turn the hides blue, but the color changes as the leather is dried and finished. Chromium tanning is relatively fast and inexpensive. However, it produces toxins that can damage the environment and cause negative health effects.

Chromium was commonly used in paint colors before the twenty-first century. Chromium compounds created bright greens, reds, and yellows. Lead chromate created a particularly bright yellow and was the compound used to make the paint color called chrome yellow. This hue was the original paint color used for American school buses and was even originally called "national school bus chrome." However, paint compounds containing chromium and other elements that may cause health problems have since been replaced by other, less harmful compounds.

Humans require trace amounts of naturally occurring chromium as a dietary supplement in order to maintain good health. Chromium helps blood glucose levels remain normal by supporting the action of the hormone insulin. The element may also be involved in the metabolism of macromolecules such as carbohydrates, proteins, and fats. Good dietary sources of chromium include broccoli, grapes, whole wheat, potatoes, and garlic. Because chromium is only needed in trace amounts, chromium deficiency is rare.

The industrial use of chromium can cause health issues for workers. Hexavalent chromium and chromates that are produced through industrial processes can cause skin irritation or burns, breathing problems, and digestive problems over time. Hexavalent chromium is a carcinogen that has been linked to lung cancer. Either form of chromium can cause death when one gram or more is accidentally consumed.

Industrial waste from ore refining, cement production, tanning industries, chrome-plating industries, and chromium-processing plants can lead to high levels of chromium in soils and water. This chromium pollution can cause health issues in people who live near waste-disposal sites.

Bibliography

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