Chromium

Chromium is a chemical element with the symbol Cr and atomic number 24. It is a strong, shiny, grey transition metal that is very hard and does not rust easily. Chromium is important because it helps make steel that stays strong and doesn’t rust, called stainless steel. This type of steel is used in many things like kitchen appliances, cars, and medical tools.

Chromium is also used to give other metals a shiny, protective coating through a process called chrome plating. When chromium is polished, it can reflect a lot of light, making surfaces very bright and shiny. The name “chromium” comes from a Greek word meaning color, because many chromium compounds have bright, vivid colors.

Chromium is made from a mineral called chromite. Most chromium is mixed with iron to make an alloy called ferrochromium, which is then used to produce stainless steel. Pure chromium is made through a special process that involves heating and treating chromite with different substances. While some forms of chromium are safe, others, like hexavalent chromium, can be harmful to health.

Physical properties

Chromium is a hard, shiny metal with a special arrangement of its tiny particles. It is one of the first elements where this arrangement breaks a usual rule in the periodic table.

Chromium is very tough and does not easily lose its shine, making it great for keeping other metals from rusting. It can scratch some rocks but can also be scratched by others. Chromium stays strong even when heated, though it melts at a lower temperature than many other metals. It also reflects light very well, especially in a type of light called infrared.

Chromium forms a thin, protective layer when exposed to air, which helps it stay strong and not rust like iron. Unlike iron, this layer does not fall off, keeping the metal safe. Chromium does not break easily when exposed to certain gases but can react with nitrogen at very high temperatures.

Main article: Isotopes of chromium

Chromium found in nature has four stable types, with one type making up most of it. Scientists have also made twenty-five unstable types, with one lasting 27.70 days before changing. These types help scientists study the early history of our solar system.

Chemistry and compounds

Chromium is part of group 6 of the transition metals. It can exist in different states called oxidation states, with +3 and +6 being the most common. Other states like +2, +1, +4, and +5 are less common but can still be found.

Chromium can form many different compounds. For example, chromium with a +3 oxidation state is found in compounds like chromium(III) nitrate and chromium(III) oxide. Chromium with a +6 oxidation state forms compounds such as chromate and dichromate, which can act as oxidizing agents. These compounds change color depending on the pH of the solution, turning from yellow to orange when an acid is added.

Common oxidation states

Chromium(0)

Chromium in the zero oxidation state can form interesting compounds. Examples include bis(benzene)chromium and chromium hexacarbonyl, which are important in organochromium chemistry.

Chromium(II)

Chromium(II) compounds are not very common because they easily change to chromium(III) in air. One example is chromium(II) chloride, which creates a bright blue solution when dissolved in water.

Chromium(III)

Many chromium(III) compounds exist, such as chromium(III) nitrate and chromium(III) oxide. These compounds can change color and form based on the environment they are in. For example, chromium(III) chloride can appear green or violet depending on how it is dissolved in water.

Chromium(III) hydroxide can dissolve in acids to form certain ions, and in basic solutions to form others. When heated, it becomes chromium(III) oxide, which is green and stable.

Chromium(VI)

Main article: Hexavalent chromium

Chromium(VI) compounds are strong oxidizing agents, especially in low or neutral pH. Chromate and dichromate are the main ions in this state, and they change based on the pH level. For example, adding acid to a solution of potassium chromate changes its color from yellow to orange.

Chromium(VI) compounds can be detected by adding hydrogen peroxide, forming a dark blue compound that can be stabilized.

Chromic acid is a strong oxidizing agent and is often sold as "chromic acid," produced by mixing sulfuric acid with dichromate.

Other oxidation states

See also: Organochromium chemistry

Chromium in the +5 oxidation state is rare but exists in compounds like chromium(V) fluoride. Chromium(IV) compounds are slightly more common and can be made by treating chromium trihalides with halogens at high temperatures. These compounds are not stable in water.

Chromium(I) compounds are usually made by oxidizing chromium(0) complexes and can feature unique bonds between chromium atoms.

Occurrence

See also: Category:Chromium minerals

Chromium is the 21st most common element in the Earth's crust, with an average amount of 100 parts per million. Chromium compounds come from the wearing down of rocks that contain chromium and can be moved around by volcanic eruptions.

Most of the world's chromite ores come from South Africa and Kazakhstan, with India, Russia, and Turkey also producing a lot. There are still plenty of chromite deposits left, mostly in Kazakhstan and southern Africa. Rarely, deposits of pure chromium can be found. The Udachnaya Pipe in Russia has samples of this pure metal. This mine is also rich in diamonds, and its special conditions help create both chromium and diamonds.

History

Early applications

Chromium wasn't used by people until the late 1700s, when a French chemist named Nicolas Louis Vauquelin began experimenting with it.

For many years, people thought that a method to protect metal from rusting, called chromium plating, was invented in ancient China. They believed it was used on old metal objects like arrowheads and sword blades. But in 2019, scientists discovered that the chromium on these objects came naturally from the paint on them, not from any special plating. The soil where the objects were buried helped keep them well-preserved.

In the 1700s, people in the west found a bright orange-red mineral in the Ural Mountains. They used this mineral, called crocoite, as a color in paints. Later, a French scientist named Louis Nicolas Vauquelin made pure chromium metal from crocoite and is credited with discovering the element.

In the 1800s, chromium was used in paints and for treating leather. Big discoveries of chromium-rich rocks were made first in the United States and later in Turkey. Chromium is also well-known for its shiny appearance when polished, and it is often used to coat items like car parts and furniture to make them look nice and protect them from rust. This coating method, called electroplating, became common in the 1920s.

Production

In 2013, about 28.8 million metric tons of chromite ore were made and turned into 7.5 million metric tons of ferrochromium. Ferrochromium is mostly used to make stainless steel.

South Africa made the most chromite ore, followed by Kazakhstan, Turkey, and India.

The two main products made from chromite ore are ferrochromium and metallic chromium. To make ferrochromium, chromite ore is melted in special furnaces or mixed with aluminium or silicon. To make pure chromium, the iron is removed in a two-step process. The ore is heated with calcium carbonate and sodium carbonate, and then treated with sulfuric acid to get chromates. These are turned into chromium oxide and then into pure chromium.

4 FeCr₂O₄ + 8 Na₂C O₃ + 7 O₂ → 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂

2 Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O

The dichromate is turned to chromium(III) oxide by heating with carbon and then reduced to chromium.

Na₂Cr₂O₇ + 2 C → Cr₂O₃ + Na₂C O₃ + CO

Cr₂O₃ + 2 Al → Al₂O₃ + 2 Cr

By country

Applications

Chromium is mainly used to make strong and resistant metal mixtures, accounting for 85% of its use. The rest is used in chemical, heat-resistant, and metalworking industries.

Metallurgy

Main articles: Chrome plating and Stainless steel

Chromium helps make steel stronger and prevents rust. It is important for making special steel tools and stainless steel, which does not rust easily. Stainless steel is made by adding chromium to iron. Nickel-based metals also become stronger with chromium. These metals are used in jet engines and gas turbines because they can handle very high temperatures. Chromium is also used in heating elements for toasters and space heaters.

Pigment

The mineral crocoite was used as a yellow color soon after it was found. Later, a way to make chrome yellow was created, and it became very popular. Though it does not fade in sunlight, it can darken over time. It was once used for school buses and postal services but is now less common because of safety concerns. Other chromium-based colors include chrome red and chrome green, used in glass and ceramics.

Other uses

Chromium can give rubies their red color. It was important for the first laser made in 1960. Chromium is also used to protect wood from bugs and decay. In leather tanning, chromium helps make the leather strong. It is also used in high-temperature applications and as a helper in making certain chemicals.

Uses of compounds

• Chromium(IV) oxide is used in making magnetic tape for audio.
• Chromium(III) oxide is a polish for metals.
• Chromic acid was once used to clean lab glass but is now replaced by safer solutions.
• Potassium dichromate is used in chemistry experiments.
• Chromates help stop rust in drilling muds.
• Chrome alum is used in dyeing fabrics and tanning leather.
• Zinc chromate prevents rust on aluminium, especially in airplanes.

Biological role

Chromium is a mineral that might be important for our health, but scientists are not sure how it works in our bodies. Some think it helps with how our body uses sugar, but this is not proven.

One type of chromium, called hexavalent chromium, is very harmful and can make people very sick. It has been linked to serious health problems when people drink water with too much of it.

Our bodies don’t need a lot of chromium from food. The amount we get from eating is usually small, and it can change depending on where our food comes from and how it is prepared. Some countries think chromium is important to eat, while others do not agree.

Scientists are still studying whether taking extra chromium helps with things like controlling sugar levels or losing weight, but so far, they have not found strong proof that it works.

Precautions

Some forms of chromium can be harmful to health. Chromium metal and certain chromium compounds are generally safe, but another form, chromium(VI), is known to be harmful and can cause health problems.

Chromium(VI) can be dangerous if it enters the body. It can harm important body parts like the kidneys and liver. It is also known to damage DNA, which can lead to health issues over time. Some people can also have skin reactions when they touch products that contain chromium compounds, like certain paints or leather goods.

Chromium compounds have sometimes been found in soil and water near old industrial sites, which can require special cleaning to make the environment safe.