Chromium

Chromium is a chemical element with the symbol Cr and atomic number 24. It is a strong, shiny, grey transition metal that does not rust easily. Chromium helps make steel stay strong and not rust. This steel, called stainless steel, is used in kitchen appliances, cars, and medical tools.

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

Chromium is made from a mineral called chromite. Most chromium is mixed with iron to make an alloy called ferrochromium, which is used to produce stainless steel. Pure chromium is made through a special process. Some forms of chromium can be harmful to health.

Physical properties

Chromium is a hard, shiny metal with a special arrangement of its tiny particles. It is very tough and does not easily lose its shine, making it great for keeping other metals from rusting. Chromium stays strong even when heated, though it melts at a lower temperature than many other metals. It also reflects light very well, especially infrared light.

Chromium forms a thin, protective layer when exposed to air, which helps it stay strong and not rust like iron. 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 a type of metal. It can have different levels, called oxidation states. The most common are +3 and +6. Other levels like +2, +1, +4, and +5 are less common.

Chromium can make many different substances. For example, chromium with a +3 level is in things like chromium(III) nitrate and chromium(III) oxide. Chromium with a +6 level makes substances such as chromate and dichromate. These can change color depending on the amount of acid in the liquid, turning from yellow to orange when acid is added.

Common oxidation states

Chromium(0)

Chromium with a zero level can make interesting substances. Examples are bis(benzene)chromium and chromium hexacarbonyl, which are important in organochromium chemistry.

Chromium(II)

Chromium(II) substances are not very common because they change to chromium(III) in air. One example is chromium(II) chloride, which makes a bright blue liquid when mixed with water.

Chromium(III)

Many chromium(III) substances exist, such as chromium(III) nitrate and chromium(III) oxide. These can change color and shape based on where they are. For example, chromium(III) chloride can look green or violet depending on how it is mixed with water.

Chromium(III) hydroxide can mix with acids to make some particles, and with basic liquids to make others. When heated, it becomes chromium(III) oxide, which is green and does not change easily.

Chromium(VI)

Main article: Hexavalent chromium

Chromium(VI) substances are strong cleaning agents, especially when there is little or no acid. Chromate and dichromate are the main particles in this level, and they change based on the amount of acid. For example, adding acid to a liquid of potassium chromate changes its color from yellow to orange.

Chromium(VI) substances can be found by adding hydrogen peroxide, making a dark blue substance that can be kept stable.

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

Other oxidation states

See also: Organochromium chemistry

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

Chromium(I) substances are usually made by changing chromium(0) mixtures and can have special bonds between chromium atoms.

Occurrence

See also: Category:Chromium minerals

Chromium is found in the Earth's crust. It is the 21st most common element. Chromium comes from rocks that wear down and can be moved by volcanic eruptions.

Most chromite ores come from South Africa and Kazakhstan. India, Russia, and Turkey also produce a lot. There are still many chromite deposits left, mostly in Kazakhstan and southern Africa. Very rarely, pure chromium can be found. The Udachnaya Pipe in Russia has samples of this pure metal. This mine is also rich in diamonds.

History

Early applications

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

For many years, people thought a way to protect metal from rusting, called chromium plating, was invented in ancient China. They thought it was used on old metal objects like arrowheads and sword blades. But in 2019, scientists found that the chromium on these objects came naturally from paint. 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 known for its shiny appearance when polished. 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.

By country

Applications

Chromium is mainly used to make strong and resistant metal mixtures. 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 do not know exactly how it works in our bodies. Some think it may help 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 sick. It has been linked to health problems when people drink water with too much of it.

Our bodies do not 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 controlling sugar levels or losing weight, but they have not found strong proof that it works.

Precautions

Some types of chromium can be bad for your health. Chromium metal and some chromium compounds are usually safe. But another type, called chromium(VI), can be harmful.

If chromium(VI) gets inside your body, it can hurt important parts like your kidneys and liver. It can also damage DNA, which may cause health problems later. Some people might get skin reactions if they touch things with chromium compounds, like certain paints or leather goods.

Chromium compounds have sometimes been found in soil and water near old factories. This can mean the area needs special cleaning to keep it safe.