Chromium

Chromium is a chemical element. Its symbol on the periodic table (a list of all the elements) is Cr. It always has 24 protons, so its atomic number is 24. Its mass number (number of protons and neutrons) is about 54. Most of the times it has 28 neutrons, but different isotopes have more or fewer neutrons. As a metal, it has 24 electrons. Its ions have fewer electrons.

Properties
Chromium is a shiny metal. It is reactive, but it reacts with air to produce a very thin coating of chromium(III) oxide, which prevents it from rusting any more. 

Compounds
Chromium occurs in several oxidation states; +2, +3, and +6 are most common. It forms chemical compounds that are colorful.
Chromium(II) oxide, black, rare, reducing agent
Chromium(II) chloride, blue when dissolved in water, unstable
Chromium(III) oxide, dark green
Chromium(III) chloride, green when containing water (hydrated), purple when not containing water (anhydrous)
Chromium(III) sulfate
Chromium(IV) oxide, black, used in cassettes
Chromium(VI) oxide, chromium trioxide, red, oxidizing agent, toxic. Chromic acid is formed by dissolving chromium(VI) oxide in water.
Chromyl chloride, red liquid
Chrome alum

Trivalent chromium
There are two main types of chromium ions, which are found in chromium compounds. One type of chromium ion (Cr3+, trivalent chromium ) is required by our body and by those of other animals. If we do not get enough chromium, certain parts of our body may not work properly. We get most of the chromium we need from our food we eat. Chromium is also put into vitamins to make sure that we get enough. If we get too much of this type of chromium, it can harm our body.

Hexavalent chromium
Another type of chromium (Cr6+ (chromate, hexavalent chromium), in the +6 oxidation state) can cause cancer or make people sick, even though most people do not get exposed to it much. It is found in places that make chromium metal, which is why they need to be cleaned up when they close. Hexavalent chromium can be destroyed by reacting it with reducing agents, turning it into Cr3+ (trivalent chromium). They are strong oxidizing agents.

Chromates
The chromate ion is CrO42-. Chromates are generally yellow.
Potassium chromate
Sodium chromate

Dichromates
The dichromate ion is Cr2O72-. Dichromates are red or orange.
Ammonium dichromate
Potassium dichromate
Sodium dichromate, used in chromium processing

Occurrence and preparation
Chromium occurs as chromite. Chromite is a mixture of iron(II) oxide and chromium(III) oxide. Its chemical formula is FeCr2O4. It is heated with sodium carbonate to make sodium chromate, iron(III) oxide, and carbon dioxide. The sodium chromate is reacted with sulfuric acid to produce sodium dichromate. The sodium dichromate is reduced by carbon to chromium(III) oxide. The chromium(III) oxide is reacted with aluminum to make chromium metal. The chromates and dichromates are what make chromium production areas toxic.

Uses
Chromium is used in different metal products. Stainless steel is the most common use. It is also used in "chrome plating", where it makes things shiny and stops corrosion. Some chromates were used as pigments, but they are toxic so people do not want to use them anymore. 

Safety
Chromium metal is nontoxic. But hexavalent chromium is toxic and carcinogenic, and trivalent chromium is mildly toxic in large quantities.

Related pages
Chromium compounds