Chromium
Symbol: Cr
Atomic Number: 24
Atomic Mass: 51.9961 amu (atomic mass units)
Melting Point: 1857.0°C (2130.15 K, 3374.6°F)
Boiling Point: 2672.0°C (2945.15 K, 4841.6°F)
Number of Protons 24
Number of Electrons: 24
Number of Neutrons: 28
Classification: Transition Metal
Crystal Structure: Cubic
Density @ 293 K: 7.19 grams per cubic centimeter
Color: blue/white
The element chromium is named from the Latin word “chroma” meaning color as it has many different colored compounds. The gem stone emerald owes its green color to chromium and it also imparts the red color to rubies.
It was discovered in by Louis-Nicholas Vauquelin in 1797. He was working with the mineral crocoite which is also known as Siberian red lead. He removed the lead from the mineral by dissolving it in hydrochloric acid leaving him with chromium oxide. The following year he produced a sample of pure chromium metal by reducing chromium oxide in a charcoal oven.
It is an essential trace element within glucose metabolism appearing to have an effect on the action of Insulin. In anything above trace levels, however, chromium compounds are considered highly toxic.
Chromium has four naturally occurring isotopes three of which are stable (chromium-52, chromium-53 and chromium-54) the most abundant of these at 83.789% is chromium-52. The unstable chromium-50 has an abundance of 4.345% and a half life of in excess of one hundred quadrillion years. Over twenty unstable isotopes of chromium have been produced.
Today the metal is commonly derived from the mineral ore chromite which contains a mixture of iron and chromium oxide. Chromium metal is brittle and hard. It is also very resistant to corrosion. The metal has a number of uses.
* Alloyed to steel it acts as a hardening agent. Such alloys are used in the manufacture of armor plating, cutting tools, safes and ball bearings.
* Stainless steel contains at least 10% chromium.
* Chromium is plated onto other metals where it can be polished to give a protective as well as decorative finish.
Compounds of chromium are also extremely useful.
* Lead chromate is a yellow pigment used in paints it is also known as chrome yellow.
* Another common pigment is the green produced by chromic oxide which is also known as chrome green.
* The leather tanning industry uses the compound potassium dichromate.
* A number of chromium compounds are used in the dyeing industry as mordants to fix dyes into fabrics.
* Chromium compounds are used in the anodizing process to coat aluminum with a thick protective layer. Anodized aluminum is used in the aircraft industry.
* The mineral ore chromite is used in brick-making where its high melting point and moderate thermal expansion make it ideal for use in forming molds.
