The Element Chromium
Atomic Number: 24
Atomic Weight: 51.9961
Melting Point: 2180 K (1907°C or 3465°F)
Boiling Point: 2944 K (2671°C or 4840°F)
Density: 7.15 grams per cubic centimeter
Phase at Room Temperature: Solid
Element Classification: Metal
Period Number: 4
Group Number: 6
Group Name: none
What's in a name? From the Greek word for color, chroma.
Say what? Chromium is pronounced as KROH-mee-em.
History and Uses:
Chromium was discovered by Louis-Nicholas Vauquelin while experimenting with a material known as Siberian red lead, also known as the mineral crocoite (PbCrO4), in 1797. He produced chromium oxide (CrO3) by mixing crocoite with hydrochloric acid (HCl). Although he believed a method for isolating chromium didn't yet exist, Vauquelin was pleasantly surprised in 1798 to discover that he was able to obtain metallic chromium by simply heating chromium oxide in a charcoal oven. Today, chromium is primarily obtained by heating the mineral chromite (FeCr2O4) in the presence of aluminum or silicon.
Chromium is a blue-white metal that is hard, brittle and very corrosion resistant. Chromium can be polished to form a very shiny surface and is often plated to other metals to form a protective and attractive covering. Chromium is added to steel to harden it and to form stainless steel, a steel alloy that contains at least 10% chromium. Other chromium-steel alloys are used to make armor plate, safes, ball bearings and cutting tools.
Chromium forms many colorful compounds that have industrial uses. Lead chromate (PbCrO4), also known as chrome yellow, has been used as a yellow pigment in paints. Chromic oxide (Cr2O3), also known as chrome green, is the ninth most abundant compound in the earth's crust and is a widely used green pigment. Rubies and emeralds also owe their colors to chromium compounds. Potassium dichromate (K2Cr2O7) is used in the tanning of leather while other chromium compounds are used as mordants, materials which permanently fix dyes to fabrics. Chromium compounds are also used to anodize aluminum, a process which coats aluminum with a thick, protective layer of oxide. Chromite, chromium's primary ore, is used to make molds for the firing of bricks because of its high melting point, moderate thermal expansion and stable crystal structure.
Estimated Crustal Abundance: 1.02×102 milligrams per kilogram
Estimated Oceanic Abundance: 3×10-4 milligrams per liter
Number of Stable Isotopes: 3 (View all isotope data)
Ionization Energy: 6.767 eV
Oxidation States: +6, +3, +2
3s2 3p6 3d5