chromium

Name chromium
Symbol Cr
Atomic Weight 51.9961
Atomic Number 24
CAS ID 7440-47-3
Group Number 6
Group Name (none)
Period Number 4
Origin of Name Greek, chroma = "color"
Year Discovered 1780
Discovery Credits N. L. Vanquelin at Paris, France
Class Metallic
Color Silvery metallic
Standard State Solid at 77 F & 298 K
Description Hard, blue-white metal. Soluble in HCl and H2SO4 but not HNO3, H3PO4 or HClO4 due to formation of protective layer. Resists oxidation in air. Main use in alloys, chrome plating, and metal ceramics.

Diagnostic tests: 
Chromium gives a borax bead test in the reducing flame a color which is yellow to red (hot) and yellowish green (cold). Under the reducing flame the bead color is a fine emerald green, hot and cold.
Chromium gives a Na2(NH4)(PO4) bead a dirty green color (hot) and a fine green color (cold).
In solution, the addition of NH4OH produces a green precipitate of Cr(OH)3. Note: chromium's name is in reference to the various colors a solution will have depending on the oxidation state Cr2+ (yellow), Cr3+ (green), or Cr6+ (orange).

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