selenium

Name selenium
Symbol Se
Atomic Weight 78.96
Atomic Number 34
CAS ID 7782-49-2
Group Number 4
Group Name Chalcogen
Period Number 4
Origin of Name Greek, selene = "moon."
Year Discovered 1817
Discovery Credits Discovered by J. J. Berzelius at Stockholm, Sweden.
Class Non-Metallic
Color Silvery Metallic or Red
Standard State Solid at 77 F & 298 K
Description Obtained as a silvery metallic allotrope or red amorphous powder, which is less stable. Burns in air, unaffected by water, dissolves in concentrated nitric acid and alkalies. Used in photoelectric cells, photocopier drums, solar cells, and semiconductors.

Diagnostic tests: 
Native Se and Se2-: Selenium and selenides are detected by heating the powdered mineral on charcoal and are detected by a very pronounced smell described as radishes or rotting radishes.

If the selenium is present in quantities, the blowpipe test evolves a brownish smoke which deposits a silvery coating of  SeO2 near the assay. If this coating is touched with the reducing flame, an azure-blue color is evident.

The open tube test, selenium yields a white oxide which crystallizes in radiating prisms on the sides, often tinged with reddish selenium. The coating is volatile and will advance depending on the location of the flame.

The closed tube test, selenium volatizes and condenses as black globules fused on the inside of the glass which may have a reddish tint in the thinnest portion of the sublimate.

Selenates (SeO4)2- and Selenites (SeO3)2-: Fuse a sample on charcoal mixed with with soda (Na2CO3) and charcoal dust in the reducing flame of the blowpipe. The reduction of SeO4 or SeO3 is detected by the radish odor.

Back to Elements Table Page