Selenium

Symbol: Se
Atomic number: 34
Category: nonmetals
Group: 16
Period: 4
Block: p
Atomic weight: 78.96 g/mol
Electrons per shell: 2, 8, 18, 6
CAS number: 7782-49-2
Phase: solid
Density: 4.81 kg/l (near room temperature)
Melting point: 494 K (221C, 430F)
Boiling point: 958 K (685C, 1265F)
Crystal structure: hexagonal
Oxidation states: 6, 4, 2, 1, -2
Electronegativity: 2.55 (Pauling scale)
Name's origin: Selene
Name's meaning: the Greek name for Moon, as selenium was associated with the discovery of tellurium (Tellus = Earth)
Facts:
- is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature
- conducts electricity better in the light than in the dark, and is used in photocells
- also exists in many non-conductive forms: a black glass-like allotrope, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter chemical cousin sulfur
- selenium salts are toxic in large amounts, but trace amounts of the element are necessary for cellular function in most, if not all, animals, forming the active center of some enzymes
- is toxic if ingested in excessive amounts: exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis
- is a common byproduct of copper refining, or the production of sulfuric acid