Periodic Table of the Elements: Selenium

Name: Selenium
Symbol: Se
Atomic Number: 34
Atomic Mass: 78.96
Atomic Volume: 16.42 cc/mol
Discovered By: Berzelius
Discovery Date: 1817
Origin of Name: Selenium - (Greek) Selene (moon)
Toxicity: Elementary selenium is less toxic than its compounds. Compounds are toxic, action is similar in character to that of arsenic.
Summary: Three forms:

1. Amorphous: Red-black, melting point = 343 K, density = 4.26, Therm. cond. = 0.00519
2. Crystalline: Red, monoclinic, melting point = 443 K, density = 4.48
3. Metallic: Gray-black, hexagonal, melting point = 490 K, density = 4.48

Used in photocopy toner. Selenium is a poor electrical conductor in the dark. When exposed to light, it becomes conductive.

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