Selenium
Atomic Number: 34
Symbol: Se
Atomic Weight: 78.96
Discovery: Jons Jacob Berzelius 1817 (Sweden)
Electron Configuration: [Ar] 4s2 3d10 4p4
Word Origin: Greek Selene: moon
Isotopes: There are 21 known isotopes of selenium. Natural selenium contains 6 stable isotopes.
Properties: Selenium has an atomic radius of 117 pm, a melting point of 220.5°C, boiling point of 685°C, with oxidation states of 6, 4, and -2. Selenium is a member of the sulfur group of nonmetallic elements and is similar to this element in terms of its forms and compounds. Selenium exhibits photovoltaic action, where light is converted directly into electricity, and photoconductive action, where electrical resistance decreases with increased illumination. Selenium exists in several forms, but is usually prepared with an amorphous or crystalline structure. Amorphous selenium is either red (powder form) or black (vitreous form). Crystalline monoclinic selenium is deep red; crystalline hexagonal selenium, the most stable variety, is gray with a metallic luster. Elemental selenium is fairly nontoxic and is considered an essential trace element for proper nutrition. However, hydrogen selenide (H2Se) and other selenium compounds are extremely toxic, resembling arsenic in their physiological reactions. Selenium occurs in some soils in amounts sufficient to produce serious effects on animals feeding on plants grown from those soils (e.g., locoweed).
Uses: Selenium is used in xerography to copy documents and in photographic toner. It is used in the glass industry to make ruby-red colored glasses and enamels and to decolorize glass. It is used in photocells and light meters. Because it can convert AC electricity to DC, it is widely used in rectifiers. Selenium is a p-type semiconductor below its melting point, which leads to many solid-state and electronics applications. Selenium is also used as an additive to stainless steel.
Sources: Selenium occurs in the minerals crooksite and clausthalite. It has been prepared from flue dusts from processing copper sulfide ores, but the anode metal from electrolytic copper refineries is a more common source of selenium. Selenium may be recovered by roasting the mud with soda or sulfuric acid, or by smelting with soda and niter:
Cu2Se + Na2CO3 + 2O2 --> 2CuO + Na2SeO3 + CO2
The selenite Na2SeO3 is acidified with sulphuric acid. Tellurites precipitate out of solution, leaving selenous acid, H2SeO3n. Selenium is liberated from selenous acid by SO2
H2SeO3 + 2SO2 + H2O --> Se + 2H2SO4
Element Classification: Non-Metal
Density (g/cc): 4.79
Melting Point (K): 490
Boiling Point (K): 958.1
Appearance: soft, similar to sulfur
Atomic Radius (pm): 140
Atomic Volume (cc/mol): 16.5
Covalent Radius (pm): 116
Ionic Radius: 42 (+6e) 191 (-2e)
Specific Heat (@20°C J/g mol): 0.321 (Se-Se)
Fusion Heat (kJ/mol): 5.23
Evaporation Heat (kJ/mol): 59.7
Pauling Negativity Number: 2.55
First Ionizing Energy (kJ/mol): 940.4
Oxidation States: 6, 4, -2
Lattice Structure: Hexagonal
Lattice Constant (Å): 4.360
References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange's Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.)
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