Selenium Basic Facts
Atomic Number: 34
Atomic Weight: 78.96
Discovery: Jöns Jakob Berzelius and Johan Gottlieb Gahn(Sweden)
Electron Configuration: Ar 4s2 3d10 4p4
Word Origin: Greek Selene: moon
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
Selenium Physical Data
Density (g/cc): 4.79
Melting Point (K): 490
Boiling Point (K): 958.1
Critical Temperature (K): 1766 K
Appearance: soft, similar to sulfur
Isotopes: Selenium has 29 known isotopes including Se-65, Se-67 to Se-94. There are six stable isotopes: Se-74 (0.89% abundance), Se-76 (9.37% abundance), Se-77 (7.63% abundance), Se-78 (23.77% abundance), Se-80 (49.61% abundance) and Se-82 (8.73% abundance).
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
CAS Registry Number: 7782-49-2
- Jöns Jakob Berzelius found a red sulfur-like deposit at a sulfuric acid manufacturing facility. He originally thought the deposit was the element tellurium. After further examination, he decided he had found a new element. Since tellurium was named after Tellus, or Earth goddess in Latin, he named his new element after the Greek Moon goddess Selene.
- Selenium is used in anti-dandruff shampoos.
- Gray selenium conducts electricity better when light is shined on it. Early photoelectric circuits and solar cells used selenium metal.
- Compounds containing selenium in the -2 oxidation state are called selenides.
- A combination of bismuth and selenium can be used to replace the more toxic lead in many brass alloys. (Lead is added to brass to increase its ability to be machined)
- Brazil nuts have the highest levels of nutritional selenium. One ounce of brazil nuts contain 544 micrograms of selenium or 777% the Recommended Daily Allowance.
Quiz: Test your new selenium knowledge with the Selenium Facts Quiz.
References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange's Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.) International Atomic Energy Agency ENSDF database (Oct 2010)
Return to the Periodic Table