Selenium 34 Se 78.96(3)   Switch element to... actinium aluminium aluminum americium antimony argon arsenic astatine barium berkelium beryllium bismuth bohrium boron bromine cadmium caesium calcium californium carbon cerium cesium chlorine chromium cobalt copper curium darmstadtium dubnium dysprosium einsteinium erbium europium fermium fluorine francium gadolinium gallium germanium gold hafnium hassium helium holmium hydrogen indium iodine iridium iron krypton lanthanum lawrencium lead lithium lutetium magnesium manganese meitnerium mendelevium mercury molybdenum neodymium neon neptunium nickel niobium nitrogen nobelium osmium oxygen palladium phosphorus platinum plutonium polonium potassium praseodymium promethium protactinium radium radon rhenium rhodium roentgenium rubidium ruthenium rutherfordium samarium scandium seaborgium selenium silicon silver sodium strontium sulfur sulphur tantalum technetium tellurium terbium thallium thorium thulium tin titanium tungsten ununbium ununhexium ununoctium ununpentium ununquadium ununtrium uranium vanadium xenon ytterbium yttrium zinc zirconium Go adjacent... P S Cl As Se Br Sb Te I

The essentials

Description 

Here is a brief description of selenium.

• Standard state: solid at 298 K
• Colour: grey, metallic lustre
• Classification: Non-metallic
• Availability:

This sample is from The Elements Collection, an attractive and safely packaged collection of the 92 naturally occurring elements that is available for sale.

Selenium can be prepared with either an amorphous or crystalline structure. Crystalline monoclinic selenium is deep red; crystalline hexagonal selenium, the most stable variety, is a metallic grey (see picture above). Elemental selenium is relatively nontoxic and is considered to be an essential trace element. However, hydrogen selenide (H₂Se) and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some soils in amounts sufficient to produce serious effects on animals feeding on plants such as locoweed (an American plant) grown in such soils.

Isolation

It is not usually necessary to make selenium in the laboratory as it is commercially available. While there are several selenium ores, most selenium is made as a byproduct of copper refining. It also accumulates in the residues from sulphuric acid manufacture. Extraction is complex since the method emplyed will depend upon what other compounds or elements are present. The first step usually involves an oxidation in the presence of sodium carbonate (soda ash).

Cu₂Se + Na₂CO₃ + 2O₂ → 2CuO + Na₂SeO₃ + CO₂

The selenite Na₂SeO₃ is acidified with sulphuric acid. Any tellurites precipitateout leaving selenous acid, H₂SeO₃, in solution. Selenium is liberated from selenous acid by SO₂

H₂SeO₃ + 2SO₂ + H₂O → Se + 2H₂SO₄

	Web webelements.com