Selenium (34)

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.

•Name: Selenium
•Symbol: Se
•Atomic number: 34
•Atomic weight: 78.96 (3)
•Standard state: solid at 298 K
•CAS Registry ID: 7782-49-2
•Group in periodic table: 16
•Group name: Chalcogen
•Period in periodic table: 4
•Block in periodic table: p-block
•Color: grey, metallic luster
•Classification: Non-metallic

Historical information

Selenium was discovered by Jöns Berzelius at 1817 in Sweden. Origin of name is from the Greek word "selene" meaning "moon". Selenium (Greek- Selen, moon) was discovered by Jöns Jacob Berzelius in 1817. He reported that tellurium was present in sulphuric acid from a Swedish factory, but in the following year decided that the impurity was not tellurium but another closely related element that he subsequently identified as selenium.

Physical properties

•Melting point: 494 [or 221 °C (430 °F)] K
•Boiling point: 958 [or 685 °C (1265 °F)] K
•Density of solid: 4819 kg m-3

Orbital properties

•Ground state electron configuration: [Ar].3d10.4s2.4p4
•Shell structure: 2.8.18.6
•Term symbol: 3P2

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 employed 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 precipitate out leaving selenous acid, H2SeO3, in solution. Selenium is liberated from selenous acid by SO2:

H2SeO3 + 2SO2 + H2O → Se + H2SO4 (concentrated)