Selenium: the essentials
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.
Selenium: historical information
Selenium (Gr. 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.
Selenium around us Read more »
Selenium is essential to mammals and higher plants, but only in small amounts. Selenium is said to stimulate the metabolism. It may help protest against free radical oxidants and against some heavy metals. Livestock grown in areas containing high amounts of selenium in the soil and in which a plant called Astragalus ("loco weed" in USA) is present are poisoned. This is because Astragalus concentrates selenium. On the other hand, sheep grown in areas containing deficient amounts of selenium in the soil develop "white muscle disease".
Selenium is occasionally found as the free element. Minerals are rare and selenium is normally extracted as a by-product from copper production.
|Location||ppb by weight||ppb by atoms||Links|
|Human||50 ppb by weight||4 atoms relative to C = 1000000|
Physical properties Read more »
Heat properties Read more »
- Melting point: 494 [221 °C (430 °F)] K
- Boiling point: 958 [685 °C (1265 °F)] K
- Enthalpy of fusion: |203| kJ mol-1
Crystal structure Read more »
The solid state structure of selenium is: monoclinic.
Selenium: orbital properties Read more »
Selenium atoms have 34 electrons and the shell structure is 18.104.22.168. The ground state electronic configuration of neutral Selenium is [Ar].3d10.4s2.4p4 and the term symbol of Selenium is 3P2.
- Pauling electronegativity: 2.55 (Pauling units)
- First ionisation energy: 941.0 kJ mol‑1
- Second ionisation energy: 2045 kJ mol‑1
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
Selenium isotopes Read more »
Selenium isotopes are mainly used for the production of medical and industrial radioisotopes. Se-77 and Se-78 are used for the production of the therapeutic radioisotope Br-77. Se-80 is used for the production of the medical radioisotope Br-80m. Se-74 is used for the production of Se-75 which is used as a source in gamma radiography. Se-76 can be used for the production of the medical isotopes Br-75 and Br-76. Se-82 has been used to study double beta decay. Most of the Selenium isotopes have been used in studies into the role of Selenium as an essential nutrient.
|74Se||73.9224746 (16)||0.89 (4)||0|
|76Se||75.9192120 (16)||9.37 (29)||0|
|77Se||76.9199125 (16)||7.63 (16)||1/2||0.53506|
|78Se||77.9173076 (16)||23.77 (28)||0|
|80Se||79.9165196 (19)||49.61 (41)||0|
|82Se||81.9166978 (23)||8.73 (22)||0|