Element number 34 on the periodic table, containing 34 protons, 34 electrons and its most common isotope has 25 neutrons in the nucleus. This gives the element an atomic mass of 78.96 amu. It has a ground state configuration of [Ar]4s23d104p4.

Symbol: Se
Atomic Number: 34
Atomic Weight: 78.96
Boiling Point: 958 K
Melting Point: 494 K
Density at 300 K: 4.79 g/cm3
Covalent radius: 1.16
Atomic radius: 1.22
Atomic volume: 16.50 cm3/mol
First ionization potental: 9.752 V
Specific heat capacity: 0.32 J g-1 K-1
Thermal conductivity: 2.04 W m-1 K-1
Electrical conductivity: 8 106 Ω-1m-1
Heat of fusion: 5.54 kJ/mol
Heat of vaporization: 26.32 kJ/mol
Electronegativity: 2.55 (Pauling's)

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Selenium is an essential nutrient, which we require in tiny quantities - 70 micrograms for an adult male, 55 for an adult female, less for children. It was discovered to be a toxin causing skin lesions, diarrhoea and death almost twenty-five years before people realised that it is a nutrient; doses only ten times the RDA can have toxic effects, while selenium deficiency is relatively uncommon. When deficiency does occur it can cause Keshan Disease, a reversable condition characterised by an enlarged heart and poor heart function which may be the result of an opportunistic virus; it can also cause arthritic Kashin-Beck disease, and may impair thyroid function and make one more susceptible to cancer.

Selenium forms part of the important antioxidant enzyme glutathione peroxidase, making the element significant in the fight against cancer and other diseases caused or aggravated by free oxygen radicals.

The selenium content of food depends on the soil where it was grown, and cases of both under- and over-exposure to selenium have been blamed largely on the makeup of the soil in the regions they have occurred. We get selenium from various grains and nuts - brazil nuts and walnuts are particularly potent sources - as well as from fish, meat and cheese.

Selenium is the element used as the basis of photocopy or xerox machines (based on xerography, from Greek for 'dry writing'). Such machines take advantage of selenium's ability to conduct an electrical current only when illuminated.

Inside the machine, a drum is coated in a film of selenium and electrostatically charged. When a document is exposed, an image of it is projected onto the selenium coated drum, producing a map of high and low positive charge corresponging to the bright and dark areas on the paper. Negatively charged dry ink particles are attracted to the regions of high enough positive charge, and from there they are transferred electrostatically to the print paper as it is rolled over the drum. The ink particles are fused with heat or by application of a solvent, and the pattern of charge on the drum is dissippated by exposure to strong light, leaving the selenium film ready to record the next page.

A nonmetallic element that resembles sulfur. Metabolically, it is believed to work closely with vitamin E and is an important part of the enzyme glutathione peroxidase. Large amounts of selenium are toxic, however, and selenium poisoning (characterized by emaciation, hair loss, anemia, and bone and liver damage) is often seen in livestock that eat plants which grow in high-selenium soil.

This is based on work I did for the BioTech Dictionary at http://biotech.icmb.utexas.edu/.

Selenium is teratogenic in high quantities – its presence in the diet or tissues of a pregnant female can cause abnormalities in the offspring. Studies into tissue selenium residues in fish populations have shown an exponential relationship between selenium levels and teratogenic defects - however this was only noted in one isolated population.

Defects include curvature of the spine, head deformities, cataracts, edema, and gross deformities such as missing eyes and missing or shortened limbs - think of the thalidomide babiesthalidomide is also teratogenic. However, no studies have demonstrated that selenium is teratogenic in humans – anecdotal reports suggest that it is, but I could find no evidence to lend weight to these suggestions. Excess selenium has been demonstrated to be teratogenic in birds, producing deformities such as those listed above.


Se*le"ni*um (?), n. [NL., from Gr. σεληνη selênê the moon. So called because of its chemical analogy to tellurium (from L. tellus the earth), being, as it were, a companion to it.] Chem.

A nonmetallic element of the sulphur group, and analogous to sulphur in its compounds. It is found in small quantities with sulphur and some sulphur ores, and obtained in the free state as a dark reddish powder or crystalline mass, or as a dark metallic-looking substance. It exhibits under the action of light a remarkable variation in electric conductivity, and is used in certain electric apparatus. Symbol Se. Atomic weight 78.9.


© Webster 1913.

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