Selenium

Selenium (Greek, selēnē, “Moon”), symbol Se, semimetallic element with an atomic number of 34. Selenium is in group 16 (or VIa) of the periodic table.

Selenium was discovered in 1817 by the Swedish chemist Baron Jöns Jakob Berzelius in a sulphuric acid residue. It was so called because it was found in association with tellurium (Latin, tellus, “Earth”).

Chemically, selenium closely resembles sulphur and is related to tellurium. Like sulphur, it exists in several allotropic (distinctly different) forms: a brick-red powder; a brownish-black, glassy, amorphous mass called vitreous selenium; red monoclinic crystals of relative density 4.5; and grey, lustrous crystals called grey selenium. It forms selenious acid (H₂SeO ₃) and selenic acid (H₂SeO₄), the respective salts of which are called selenites and selenates. Grey selenium melts at 217° C (423° F), boils at about 685° C (about 1265° F), and has a relative density of 4.81. The atomic weight of selenium is 78.96.

The element occurs in a few selenide minerals, the most common of which is clausthalite, or lead selenide. It also occurs with free sulphur and in many sulphide ores; it is generally obtained as a by-product in the refining of copper-sulphide ores. The yield from by-product sources, however, is insufficient to supply the rapidly increasing industrial demand for the element. The world’s first deposit of commercial-grade ore was discovered near Baggs, Wyoming, in 1955.

Grey selenium conducts electricity; it is a better conductor of electricity in light than in darkness, the conductivity varying directly with the intensity of light. It is therefore used in many photoelectric devices (see Photoelectric Cell; Xerography). In the form of red selenium or as sodium selenide the element is used to impart a scarlet red colour to clear glass, glazes, and enamels. It is also used to a great extent as a decolorizer of glass because it neutralizes the greenish tint produced by iron (ferrous) compounds. Small amounts of selenium are added to vulcanized rubber to increase its resistance to abrasion. Sodium selenate is an insecticide used to combat insects that attack cultivated plants, particularly chrysanthemums and carnations; the insecticide is scattered around the roots and is carried by the sap throughout the plant. Selenium sulphide is used in the treatment of dandruff, acne, eczema, seborrhoeic dermatitis, and other skin diseases. Selenium is also an essential micronutrient for animals and humans and it is found naturally in some soils. However, in larger amounts this element is toxic to animals, humans, and nearly all plants.