I.

Introduction

Selenium, symbol Se, semimetallic element with an atomic number of 34. Selenium is in group 16 (or VIa) of the periodic table (see Periodic Law).

Selenium was discovered in 1817 by the Swedish chemist Baron Jöns Jakob Berzelius in a sulfuric acid residue. It was named after the Greek word selēnē, “moon”, so called because it was found in association with the element tellurium (from Latin tellus, “earth”).

II.

Properties and Occurrence of Selenium

Chemically, selenium closely resembles sulfur and is related to tellurium. Like sulfur, it exists in several allotropic (distinctly different) forms: a brick-red powder; a brownish-black, glassy, amorphous mass called vitreous selenium of specific gravity 4.3; red monoclinic crystals of specific gravity 4.5; and gray, lustrous crystals called gray selenium of specific gravity 4.8. The element forms selenious acid (H₂SeO₃) and selenic acid (H₂SeO₄), the respective salts of which are called selenites and selenates. Gray selenium melts at 221°C (430°F) and boils at about 685°C (about 1265°F). 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 sulfur and in many sulfide ores. Most selenium for industrial use is obtained as a byproduct in the refining of copper ores.

III.

Uses of Selenium

Gray 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 was therefore used in many photoelectric devices (see Photoelectric Cell; Xerography). In the form of red selenium or as sodium selenide the element has been used to impart a scarlet red color to clear glass, glazes, and enamels. It has also been 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. It is also used in fertilizers and dietary supplements. 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 sulfide is used in the treatment of dandruff, acne, eczema, seborrheic dermatitis, and other skin diseases.