Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


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 sulfuric acid residue. It was so called because it was found in association with tellurium (Latin tellus, “earth”).


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; red monoclinic crystals of specific gravity 4.5; and gray, lustrous crystals called gray selenium. It forms selenious acid and selenic acid, the respective salts of which are called selenites and selenates. Gray selenium melts at 217°C (423°F), boils at about 685°C (about 1265°F), and has a specific gravity 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 sulfur and in many sulfide ores; it is generally obtained as a by-product in the refining of copper-sulfide ores. The yield from by-product sources, however, is insufficient to supply the rapidly increasing industrial demand for the element.


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 is therefore used in many photoelectric devices. In the form of red selenium or as sodium selenide the element is used to impart a scarlet red color 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 sulfide is used in the treatment of dandruff, acne, eczema, seborrheic dermatitis, and other skin diseases.