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.