Selenium is the third element in Group 16 of the periodic family. The members of this family are sometimes referred to as the chalcogens, meaning "ore-forming." Selenium has an atomic number of 34, an atomic mass of 78.96, and a chemical symbol of Se.

Properties

Selenium exists in a number of allotropic forms. One allotrope is an amorphous red powder, while a second allotrope has a bluish, metallic appearance. Other allotropes have properties intermediate between these two allotropes. The crystalline form of selenium has a melting point of 423°F (217°C), a boiling point of 1,265°F (685°C), and a density of 4.5 grams per cubic centimeter.

Chemically, selenium is a metalloid. It is a fairly reactive element that combines readily with hydrogen, fluorine, chlorine, and bromine. It also combines with a number of metals to form compounds known as selenides as, for example, magnesium selenide (MgSe). Selenium burns in...