Selenium belongs to Group VIa of the periodic table of the elements. It has atomic No. 34 and most closely resembles sulfur and tellurium just above and below it in the table. In small quantities, selenium is essential to proper enzymatic function. Selenium is usually separated as a byproduct from other substances with which it is found.

Allotropes

Selenium exists in a number of allotropic forms. For instance, selenium atoms can form a trigonal polymer, eight-member rings or even an amorphous variety. Selenium was once often used as a light-sensitive semiconductor material, though other materials now generally are substituted.

Physical Properties

The red to purple to black solid has a density at or greater than 4.2, depending upon the allotrope. It melts at 221 degrees C. Selenium is diamagnetic, which means if it is placed in a magnetic field, it generates its own, counter magnetic field.

In the Environment

Selenium forms oxy-acids analogous to sulfur. As sulfuric acid forms sulfites and sulfates, selenium forms selenites and selenates. Selenites and selenates are found in certain selenium-rich soils.

Chemical Application

Selenium forms many organic compounds analogous to those of sulfur. Although used mostly in glasses and ceramics to impart a characteristic red color, selenium is also used in catalysis, such as in the liquefaction of coal.

Advisories

In other than trace quantities, selenium is toxic to humans. The National Institute of Health indicates 400 micrograms per day poses health risks for healthy adults, and less for others. Garlic odor of the breath is one indicator of selenium overexposure.

Source:

University of Wisconsin: Sulfur and Selenium Chemistry

NIH Office of Dietary Supplements: Dietary Supplement Fact Sheet--Selenium

Free Patents Online: Coal Liquefaction with a Selenium Catalyst

More Information:

Rare Earth Magnets: Diamagnetism

Georgia State University: Magnetic Properties of Solids--Diamagnetism

BC Cancer Agency: Selenium