Introduction to Selenium
Selenium, identified by the chemical symbol Se, is an element found on the periodic table with atomic number 34. It is often classified as a nonmetal, although it exhibits some properties characteristic of metalloids, which are elements that have properties intermediate between metals and nonmetals. In its pure form, Selenium can exist in several different structural arrangements, known as allotropes, each with distinct appearances and characteristics.
The Story of its Discovery and Name
Who Discovered It?
Selenium was discovered in 1817 by the Swedish chemist Jöns Jacob Berzelius. Berzelius was investigating a method for producing sulfuric acid at a factory in Gripsholm, Sweden. During this process, he observed a reddish-brown residue that contained an unknown substance. Initially, he suspected it might be the element tellurium, which had been discovered just a few decades earlier. However, further analysis revealed it to be a new, distinct element.
What Does its Name Mean?
Berzelius decided to name this new element “Selenium.” The name originates from the Ancient Greek word “selene” (σελήνη), which means “moon.” This naming choice was a direct reference to the element tellurium, which had been named after “tellus,” the Latin word for Earth. Since selenium displayed chemical similarities to tellurium, Berzelius chose a celestial body name to reflect this connection, placing the “moon” (selenium) in relation to the “Earth” (tellurium).
Quick Facts About Selenium
- Diverse Physical Forms: Selenium can exist in various allotropes, including a red amorphous powder, a black vitreous (glass-like) form, and a stable grey crystalline form. The grey metallic form is notable for its electrical conductivity.
- Semiconductor Properties: Grey selenium is a photoconductor, meaning its electrical conductivity increases significantly when exposed to light. This property has made it useful in devices such as photocopiers, laser printers, and some solar cells, though its use has largely been replaced by other materials in newer technologies.
- Biological Importance: Selenium is an essential trace element for humans and many other organisms, meaning it is required in very small amounts for proper biological function. It is a key component of selenoproteins, which play roles in antioxidant defense and thyroid hormone metabolism.
- Dual Nature of Toxicity: While essential, selenium can be toxic in higher concentrations. Both selenium deficiency and selenium toxicity can lead to health problems. For instance, areas with very low selenium levels in the soil, such as parts of the Keshan region in China, have historically been associated with a specific heart disease. Conversely, areas with naturally high selenium in soil can lead to chronic poisoning in livestock and humans.
- Industrial Applications: Beyond electronics and biology, selenium is used in glass manufacturing to decolorize glass or impart a red color. It also finds application in some types of pigments, as a component in certain metal alloys, and in producing some antifungals for shampoos.