Selenium (Se)
Selenium: The Photovoltaic Semi-Metal
Selenium is a semi-metal that can appear either as a shiny, silvery solid or as a red powder. Its name comes from the Greek word selene (moon), chosen because it was discovered just after tellurium (named after Earth). Selenium’s unique electrical and optical properties make it useful in technology, glassmaking, and even medicine.
Why Is Selenium Useful?
Selenium has a wide range of specialized applications:
Electronics: Selenium shows both photovoltaic action (turns light into electricity) and photoconductivity (its resistance drops in light). These properties made it essential in early photocopiers, solar cells, photocells, and rectifiers that convert AC to DC power.
Glass & Pigments: Small amounts of selenium remove the green tint from glass, while larger amounts give glass a deep red or bronze color. It’s also used to make pigments for ceramics, paints, and plastics.
Medicine: Selenium sulfide is toxic to the scalp fungus that causes dandruff, so it’s a key ingredient in anti-dandruff shampoos.
Alloys: Selenium is sometimes added to stainless steel to improve its properties.
Biological Role
Selenium is an essential trace element for humans and many other organisms. The average human body contains about 14 milligrams, with each cell holding more than a million selenium atoms.
Too little selenium can cause health problems.
Too much selenium is toxic—it can cause birth defects, is carcinogenic, and even causes a characteristic “garlic breath” odor.
Natural Abundance
Selenium is relatively rare. Most commercial selenium is obtained as a by-product of copper refining. During the electrolytic process, it collects in the anode muds, which are later processed to extract selenium.
History of Discovery
1817: Swedish chemist Jöns Jacob Berzelius discovered selenium while investigating a strange red-brown residue from a sulfuric acid factory.
At first, he thought it was tellurium, since heating it gave off a radish-like smell. But closer study proved it was a new element similar to both sulfur and tellurium.
Fun fact: Berzelius himself reportedly developed bad breath from absorbing selenium through his skin while working with it!