Selenium is a non-metal element with unique chemical properties, discovered unexpectedly through an industrial accident. Today, this trace element is recognized as essential for human health, playing a role in various metabolic processes. Its ability to conduct electricity when exposed to light makes it a valuable component in modern electronics. Early chemists initially confused the material for tellurium, highlighting the subtle nature of its chemical identity.
Setting the Scene: The Industrial Context
The discovery of selenium was an unexpected result of a manufacturing problem at a Swedish chemical works in the early 19th century. The facility was dedicated to the production of sulfuric acid, a process which utilized the lead chamber method. This method involved oxidizing sulfur dioxide, often derived from roasted iron sulfide ores, or pyrite.
The owners noticed that when they began using pyrite sourced from the Falun Mine, a mysterious, reddish-brown deposit began accumulating in the lead chambers. This sludge was a cause for concern as it was initially suspected to be an arsenic compound, which is highly toxic. To avoid the presumed arsenic contamination, the facility temporarily ceased using the local Falun pyrite.
This industrial problem attracted the attention of scientists invested in the factory’s operation. Initial analysis suggested the substance was tellurium, an element chemically similar to sulfur known to be present in some ores. The investigation was necessary to determine if the local pyrite could be safely used for acid production.
Identifying the New Element: Location and Discoverers
The investigation that led to the isolation of selenium took place at a chemical plant near Gripsholm, Sweden, which drew its raw materials from the Falun Mine. The two scientists responsible for the breakthrough were the renowned Swedish chemist Jöns Jacob Berzelius and his colleague, mineralogist Johan Gottlieb Gahn. Berzelius and Gahn began analyzing the red sludge in 1817.
Their initial experiments seemed to confirm the presence of tellurium, especially since a sample of the substance, when heated, gave off a distinct odor resembling rotting horseradish. This smell was a known characteristic of tellurium compounds. Berzelius, however, remained skeptical because tellurium was not known to occur in the minerals from the Falun region.
Through meticulous chemical analysis, Berzelius proved that the red deposit was a new element, not tellurium. He found that the substance possessed properties intermediate between those of sulfur and tellurium. This unique chemical signature established its identity as a new addition to the periodic table in 1817.
Naming Convention and Early Observations
Berzelius gave the new element the name “Selenium,” which he derived from the Greek word selene, meaning “Moon.” This naming choice was a deliberate nod to the element it so closely resembled, tellurium. Tellurium had been named after tellus, the Latin word for “Earth,” making the celestial pairing of Earth and Moon a fitting convention for the two chemically similar elements.
Early observations noted that the element could exist in different forms, including the reddish-brown powder of the original sludge and a metallic-looking gray form. Berzelius also noted that the impure element retained the characteristic horseradish smell upon heating. Later, it was shown that the electrical conductivity of the gray form was greatly enhanced by exposure to light, a property known as photosensitivity. This light-dependent electrical behavior established its technological importance.