Selenium refers to a chemical element with the symbol Se and atomic number 34. It is a nonmetal, chemically related to sulfur, tellurium, and polonium, and it belongs to the chalcogen group on the periodic table.
Selenium was discovered in 1817 by Swedish chemist Jöns Jacob Berzelius, who found it as an impurity in sulfuric acid. He named it after Selene, the Greek goddess of the moon, because of its similarity to tellurium, named after the Earth. Initially, the element was mistaken for arsenic, but further investigation revealed its unique properties.
Selenium can exist in several oxidation states, with -2, +4, and +6 being the most common in its compounds. It forms selenides with metals, analogous to sulfides, and can also form selenates (SeO42-) and selenites (SeO32-) when it reacts with oxygen.
While selenium is an essential nutrient, high levels can be toxic. Selenium poisoning can occur from industrial exposure or from dietary excess. Conversely, selenium deficiency is linked to various health issues like Kashin-Beck disease and Keshan disease.
The primary commercial source of selenium is as a byproduct of the refining of copper, where it is found in copper anode slimes. Other sources include the processing of sulfide ores containing selenium.