- Selenium (Se) is a nonmetal with atomic number 34, positioned in Group 16 of the periodic table alongside oxygen, sulfur, tellurium, and polonium.
- It exhibits properties that bridge those of sulfur and tellurium and can exist in several allotropic forms, ranging from gray metallic-looking crystals to red amorphous powders. Selenium has six valence electrons—two in the 4s subshell and four in the 4p subshell—and most commonly exhibits oxidation states of −2, +4, and +6. Its atomic structure consists of thirty-four protons, typically forty-five neutrons, and thirty-four electrons arranged in four shells.
- Naturally occurring selenium is composed of six stable isotopes: selenium-74 (⁷⁴Se, 0.89%), selenium-76 (⁷⁶Se, 9.37%), selenium-77 (⁷⁷Se, 7.63%), selenium-78 (⁷⁸Se, 23.77%), selenium-80 (⁸⁰Se, 49.61%), and selenium-82 (⁸²Se, 8.73%). Selenium-79 (⁷⁹Se) is a long-lived radioactive isotope formed in nuclear fission.
- Selenium is relatively rare in Earth’s crust, occurring at about 0.05 parts per million, and is not usually found in elemental form. It is often associated with sulfide ores of copper, lead, silver, and nickel, where it substitutes for sulfur in mineral lattices. The chief commercial source of selenium is as a by-product of copper refining, where it is recovered from anode slimes. Major producers include Japan, Belgium, China, Russia, and the United States.
- Selenium was discovered in 1817 by the Swedish chemist Jöns Jacob Berzelius, who identified it while investigating impurities in sulfuric acid produced at a factory. Initially confused with tellurium due to similar properties, selenium was recognized as a distinct element and named after the Greek word selēnē, meaning “moon,” because of its relation to tellurium (named after Earth).
- Selenium has a range of important industrial and technological applications. In the past, it was widely used in photocopiers and light meters due to its photoconductive properties—its electrical conductivity increases dramatically under light. Although modern electronics have largely replaced selenium with silicon-based devices, selenium remains important in glassmaking, where it decolorizes glass by neutralizing green tints from iron impurities and can also impart a red hue to certain glasses and enamels. In metallurgy, small amounts of selenium are used to improve the machinability of stainless steel and copper alloys.
- In electronics and solar technology, selenium is still used in thin-film photovoltaic cells, particularly in copper indium gallium selenide (CIGS) solar panels, which offer high efficiency and flexibility. Selenium compounds such as selenium sulfide are used in medicinal shampoos to treat dandruff and certain skin conditions due to their antifungal properties. Sodium selenite and sodium selenate are used in animal feeds and nutritional supplements in trace amounts.
- Chemically, selenium resembles sulfur, forming selenides (analogous to sulfides), selenates, and selenites. It can bond with hydrogen to form hydrogen selenide (H₂Se), a highly toxic gas with a foul odor. Elemental selenium is relatively inert in dry air but can slowly oxidize to selenium dioxide (SeO₂) when heated. Selenium dioxide is a valuable catalyst in organic synthesis, especially for oxidations.
- Biologically, selenium is an essential trace element for humans and many animals, playing a key role in enzymes such as glutathione peroxidase, which protects cells from oxidative damage. The required dietary intake is very small, and both deficiency and excess can be harmful. Selenium deficiency can lead to Keshan disease, a cardiomyopathy observed in certain low-selenium regions of China, while excessive intake can cause selenosis, characterized by hair loss, nail brittleness, and neurological abnormalities.
- From an environmental perspective, selenium is naturally present in soils, but agricultural runoff, mining operations, and industrial discharges can lead to elevated levels that are toxic to wildlife, especially aquatic organisms. Careful management of selenium-containing waste is necessary to avoid ecological damage.
