Se

Se is the chemical element with symbol Se and atomic number 34. In the periodic table it sits in group 16, commonly described as a nonmetal with metalloid-like properties. Selenium occurs in several allotropes, notably a gray crystalline form and red and black forms. The gray form is a photoconductive semiconductor.

Occurrence and production: Selenium is found in trace amounts in the Earth's crust, mainly in sulfide minerals as selenides and in selenates and selenites. It is largely recovered as a byproduct of refining copper and other nonferrous ores, and from fossil-fuel processing.

Isotopes and properties: Selenium has multiple isotopes, including several stable ones. It behaves as a semiconductor with photosensitivity and forms compounds in oxidation states of -2, +4, and +6, such as selenides, selenites, and selenates.

Applications: Selenium is used in photoconductors, notably in early xerography; in glassmaking to decolorize or tint glass; and as a pigment. It is also used in electronics and thin-film solar devices. In biology, selenium is an essential trace element, incorporated into selenoproteins that support antioxidant defense and thyroid function.

Safety and history: Selenium is essential in small amounts but toxic in excess. Overexposure can cause selenosis, with symptoms such as hair loss and nail changes. It was discovered in 1817 by Jöns Jakob Berzelius and named after the Greek moon, selene.