Selenium (Se)

Selenium is a nonmetal element with atomic number 34, located in group 16 (chalcogens) of the periodic table. It was discovered in 1817 by Jöns Jacob Berzelius and named after the Greek word "Selene," meaning "Moon." Selenium can exist in various allotropes and is particularly known for its roles in electronics, glass manufacturing, and biological systems.

Classification and Basic Properties

Selenium (Se) is one of the elements in the fourth period and group 6A (or group 16) of the periodic table. Its chemical properties resemble those of sulfur and tellurium. It is a nonmetal, but some of its forms exhibit semiconducting properties. Its electron configuration is [Ar] 3d¹⁰4s²4p⁴. Selenium occurs in several allotropes; the most common are red amorphous selenium (powder) and gray crystalline (metallic appearance) selenium. It is solid at room temperature. The density of gray selenium is approximately 4.809 g/cm³.

Discovery

Selenium was discovered in 1817 by Swedish chemists Jöns Jacob Berzelius and Johan Gottlieb Gahn while examining a red precipitate that had accumulated in the lead chambers of a sulfuric acid factory in Gripsholm, Sweden. Initially, the precipitate was thought to contain tellurium, which Berzelius had previously identified in pyrite ore used for sulfuric acid production at the same facility. However, more detailed analysis revealed to Berzelius that this was a new element resembling tellurium.

Red Amorphous Selenium Powder and Gray Metallic Crystalline Selenium (Generated by Artificial Intelligence.)

Etimology

Jöns Jacob Berzelius named the newly discovered element "selenium," inspired by the Greek word "Selene" (Σελήνη), meaning "Moon." This naming created a parallel with tellurium, whose name derives from the Latin "Tellus" meaning "Earth," reflecting their chemical similarities.

Natural Occurrence

Selenium is a relatively rare element in the Earth's crust. It is rarely found in its free state and typically occurs in trace amounts associated with sulfide minerals. It can be found as selenites or selenides within sulfide deposits of copper, lead, silver, and nickel. Important selenium minerals include clausthalite (PbSe) and berzelianite (Cu₂Se). Commercially, most selenium is obtained as a byproduct during the electrolytic refining of copper, lead, and nickel, where it is recovered from anode slimes.

Physical and Chemical Properties

The most stable form of selenium, gray (metallic) selenium, has a silvery luster, is brittle, and exhibits semiconducting properties. Its electrical conductivity increases when exposed to light (photoconductivity), and it can directly convert light energy into electricity (photovoltaic effect). Red selenium may exist in amorphous or monoclinic crystalline forms and acts as an insulator. The melting point (for gray selenium) is 220.8 °C and the boiling point is 685 °C. The atomic radius is approximately 190 pm and the electronegativity is 2.55. Its electron affinity is reported as 194.965 kJ/mol. Selenium readily reacts with halogens and oxygen. When burned in air, it produces blue flames and selenium dioxide (SeO₂), emitting a characteristic odor reminiscent of rotten horseradish. It dissolves in acids. In its compounds, selenium commonly exhibits oxidation states of -2, +2, +4, and +6.

Isotopes

Selenium has six naturally occurring stable or very long-lived isotopes: ⁷⁴Se, ⁷⁶Se, ⁷⁷Se, ⁷⁸Se, ⁸⁰Se, and ⁸²Se (radioactive with a very long half-life). The isotope ⁸⁰Se is highlighted in the source as significant. Many artificial radioactive isotopes are also known. ⁸⁰Se is the most abundant naturally occurring isotope of selenium, making up approximately 49.6%.

Applications

Selenium has various applications due to its unique physical and chemical properties:

• Glass Manufacturing: It plays a significant role in the glass industry. In small amounts, it neutralizes the greenish tint caused by iron compounds, making glass colorless (as a decolorizer). At higher concentrations, it imparts a ruby red color to glass, making it useful in the production of signal lamps and decorative glassware.
• Electronics and Semiconductors: Due to its photoconductive and photovoltaic properties, selenium is used as a photoreceptor in light meters (photometers), solar cells, photocopiers, and laser printers. It was also widely used in rectifiers because it can convert alternating current (AC) to direct current (DC), although silicon-based devices have largely replaced it in this application.
• Pigments: Selenium compounds such as cadmium sulfoselenide are used as pigments to produce colors ranging from bright red to maroon in ceramics, paints, and plastics.
• Metallurgy: It is added as an alloying agent to improve the machinability of stainless steels and certain copper alloys. It also enhances the durability of lead sheets.
• Dandruff Shampoos: Selenium sulfide (SeS₂) is added to some anti-dandruff shampoos because of its antifungal properties, which help control the scalp fungus Malassezia globosa responsible for dandruff.
• Other Applications: It has been used as a vulcanizing agent in rubber production, as a nutritional supplement in animal feed, and in certain photographic processes.

Biological Importance and Effects

Selenium is an essential trace element for many living organisms, including humans. It is a structural component of various enzymes, particularly the antioxidant enzyme glutathione peroxidase, which protects cells from oxidative damage. It is estimated that the human body contains approximately 14 milligrams of selenium, with more than a million selenium atoms present in each cell.

Selenium deficiency can lead to various health problems, such as Keshan disease, a cardiomyopathy. However, the range between the amount required for health and the amount that becomes toxic is narrow. Excessive intake of selenium is hazardous and can cause a condition known as selenosis. Symptoms include hair loss, nail abnormalities, skin lesions, neurological disturbances, and a distinctive garlic-like odor on the breath. High-level exposure may increase cancer risk and adversely affect embryonic or fetal development.