Properties and Overview of Selenium

Overview:

Selenium (Se) is a nonmetallic chemical element with the symbol Se and atomic number 34. It is positioned in group 16 of the periodic table, also known as the chalcogen group, along with oxygen, sulfur, tellurium, and polonium. Discovered in 1817 by the Swedish chemist Jöns Jakob Berzelius, selenium was named after the Greek word "selene," meaning moon, because of its association with tellurium, which is named after the Earth. Selenium is relatively rare in the Earth's crust, with an average concentration of about 0.05 parts per million. It is commonly found in metal sulfide ores such as pyrite, which partially substitutes for sulfur. Physically, selenium exhibits several allotropes, including gray (metallic), red, and black forms, each with distinct properties. The most stable form at room temperature is the gray allotrope, which has a hexagonal crystal structure and is a good photoconductor. Its electrical conductivity increases when exposed to light. This gray, metallic form is brittle, has a density of 4.81g/cm³, and has a melting point of about 221°C, with a boiling point of 685°C. The red and black allotropes of selenium are less stable and typically occur as powders or vitreous forms. Selenium is also a semiconductor, allowing it to conduct electricity under certain conditions, particularly when exposed to light, a property exploited in various electronic and photocopying applications.
Chemically, selenium exhibits a wide range of oxidation states, including -2, +4, and +6, making it versatile in chemical reactions. It forms compounds with both metals and nonmetals, displaying behavior intermediate between sulfur and tellurium. Selenium dioxide (SeO₂) is one of its most common compounds, a white crystalline solid that is highly soluble in water and forms selenous acid (H₂SeO₃) in aqueous solution. Selenium can also form hydrogen selenide (H₂Se), a toxic and foul-smelling gas that is an analog of hydrogen sulfide (H₂S). Selenium compounds are known for their ability to form both organic and inorganic derivatives, including selenates and selenides, which have applications in various industrial processes. Due to its chemical similarity to sulfur, selenium can replace sulfur in many compounds, resulting in a range of seleno-substituted organic and inorganic molecules.
It's crucial to note that safety considerations are paramount when dealing with selenium and its compounds, as they can be toxic at high concentrations. Elemental selenium and many of its compounds are considered hazardous, particularly hydrogen selenide, which is extremely toxic and can cause respiratory distress, dizziness, and even death at high concentrations. Selenium is also a bioaccumulative element, meaning it can build up in living organisms and potentially cause chronic health effects such as selenosis, a condition characterized by hair loss, nail brittleness, and neurological damage if ingested in large amounts over an extended period. However, selenium is also an essential trace element for humans and animals, playing a critical role in the function of several enzymes, such as glutathione peroxidase, which helps protect cells from oxidative damage. The recommended daily intake for selenium is typically around 55 micrograms for adults, but the margin between beneficial and harmful levels is relatively narrow, necessitating careful regulation of selenium intake.

Production:

Selenium is primarily produced as a byproduct of refining sulfide ores, particularly those of copper, lead, and nickel. During the electrolytic refining of copper, for instance, selenium accumulates in the anode slimes, which are then treated with sulfuric acid or other reagents to recover selenium. The process involves several steps, including roasting the slimes to convert selenium to selenium dioxide, which is then reduced to elemental selenium using reducing agents such as sulfur dioxide. Once recovered, selenium can be purified further by distillation or recrystallization to obtain high-purity selenium for industrial use. Due to its occurrence in trace amounts within ores, selenium production is closely tied to the production rates of other metals, particularly copper, which serves as the primary source of commercial selenium.

Applications:

Selenium has a variety of applications across different industries, owing to its unique semiconductor properties and ability to form stable compounds with many elements. One of its most significant uses is in the glass manufacturing industry, where selenium compounds are used to decolorize and produce ruby-colored glass. Selenium acts as a decolorizing agent by neutralizing the green tint caused by iron impurities in glass, and its addition to glass formulations can produce a range of colors from red to pink. In electronics, selenium is used to manufacture photovoltaic cells and photocopiers due to its photoconductive properties. Selenium's ability to convert light into electricity makes it an ideal material for solar cells, although it has largely been replaced by silicon in modern solar technology. In photocopiers and laser printers, selenium's photoconductive properties are utilized in the drum to transfer images onto paper.
Selenium also plays a vital role in metallurgy, which is used as an additive to improve the machinability of stainless steel and copper. Adding small amounts of selenium to these metals enhances their machinability without significantly affecting their other mechanical properties, making them easier to work with during manufacturing processes. In the chemical industry, selenium is used as a catalyst in various reactions, including producing chemicals such as acrylonitrile, a synthetic fibers and plastics precursor. Additionally, selenium compounds are used as vulcanizing agents in the rubber industry to improve the durability and elasticity of rubber products.

Summary:

Selenium's unique physical and chemical properties, such as its ability to function as a semiconductor and its photoconductive properties, make it a versatile element with a wide range of applications. These properties have led to its use in photovoltaic cells and photocopiers, although advances in technology have seen silicon largely replace selenium in solar cells. Safety precautions are essential when handling selenium and its compounds due to their potential toxicity, though selenium is also recognized as a vital nutrient in trace amounts necessary for human health. The production of selenium is closely linked to the refining of sulfide ores, particularly copper, with the element being recovered as a byproduct. With its wide range of applications and essential biological roles, selenium remains an important element in both industrial and scientific contexts.

See a comprehensive list of atomic, electrical, mechanical, physical and thermal properties for selenium below: