Selenium (Se) is classified as a metalloid, meaning it exhibits properties of both metals and nonmetals. Selenium presents a complex profile due to its ability to exist in multiple structural forms. The most frequently cited value for the melting point of selenium is approximately \(221^{\circ}C\) (\(430^{\circ}F\)). This specific temperature, however, is not universally applicable across all samples of the element. The precise melting behavior depends entirely on the element’s atomic arrangement, which differs significantly depending on how the material was processed or formed.
The Variable Nature of Selenium’s Melting Point
The reason selenium’s melting point is variable lies in its capacity to form various allotropes, which are different structural modifications of the element. The most stable and well-ordered form is gray, metallic selenium, which possesses a crystalline, hexagonal lattice structure composed of long, helical chains of atoms. This specific, thermodynamically stable gray allotrope is associated with the sharp melting point of \(221^{\circ}C\).
Other common forms are amorphous, meaning they lack an ordered crystalline structure. These include the black, vitreous (glassy) form and the red powder form. Amorphous allotropes do not have a distinct melting point but instead soften over a range of temperatures. For example, the black, vitreous form begins to soften as low as \(50^{\circ}C\) and converts entirely into the more stable gray form around \(180^{\circ}C\). The quoted \(221^{\circ}C\) value represents the melting point of this most stable, metallic arrangement.
Unique Electrical and Optical Properties
Selenium possesses unique electrical and optical characteristics that stem from its status as a semiconductor. The most notable property is photoconductivity, where the material increases its electrical conductivity when exposed to light. Light energy excites the electrons, allowing them to migrate freely and conduct current far more effectively than they do in the dark. This light-sensitive behavior can cause the electrical conductivity of crystalline selenium to increase by over a thousand times upon illumination.
Selenium also exhibits rectifying properties, meaning it can efficiently convert alternating current (AC) into direct current (DC). Early selenium rectifiers utilized this behavior to control the flow of electricity. These features are fundamental to its application in various technologies, allowing it to act as a light-controlled switch and a one-way electrical valve. The conductivity of selenium is highly dependent on variables such as impurities, pressure, and temperature.
Primary Industrial Applications
The unique properties of selenium have made it indispensable in several industrial sectors. One of the largest historical applications was in xerography, the technology used in photocopiers and laser printers. The light-sensitive coating on the photoreceptor drum is traditionally composed of amorphous selenium. This coating holds an electrostatic charge in the dark but allows the charge to dissipate when exposed to light, forming the image.
In the glass industry, selenium is used extensively for two distinct purposes. It acts as a decolorizer, neutralizing the subtle green tint that iron impurities impart to glass, resulting in clear, colorless products. Conversely, at higher concentrations, selenium gives glass and enamels a deep, ruby-red color. Its semiconductor properties also led to its use in the production of rectifiers for current conversion and in certain types of solar cells.