Tellurium (Te), atomic number 52, is not classified as a metal. It is found in Group 16 of the periodic table, alongside oxygen and sulfur. Tellurium occupies a unique position, which is why its properties are often confusingly described as “metallic.” The correct classification, which accounts for its dual nature, is that of a metalloid.
Defining the Metalloid Category
The term metalloid, also known as a semi-metal, describes elements that display properties intermediate between those of true metals and nonmetals. These elements are physically located along the “stair-step line” on the periodic table, separating metals on the left from nonmetals on the right. Tellurium sits directly on this boundary, which dictates its ambiguous behavior.
Metalloids exhibit variable behavior, sometimes acting like a metal and other times like a nonmetal, depending on the chemical environment. For example, tellurium can form compounds carrying a positive charge (like a metal) or adopt a negative charge (a characteristic of nonmetals). This flexibility distinguishes it from pure metals (lustrous and highly conductive) or nonmetals (often gases or brittle solids).
Tellurium’s placement in the chalcogen group (Group 16) links it chemically to nonmetals like sulfur and selenium, yet it exhibits a metallic appearance. This intermediate nature requires the metalloid category for a precise description of its elemental nature.
Distinct Physical and Electrical Characteristics
In its crystalline form, tellurium exhibits a silvery-white color and a distinct metallic luster, leading many to initially mistake it for a true metal. However, its physical behavior contrasts sharply with this metallic appearance; solid tellurium is brittle and can be easily pulverized into a powder, traits typical of nonmetals. Unlike malleable metals that can be hammered into sheets, tellurium lacks the ability to be drawn into wires or shaped without breaking.
Tellurium functions as a semiconductor, which is definitive evidence of its metalloid status. Specifically, it is a p-type semiconductor, meaning its electrical conductivity is significantly lower than that of metals but higher than that of insulators. Its conductivity increases when it is exposed to light, a phenomenon known as photoconductivity. Furthermore, tellurium’s conductivity also increases with a rise in temperature, which is the opposite of how true metals behave.
The element also displays anisotropic conductivity, meaning electricity flows more easily along certain crystal directions than others. This directional preference for electrical current is a result of its unique crystalline structure.
Essential Roles in Modern Technology
Tellurium’s unique electrical properties make it a foundational material in several advanced technological fields. One significant application is in the production of thin-film solar cells, particularly those made from cadmium telluride (CdTe). These cells leverage tellurium’s semiconductor nature to efficiently convert sunlight into electricity, offering a cost-effective alternative to traditional silicon-based solar panels.
The element is also indispensable in thermoelectric devices, where it is often alloyed with bismuth to create bismuth telluride (Bi₂Te₃). These materials are used to generate electricity from a temperature difference or to create cooling using an electrical current. These applications rely directly on tellurium’s thermal and electrical properties.
Tellurium also serves as an alloying agent in traditional metallurgy. Adding a small percentage of tellurium, often around 0.04%, to materials like stainless steel and copper significantly improves their machinability. This allows the metals to be cut and shaped more easily without compromising their structural integrity or electrical conductivity.