TELLURIUM

Name: Tellurium (from the Latin "tellus," "Earth")

Symbol: Te

Group: 16

Period: 5

Block: p

Category: Metalloids

Atomic Number: 52

Atomic Mass: 127.60 u

Electrons per Shell: 2, 8, 18, 18, 6

Electronegativity: 2.1

Density: 6.24 g/cc

Melting Point: 449.51°C

Boiling Point: 988°C

Thermal Conductivity: 3 W (m K)

Electrical Conductivity: 10,000 S/m

Magnetic Order: Diamagnetic

Ordinary State: Solid

Oxidation States: 6, 5, 4, 3, 2, 1, -1, -2

Mohs Hardness: 2.25

Vickers Hardness: No data

Brinell Hardness: 180 MPa

Most stable isotopes: Te-120 (0.09%), Te-122 (2.55%), Te-123 (0.89%), Te-124 (4.74%), Te-125 (7.07%), Te-126 (18.84%), Te-128 (31.74%), Te-130 (34.08%)

Discoverer: Martin Klaproth, German (1782)

Tellurium (Te), with atomic number 52, is a metalloid of group 16 (chalcogens), with a density of 6.24 g/cm³ and an abundance of approximately 0.001 ppm in the Earth's crust, making it extremely rare. Its most distinctive characteristic is its ability to form compounds with gold (Au), a noble metal that rarely combines chemically, underscoring tellurium's unique chemistry within its group. Discovered in 1782 by the German chemist Martin Heinrich Klaproth, tellurium was named in honor of the Latin term Tellus, meaning Earth, reflecting its presence in terrestrial minerals. This name complements selenium (Se), named by Jöns Jacob Berzelius in 1817 in reference to Selene (the Moon in Greek mythology), highlighting the relationship between these two group 16 chalcogens.

Tellurium is found in minerals such as calaverite (AuTe₂), a gold telluride, and sylvanite (AuAgTe₄), notable compounds because gold, with atomic number 79, is chemically inert and does not form oxides (like copper, Cu, with CuO or Cu₂O) or sulfides (like silver, Ag, with Ag₂S) under normal conditions. The formation of tellurides with gold is due to tellurium's high atomic mass (Z=52), which gives it a unique reactivity among chalcogens. While reactive elements to the left of group 8 (iron, Fe) tend to form oxides, and noble metals from group 8 onwards form sulfides or, in the case of gold, tellurides, this trend reflects the relationship between the atomic number of the metal and its non-metallic partner: copper (Z=29) with oxygen (Z=8), silver (Z=47) with sulfur (Z=16), and gold (Z=79) with tellurium (Z=52). Calaverite, one of the few natural gold compounds, is especially valuable in mining and geochemical studies.

Although known since the 18th century, tellurium had no significant applications until the 20th century due to its rarity and moderate toxicity. In 2025, its global production (500 annual tons) focuses on electronic applications, such as semiconductors (cadmium telluride, CdTe, in solar cells) and alloys, where its cost (50–100 USD/kg) reflects its scarcity. The history of tellurium, from its discovery to its association with gold, highlights its chemical uniqueness and its symbolic connection to the Earth, complementing selenium in the chalcogen group.

Tellurium, with atomic number 52, is a metalloid of group 16 (chalcogens), with a density of 6.24 g/cm³ and an abundance of approximately 0.001 ppm in the Earth's crust, making it one of the rarest elements – an irony given its name derived from the Latin Tellus (Earth). Although it can be found in native form, this occurrence is extremely rare; it is more common in minerals such as calaverite (AuTe₂) or sylvanite (AuAgTe₄). In its pure state, tellurium exhibits a metallic grey luster, with an appearance similar to antimony (Sb), but its soft (2–2.5 on the Mohs scale) and brittle nature distinguishes it, with a low compressive strength (2 GPa), making it easily pulverizable.

Chemically, tellurium is relatively inert in dry air at room temperature, forming a thin layer of tellurium oxide (TeO₂) that partially protects it from oxidation. However, in humid environments or at elevated temperatures, it is more reactive, being attacked by oxidizing acids such as nitric (HNO₃) and hydrofluoric (HF), as well as strong alkalis like sodium hydroxide (NaOH). Compared to arsenic (As) and antimony, tellurium is less resistant to corrosive media, limiting its applications in aggressive environments. Its thermal conductivity (3 W/(m·K)) and electrical conductivity (1–10 S/m) are poor, but its band gap of ~0.33 eV makes it valuable as a semiconductor in specific applications.

Tellurium is fragile and lacks mechanical robustness, which restricts its use in its pure state. However, its ability to form compounds with noble metals like gold (Au) and its relatively low toxicity compared to arsenic make it relevant in specialized applications. In 2025, the global production of tellurium (500 annual tons) and its cost (50–100 USD/kg) reflect its scarcity and its importance in technologies such as solar cells and alloys, where its chemical reactivity and electronic properties are exploited despite its mechanical limitations.

Tellurium, with atomic number 52, is a metalloid of group 16 (chalcogens), with a density of 6.24 g/cm³ and an abundance of approximately 0.001 ppm in the Earth's crust, making it one of the rarest elements. Its global production, about 500 annual tons in 2025, and its cost (50–100 USD/kg) reflect its rarity and its limited use in industrial applications. Due to its fragility (2–2.5 Mohs), low mechanical strength (~2 GPa), and reactivity in corrosive media, tellurium has few applications in its pure state or as a compound, standing out mainly in metallurgy and specialized technologies, where its moderate toxicity and unique chemical properties are leveraged in specific contexts.

In metallurgy, tellurium is used in small quantities to improve the properties of certain alloys. In carbon steels and stainless steels, it is added (0.01–0.1%) to increase machinability, facilitating cutting and machining by reducing friction and forming more manageable chips, although its use is limited due to its impact on the material's toughness. In copper alloys (Cu-Te), tellurium (0.5–2%) improves wear resistance and moldability, being useful in electrical connectors and precision parts, but its application is restricted by its cost and the availability of alternatives such as phosphorus (P). In lead alloys (Pb-Te), tellurium acts as a hardening agent, increasing rigidity and resistance to sulfuric acid (H₂SO₄), which is valuable in batteries and chemical coatings. However, its effect is marginal compared to antimony (Sb) or bismuth (Bi), which are more common and economical.

Outside of metallurgy, tellurium has specialized applications as a compound. Cadmium telluride (CdTe) is a key semiconductor in thin-film solar cells, with a band gap of ~1.45 eV that optimizes solar energy conversion. It is also used in infrared and gamma ray detectors, leveraging its high sensitivity to specific wavelengths (1–10 µm). Tellurium oxide (TeO₂) is used in acousto-optic crystals for light modulation devices, such as lasers and spectrometers. Despite these applications, tellurium is still considered an exotic element, with limited use due to its scarcity, moderate toxicity (less severe than arsenic, but still regulated), and the preference for more abundant materials like silicon (Si) or germanium (Ge). Its ability to form tellurides, such as calaverite (AuTe₂) with gold, remains of interest in mining and geochemistry, but does not translate into significant industrial applications.