Tellurium is a chemical element with symbol Te and atomic number 52 It lies in group 16 (the chalcogens) of the periodic table, period 5, in the p-block. At room temperature it is a solid, a brittle silvery-gray metalloid Its standard atomic weight is 127.60 g/mol and it typically shows oxidation states –2, +2, +4 and +6 (with +4 most common) Tellurium is dense (~6.24 g/cm³) and has relatively high melting and boiling points among the chalcogens (about 722.7 K = 449.5 °C and 1261 K = 988 °C, respectively) In its pure form Te has a lustrous metallic appearance; it forms two allotropes – a silvery-white crystalline form that is brittle and easily powdered, and a black amorphous powder obtained by chemical precipitation Its electronic configuration is [Kr] 4d¹⁰ 5s² 5p⁴ giving it six valence electrons (like sulfur or selenium). As expected for a heavy chalcogen, tellurium has a moderate electronegativity (~2.1 on the Pauling scale) and a first ionization energy around 9.01 eV (≈ 870 kJ/mol) Its covalent radius is about 136 pm (van der Waals radius ≈200 pm), reflecting its large size compared to lighter chalcogens.

Atomic Structure and Chemical Trends

Tellurium
Atomic number	52
Symbol	Te
Group	16 (chalcogens)
Boiling point	988 °C
Electron configuration	[Kr] 4d10 5s2 5p4
Density	6.24 g/cm^3
Period	5
Melting point	449.5 °C
Phase STP	Solid
Block	p
Oxidation states	−2, +4, +6
Wikidata	Q1100

The electronic structure of Te (with filled 4d and 5s shells and a half-filled 5p shell) makes it a p-type semiconductor. Its single-crystal structure consists of parallel helical chains of Te atoms (three atoms per turn) in a trigonal lattice This chain structure leads to highly anisotropic properties: for example, the electrical conductivity is higher along the chains, and pure Te is photoconductive (conductivity increases slightly under illumination) The narrow band gap (∼0.33 eV) gives Te metal-like luster but relatively poor conductivity. In practical terms, Te is a poor thermal conductor (∼2 W·m⁻¹·K⁻¹ at room temperature and is diamagnetic. Chemically, Te is the heaviest stable chalcogen and trends in the group: its atoms are larger and more polarizable than selenium’s, while its electronegativity (2.1) is lower than Se (2.55) but higher than metallic polonium (∼2.0). Its ionic radius (for Te²⁻) is large (about 221 pm in Na₂Te), consistent with its –2 oxidation state in telluride salts. The first ionization energy (∼9.0 eV is smaller than that of selenium (9.75 eV) and sulfur (10.36 eV), following the expected decrease in ionization energy down the group.

Isotopes and Nuclear Properties

Naturally occurring tellurium consists of eight stable or quasi-stable isotopes: ^120Te, ^122Te, ^123Te, ^124Te, ^125Te, ^126Te, ^128Te, and ^130Te The lighter six (^120–^126) are essentially stable (with infinite or effectively stable half-lives), whereas ^128Te and ^130Te undergo extremely slow double-beta decay with half-lives on the order of 10^24 years In fact ^128Te (half-life ≈2.2×10^24 y) has the longest known nuclear half-life of any nuclide Overall, only about 33% of natural Te atoms are from “truly stable” isotopes; the rest are the long-lived radioisotopes ^128Te (31.7%) and ^130Te (34.1%) Humans artificially produce many tellurium radioisotopes (dozens are known, with mass numbers 104–142); none are used for long-term dating, though trace alpha decays of ^106Te–^110Te are of research interest Two minor tellurium isotopes (^123Te and ^125Te) have nuclear spin 1/2; the other natural isotopes have spin 0 Notably, tellurium is the lightest element with any alpha-emitting isotopes (for ^106–^110Te) though this is only seen in the lab.

In nuclear applications, tellurium’s isotopes play useful roles. For example, neutron irradiation of stable Te produces iodine-131 (via ^130Te(n,γ)^131Te→^131I), which is widely used in medical diagnostics and thyroid therapy Tellurium-130 is a prominent candidate in double-beta decay research (neutrino physics). The long-lived isotopes (like ^128Te/^130Te) are occasionally used in proto-solar system dating or as trace markers in geochemistry. Additionally, two isotopes (^123Te and ^125Te) are NMR-active (spin 1/2) and are used in solid-state NMR studies of tellurium compounds.

Allotropes and Compounds

Tellurium has two allotropes: a crystalline form and an amorphous form Crystalline tellurium is silvery-white with a metallic luster; its structure is trigonal (space group 152), consisting of spiral chains of Te atoms This form is relatively hard and brittle. The amorphous allotrope is a black-brown powder obtained, for example, by precipitation from tellurous or telluric acid solutions Amorphous Te is semiconducting but is less commonly encountered.

Chemically, Te behaves as a heavy chalcogen. It forms a variety of compounds analogous to those of sulfur/selenium. The most important binary compounds include:

Hydrides: The tellurium hydride H₂Te (tellurane) is analogous to H₂S and H₂Se. It is a very unstable, colorless, and highly toxic gas. (In the lab it is generated by acidifying metal telluride salts, e.g. ZnTe + 2 HCl → H₂Te + ZnCl₂.) Like H₂Se, H₂Te decomposes to elemental Te and hydrogen.
Oxides and oxyacids: Tellurium trioxide and dioxide are known. The stable oxide TeO₂ (tellurium dioxide) is a white or pale-yellow solid; it is amphoteric (dissolves in strong base to give tellurite ions TeO₃²⁻, and in strong acid to give tellurium(IV) species). Higher oxide TeO₃ can be described as H₆TeO₆ (telluric acid) which yields tellurate ions TeO₄²⁻ in basic solution. Tellurous acid H₂TeO₃ (derived from TeO₂) also exists.
Halides: Tellurium forms halides in +4 and +6 oxidation states. For example, TeCl₄, TeBr₄ and TeI₄ are tetrachloride/bromide/iodide (white or pale solids), while TeF₆ is a volatile colorless gas (tellurium hexafluoride, Te in +6). Under controlled conditions TeF₄ (sublimable solid) is also obtained. Thus Te reacts vigorously with halogens: for instance, F₂ yields TeF₆ (or TeF₄ under milder conditions) and Cl₂/Br₂ form the tetrahalides
Metal tellurides: The –2 state appears in numerous inorganic tellurides. For example, zinc telluride (ZnTe), cadmium telluride (CdTe), mercury telluride (HgTe), silver/nickel tellurides (Ag₂Te, NiTe₂), and bismuth telluride (Bi₂Te₃) are well-known semiconductors. Bi₂Te₃ and related compounds (like PbTe, Sb₂Te₃) are used in thermoelectrics. CdTe is a key photovoltaic semiconductor. Many metals (e.g. Au, Ag, Cu, Ni) form low-volatility tellurides in which Te is –2. Under acid these “telluride minerals” release H₂Te. Some mixed chalcogenides (selenotellurides) also exist, combining Se and Te (often in glasses or as complex salts).
Organotellurium: Although less common, Te forms organic compounds (e.g. dimethyl telluride) by analogy to thiols. Metabolism in organisms (and in industry) often methylates tellurium, giving volatile di-alkyl tellurides.

Overall, tellurium compounds parallel those of selenium and sulfur, especially in oxidation states +4 and +6. But because Te is larger and less electronegative, its compounds are often more covalent and less oxidizing than analogous sulfur compounds.

Physical Properties

Solid tellurium is a brittle material with a metallic shine. Crystalline Te adopts a trigonal lattice of parallel Te–Te chains (bond length ~286 pm which accounts for its anisotropy and brittleness. The element resists oxidation in air at room temperature and is not volatile. Molten Te, however, is highly corrosive to common metals (it attacks copper, steel and stainless steel) Compared to metals, solid Te has relatively low thermal and electrical conductivity. Its thermal conductivity at 300 K is ~2 W·m⁻¹·K⁻¹ (much lower than typical metals), reflecting its semiconducting nature. As noted, Te is a p-type semiconductor, so its electrical resistivity is moderate (and decreases with heating); it shows photoconductivity under light It is diamagnetic and has only inverse pyroelectric/optic effects.

Key numerical properties include a density of about 6.24 g/cm³ Its melting point (449.5 °C and boiling point (988 °C are the highest in group 16. The Dulong–Petit heat capacity is on the order of 0.2 J·g⁻¹·K⁻¹ On the Mohs hardness scale, tellurium is quite soft (around 2), and its Brinell hardness is ~25 MPa Optically, elemental Te is opaque, but its oxide (TeO₂) forms colorless crystals that are transparent in the infrared; specialized optical glasses containing Te (mixed with Se or Ge) are prized for high refractive index in IR fibers. Spectroscopically, Te has many atomic emission lines (especially in the near-infrared), but these are more of academic interest; no single “signature line” is commonplace in laboratory analysis outside of specialized instruments.

Chemical Reactivity

Tellurium is chemically less reactive than sulfur or selenium but readily forms compounds. In air it combines with oxygen when heated:

<code>2 Te + O₂ → 2 TeO₂
</code>

so it burns to tellurium(IV) oxide At ambient conditions Te metal is quite inert: it does not react appreciably with water and is unaffected by non-oxidizing acids such as HCl However, oxidizing agents attack it; for example hot concentrated nitric acid and sulfuric acid dissolve Te, producing tellurate species. Molten tellurium (or TeO₂) dissolves in strong alkali to give tellurite (TeO₃²⁻) or tellurate (TeO₄²⁻) ions. Elemental Te does react with halogens: fluorine reacts vigorously (even at 0 °C with a diluent) to give TeF₄ or TeF₆ and chlorine, bromine or iodine (under heating) give the tetrahalides (TeCl₄, TeBr₄, TeI₄) In solution, Te(IV) and Te(VI) species are quite oxidizing (e.g. telluric acid is a strong oxidizer, capable of converting iodide to iodine).

In its –2 state, tellurium (as telluride Te²⁻) is a reducing agent. For example, zinc telluride decomposes with acid to yield hydrogen telluride H₂Te (analogous to H₂S) Metal tellurides generally behave like salts of Te²⁻: they often hydrolyze in acid to form Te or TeO₂. In alloys, tellurium shows interesting behavior: even small additions improve the machinability of copper and steel. (It tends to form fine, brittle inclusions that act as stress concentrators for cutting tools, without greatly affecting the metal’s conductivity.) All of this chemistry places tellurium in line with heavier chalcogens: it can span –2 to +6, forming covalent or ionic bonds depending on partner.

Occurrence and Production

Tellurium is extremely rare in Earth’s crust – about 1 part per billion by weight (roughly comparable to platinum). Its cosmic abundance is much higher, but Te’s formation of volatile hydrides in the early solar nebula caused it to be largely lost from the Earth In nature Te is found mostly as metal telluride minerals, rather than as free element. Notable examples include: calaverite and krennerite (AuTe₂ polymorphs), sylvanite (AgAuTe₄), and petzite (Ag₃AuTe₂) – ores associated with gold and silver. Telluride minerals of more common metals also occur (e.g. melonite NiTe₂, Bi₂Te₃). Very rarely one finds native (elemental) tellurium. Secondary minerals such as tellurites (TeO₃²⁻) and tellurates (TeO₄²⁻) form by oxidation of tellurides near the surface Unlike selenium, tellurium seldom substitutes for sulfur in sulfide minerals.

Commercially, almost all tellurium is produced as a by-product of copper or lead refining In electrolytic copper refining, for example, Te (and Se) accumulate in the sludges of the anodes. These sludges (containing metal tellurides of Cu, Ag, Au, etc.) are roasted with alkali, converting telluride to tellurite (Na₂TeO₃) while metals are precipitated The tellurite is then leached, the tellurate filtered out (or converted to TeO₂ by acid precipitation), and finally elemental Te is recovered (for instance by electrolysis or by reduction with SO₂/H₂SO₄) Roughly 1000 tonnes of copper ore yields only ~1 kg of Te underscoring the element’s scarcity. Major tellurium producers today include China (by far the leader), followed by Russia, Japan, Canada, and a few others. For example, in 2022 China’s output was ~340 tonnes, while Russia and Japan produced on the order of 70–80 t each (China is unique in actively mining Te concentrates; other countries rely entirely on by-products.) Prices and supply are volatile, as global demand (for PV and thermoelectrics) has risen faster than refining can keep up

Applications and Technology

Most tellurium is now used in high-technology applications. The largest single end-uses are in photovoltaic and thermoelectric devices. Approximately 40 % of Te goes into cadmium telluride (CdTe) thin-film solar cells, which are valued for low-cost, high-efficiency conversion of sunlight Another ~30 % is used in thermoelectric devices – notably bismuth telluride (Bi₂Te₃) alloys – which convert heat to electricity (or act as solid-state Peltier coolers) Both of these “green” technologies have driven a surge in Te demand in the 21st century.

Another major application (∼15–20 %) is metallurgical additives. Small additions of Te greatly improve the machinability of copper, steel and lead alloys. For instance, “tellurium copper” (with ~0.5 % Te) can be machined more easily without losing electrical conductivity. Similarly, tellurium makes lead and malleable iron more resistant to vibration and fatigue. In lead–tin–copper alloys for bearings and white metal, Te refines grain structure and hardness. In short, Te is a specialty alloying agent for conventional metals

Other specialized uses include: vulcanization of rubber (Te-based catalysts yield faster curing and improved mechanical/thermal properties compared to sulfur alone semiconductor detectors (CdZnTe crystals for X-ray and gamma-ray sensors); optical materials (tellurium oxides and selenotellurite glasses for high refractive-index IR optics); ceramic pigments (some Te-based compounds produce ruby-red and orange colors). In chemical industry, tellurium oxides are used as oxidation catalysts (e.g. in acrylonitrile production), and Te salts serve as specialty reagents. Finally, irradiation of Te to produce radioactive iodine (as noted above) is an important medical application Te has also found niche uses in laboratory science – for example, it can “bulk dope” certain semiconductors, and it has been researched in new thermoelectric materials and topological quantum devices.

Biology, Environment, and Safety

Tellurium has no known essential function in biological systems. In fact, it is generally toxic (“it has great biological potency” at high levels In the environment, tellurium is extremely dilute. Certain microorganisms can interact with it: some bacteria (e.g. Pseudomonas spp.) and yeasts can take up soluble tellurite (TeO₃²⁻) and reduce it to elemental tellurium, producing black intracellular deposits Fungi can inadvertently incorporate Te in place of sulfur or selenium in amino acids (forming telluro-cysteine/methionine) However, Te does not bioaccumulate in food chains to any significant extent, and its ecological cycling is negligible compared to major elements.

For human health, tellurium is toxic at relatively low doses. Effectively any intake above trace levels can cause illness. Acute poisoning is rare, but chronic exposure (e.g. via dust or fumes) can lead to: gastrointestinal distress, weight loss, fatigue, kidney and liver damage, and peripheral neuropathy. The most distinctive symptom is the so-called “tellurium breath”: the body metabolizes tellurium into dimethyl telluride (CH₃)₂Te, a volatile compound with a strong garlic-like odor This odor can persist for weeks. Physiologically, Te interferes with selenium metabolism and sulfhydryl enzymes, which underlies much of its toxicity.

Regulators treat tellurium as a hazardous material. U.S. occupational exposure limits are on the order of 0.1 mg/m³ as a time-weighted average NIOSH identifies 50 mg/m³ as an immediately dangerous to life/health (IDLH) benchmark for acute exposure Because Te compounds can deposit in the body and chelation is not effective, chronic exposure should be strictly avoided. In practice, laboratory and industrial handling require fume hoods and protective equipment. Personal exposure even to elemental Te dust is minimized because of its odor hazard. Finally, environmental releases of tellurium (for example from mining or refining) are of limited concern due to its rarity and the fact that any Te entering water or soil is rapidly converted to insoluble or volatile forms with minimal bioavailability. Nonetheless, its toxicity warrants careful waste treatment of Te-containing effluents.

History and Etymology

Tellurium’s story began in 1782, when Hungarian mineralogist Franz-Joseph Müller von Reichenstein discovered “a new metal” in a Transylvanian gold ore Müller initially thought it was antimony, but after years of tests (noting an odd garlic “radish” smell on heating) he realized it was a new element. In 1798 the German chemist Martin Heinrich Klaproth independently isolated the element and named it tellurium, from the Latin tellus meaning “earth” (The town of Telluride, Colorado, was later named for gold-telluride minerals once sought there Earlier nicknames for Te included aurum paradoxum (“paradoxical gold”) because it was often encountered in gold minerals.

In the 20th century tellurium found strategic uses. During World War II it was used as an alloying agent (a “chemical bonder”) in the outer shell of the first atomic bomb After the war, Te’s applications in electronics emerged. The 1960s saw a surge in thermoelectric research (bismuth telluride Peltier coolers) and the introduction of “free-machining” Te-copper and Te-steel alloys, quickly making Te metallurgy the element’s dominant use In recent decades, the rise of solar photovoltaics and materials science has turned tellurium into a “critical” technology element: it is now valued more for its electronic and optical applications than ever before

Data Table

Property	Value
Atomic number (Z)	52
Symbol	Te
Atomic weight	127.60 (standard atomic weight)
Period, group, block	Period 5, Group 16 (Chalcogens), p-block
Element category	Metalloid (silvery-gray)
Electron config.	[Kr] 4d¹⁰ 5s² 5p⁴
Oxidation states	–2, +2, +4, +6 (common: –2, +4, +6)
Valence electrons	6
Electronegativity (Pauling)	2.1
First ionization energy	9.01 eV (869 kJ/mol)
Atomic radius	~135 pm (atomic), 136 pm (covalent)
Van der Waals radius	~200 pm
Density (solid, 20 °C)	6.24 g/cm³
Melting point	722.7 K (449.5 °C)
Boiling point	1261 K (988 °C)
Heat capacity (solid)	~0.2 J·g⁻¹·K⁻¹
Thermal conductivity	~2 W/(m·K) at 300 K
Electrical conductivity	p-type semiconductor (~10³ S/m)
Crystal structure	Trigonal (helical chains, space grp 152)
Magnetic order	Diamagnetic
Isotopes (natural)	8 stable (120–126) + 2 long-lived (128,130)
Key compounds	H₂Te; TeO₂, H₂TeO₃ (tellurous acid), H₆TeO₆ (telluric acid); TeCl₄, TeF₆; many metal tellurides (Bi₂Te₃, CdTe, ZnTe…)
Discovery	1782 (Müller von Reichenstein)
Etymology	Named after Latin tellus (“earth”) in 1798 (Klaproth)