Atomic number 52

Tellurium

Te
+ phase
− phase

Shells Orbitals 4d 5s 5p

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52 electrons in 5 shells: 2, 8, 18, 18, 6.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Metalloid
Period: 5
Group: 16
Block: p
Standard atomic weight: 127.60(3) Da
Electron configuration: [Kr] 4d¹⁰ 5s² 5p⁴
Phase at 25 °C: solid
Melting point: 449.51 °C
Boiling point: 988 °C
Density: 6.232 g/cm³
Electronegativity (Pauling): 2.1
Discovered: 1782, Franz Joseph Müller von Reichenstein

Neighbours in the table

Sb I

Tellurium was found, mistaken for antimony and then forgotten. In 1782 Franz Joseph Müller von Reichenstein extracted a new metal from a gold ore called aurum album. The Hungarian chemist Paul Kitaibel found the element independently in 1789, according to CIAAW. The German chemist Martin Heinrich Klaproth isolated it in 1798 and named it after tellus, Latin for earth. He gave Reichenstein full credit, Jefferson Lab says.

Of tellurium’s 52 electrons ([Kr] 4d¹⁰ 5s² 5p⁴), the shells hold 2, 8, 18, 18 and 6. That outer count of six largely sets its valence, its combining power, in Chem1’s account. Group 16 atoms reach an s² p⁶ outer shell by gaining or sharing two electrons. Its oxidation states, bookkeeping charges as if all bonds were ionic, are +6, +4 and −2 in Jefferson Lab’s list. Los Alamos gives +6 through +1, −1 and −2.

Pure crystalline tellurium is silvery white with a metallic lustre, yet Los Alamos finds it brittle and easily ground to powder. It is a semiconductor, and it conducts better in some directions than others, depending on how its atoms are aligned, in Los Alamos’s description. Light raises that conductivity slightly. Its electronegativity, its pull on the electrons it shares in a bond, is 2.1, below the value of about 2.2 that Chem1 associates with metallic character.

Where it is found

Tellurium is scarce in rock: Jefferson Lab’s figure for Earth’s crust is 1 × 10⁻³ milligrams per kilogram. Los Alamos finds it native now and then, but more often bound to gold as calaverite or combined with other metals. Its three usual ores are sylvanite, AgAuTe₄, and calaverite and krennerite, both AuTe₂, per Jefferson Lab.

The metal itself is recovered as a byproduct. More than 90 per cent has come from anode slimes, in the US Geological Survey’s figures: the residue left from copper anodes during electrolytic refining. The rest comes from lead-refinery skimmings and from flue dusts of bismuth, copper and lead-zinc smelting. In 2024 two American copper refineries, in Texas and Utah, turned their slimes into copper telluride and exported it for refining abroad.

China led refined output in 2024 with about 75 per cent of the world estimate. Its production grew over ten years alongside a rise of about 60 per cent in Chinese electrolytic copper refining capacity, in the Survey’s account. Because tellurium is a byproduct, mainly of copper, its supply is directly affected by the supply of copper, the Survey notes.

Why it matters

Most tellurium now ends up in solar panels. Solar cells took 60 per cent of world consumption in 2024, mainly as thin films of cadmium telluride, CdTe, by the US Geological Survey’s estimate. Thermoelectric devices took 20 per cent, using bismuth telluride to cool or to generate power from heat. Amorphous silicon and copper indium gallium diselenide compete with CdTe in thin-film cells.

Metallurgy took another 15 per cent. A small addition makes copper alloys easier to machine without lowering their conductivity, and it helps lead alloys resist vibration and fatigue. In lead, tellurium also reduces attack by sulfuric acid and raises hardness, according to Los Alamos. In cast iron it controls the depth of chill, and steel machines more easily with it. Rubber vulcanising, catalysts for synthetic fibres and colours in glass and ceramics account for part of the rest.

Four tellurium isotopes are starting materials for medical iodine isotopes, in IUPAC’s notes. Tellurium-123 yields iodine-123 for thyroid imaging, and tellurium-124 yields iodine-123 and iodine-124, the latter for positron emission tomography (PET); tellurium-120 and -122 serve imaging too. Its metallurgical and chemical uses scatter it too thinly to leave much scrap, in the Survey’s account. CdTe panels are recycled in the United States, but little so far, since most have not reached the end of their working lives.

Properties

Properties of tellurium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	52
Standard atomic weight	127.60(3) Da	Da	IUPAC/CIAAW 2024
Melting point	449.51 °C	°C	Los Alamos: 449.51 °C
Boiling point	988 °C	°C	Los Alamos: 988 °C
Density	6.232 g/cm³	g/cm³
Electronegativity (Pauling)	2.1
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁴
Phase at 25 °C	solid

Safety

Elemental tellurium is harmful: it carries the signal word Danger under the Globally Harmonized System (GHS) of hazard labels, in PubChem’s safety record. The European Union’s harmonised classification says it may damage the unborn child, is suspected of damaging fertility and may harm breast-fed children. For workers, its target organs are the skin, central nervous system and blood, in NIOSH’s listing. Details are in the PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide card on the element.

Questions

Is garlic breath a sign of tellurium exposure?

Haz-Map, the occupational health database quoted in PubChem's safety record, lists a garlic odour on the breath, in urine and in sweat as signs that the body has absorbed tellurium. Separately, inside the body, tellurium can do harm by taking the place of sulfur, an element the body needs, in an EPA summary quoted in the same record.

Why is tellurium named after the Earth?

Klaproth chose the name in 1798 from the Latin tellus, earth; CIAAW, IUPAC's atomic-weights commission, ties it to Tellus, the Roman goddess of the Earth. Selenium, found by Berzelius in 1817 alongside tellurium, was then named for Selene, the Moon, according to Los Alamos. It also goes by two Latin names, aurum paradoxum and metallum problematum, in the NIOSH Pocket Guide.

Is tellurium a metal or a metalloid?

Tellurium is a metalloid in PubChem's classification and a semi-metal in Jefferson Lab's. Pure crystals have a metallic lustre but are brittle. In air it burns with a greenish-blue flame to form the dioxide, per Los Alamos. It does not dissolve in water, and concentrated sulfuric acid turns it into a red solution, according to a CAMEO entry in PubChem.

Is tellurium radioactive?

Only very weakly. Natural tellurium is a mix of eight isotopes, per Los Alamos, and two of them decay extremely slowly. Tellurium-130, about 34 per cent of the natural element in CIAAW's figures, has a half-life of 7.91 × 10²⁰ years in the Atomic Mass Data Center figures, and that decay is used to date when tellurium minerals began to retain gas, IUPAC notes. Tellurium-128 has a half-life of 2.25 × 10²⁴ years, in IAEA's Atomic Mass Data Center figures.