Tellurium

Tellurium is a rare, silver-white metalloid that smells of garlic when inhaled, a smell that can persist in a person's breath and sweat for weeks after even tiny exposure. It is a semiconductor used in thermoelectric devices and is the "Te" in CdTe solar cells, one of the most successful thin-film solar technologies.

• Atomic Number5252 protons, 52 electrons
• Atomic Mass127.6 u52× heavier than hydrogen
• State at Room TempSolidSolid
• Density6.232 g/cm³
• Melting / Boiling449.5°C / 987.9°C
• Discovered1782

What is Tellurium?

Tellurium is a metalloid in Group 16 of the periodic table, sitting below selenium. With 52 protons, tellurium has a metallic appearance but conducts electricity only moderately. It is brittle and burns in air with a blue-green flame. Like selenium, it exists in several allotropes. Its most common oxidation states are +4 and +6.

Tellurium is named after Tellus, the Latin name for the Earth, chosen by Martin Heinrich Klaproth when he confirmed the element's identity in 1798. Franz-Joseph Müller von Reichenstein had first identified it in a gold ore from Transylvania in 1782, calling it aurum paradoxum (paradoxical gold) because it looked like gold but was not. Klaproth confirmed and named it 16 years later. Selenium was later named after Selene (the Moon) as a complement.

Fact Tellurium has one of the most distinctive symptoms of overexposure of any element: "tellurium breath", a persistent, powerful garlic-like odour caused by the body excreting volatile dimethyl telluride through the lungs and skin. The odour can last for weeks and is detectable even after microscopic exposures.

Where you find Tellurium

In space

Tellurium is produced in stellar explosions (supernovae) and found in the Sun in tiny amounts.

On Earth

Tellurium is one of the rarest stable elements in the Earth's crust, only approx. 0.001 parts per million.

• Gold-telluride ores. Tellurium occurs in gold and silver telluride minerals such as calaverite and sylvanite. These were important gold ores in Transylvania and in Colorado.
• By-product of copper refining. Almost all commercial tellurium is recovered from the anode sludge produced during electrolytic copper refining. Canada, Japan, the USA and Peru are significant producers.

How we use Tellurium

• CdTe solar cells.. Cadmium telluride thin-film solar panels are the second most widely deployed solar technology. Each panel contains a thin layer of cadmium telluride that absorbs sunlight.
• Thermoelectric devices.. Bismuth telluride thermoelectric generators convert heat directly to electricity and are used in spacecraft power systems, portable coolers and waste heat recovery.
• Metallurgy.. Adding small amounts of tellurium to steel and copper improves machinability.
• Rewritable optical discs.. Phase-change materials including tellurium alloys were used in rewritable CDs and DVDs.

Did you know? Tellurium is actually rarer than gold in the Earth's crust, yet it costs far less, because gold concentrates in accessible placer deposits while tellurium disperses evenly through the deep crust. The growing demand for CdTe solar cells is making tellurium supply an increasing concern for the renewable energy industry.

How it was discovered

Franz-Joseph Müller von Reichenstein identified a new element in gold ore from Zlatna (Transylvania, now Romania) in 1782 but could not characterise it fully. Martin Klaproth received samples in 1796, confirmed the new element and named it tellurium from the Latin for Earth in 1798.

Deeper dive: tellurium chemistry and applications

Bismuth telluride (Bi₂Te₃) thermoelectric materials are the most efficient at converting small temperature differences to electricity near room temperature. In a thermoelectric generator, heat flowing across a Bi₂Te₃ module drives charge carriers from the hot side to the cold side, creating a voltage. These devices are solid-state, have no moving parts and run quietly and reliably. The RTGs (Radioisotope Thermoelectric Generators) on NASA spacecraft like Voyager, Cassini and New Horizons use plutonium-238 heat and thermoelectric modules (not Bi₂Te₃, which works best near room temperature, but silicon-germanium at higher temperatures) to power instruments far from the Sun.

Moving to 53 protons on the periodic table brings us to Iodine.