Tellurium – facts and properties

Position in the periodic table

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Occurrence and extraction

Tellurium is about as rare in the Earth's crust as platinum. It sometimes occurs native, but mostly in tellurides, which are compounds with metals such as gold, silver or lead. One of these minerals is calaverite, a gold telluride.

The discovery was made in 1782 by the mining official Franz Joseph Müller von Reichenstein. He examined a gold ore from Transylvania that did not behave as expected and found a new element in it. The Berlin chemist Martin Heinrich Klaproth confirmed the find in 1798 and named it tellurium, after the Latin word tellus for Earth.

Today tellurium is obtained almost only as a by-product of copper refining. When copper is purified by electrolysis, an anode slime is left at the bottom, and tellurium is extracted from it. Only several hundred tonnes are produced each year, and demand grows with solar technology, so the United States, for example, lists tellurium as a critical mineral.

Properties and reactions

Tellurium is in period 5 and group 16, below selenium and above polonium. In the period, antimony lies to its left and iodine to its right. Its electron configuration is [Kr] 4d¹⁰ 5s² 5p⁴ and its electronegativity is 2.1. The element has 6 valence electrons. The density is 6.232 g/cm³, the melting point 449.51 °C and the boiling point 988 °C.

Tellurium is a metalloid. It looks metallic but is brittle and can be ground to a powder. It conducts electricity poorly, but better when heated or lit, which is why it counts as a semiconductor. It is fairly stable in air. In a flame it burns with a greenish-blue colour to white tellurium dioxide, TeO₂. In compounds the oxidation numbers minus two, plus four and plus six occur. The hydrogen compound H₂Te is unstable, decomposes easily and smells unpleasant.

Tellurium and its compounds are weakly to moderately toxic. Their effect on the breath is well known: the body converts absorbed tellurium into dimethyl telluride, a volatile compound with a penetrating garlic smell. The smell can last for weeks and is unpleasant for those affected, but above all it is a warning sign that tellurium has been absorbed. Tellurium compounds do not belong in school experiments.

Uses in everyday life and technology

The most important use is solar cells made of cadmium telluride, CdTe. They are thin-film cells: a semiconductor layer only a few micrometres thick is coated onto glass and turns sunlight into electricity. After silicon cells, this is the second most common type, though far behind. Because cadmium is toxic, recycling the modules is especially important.

Bismuth telluride, Bi₂Te₃, is a thermoelectric material. When current flows through it, one side becomes cold and the other warm. This is how Peltier elements work, for example in small cool boxes and in devices that regulate temperature. The effect can also be reversed: a temperature difference produces electricity.

Compounds of germanium, antimony and tellurium switch between an ordered and a disordered form under laser light or electrical pulses. This is used in rewritable optical discs and in phase-change memory.

Small amounts of tellurium make steel and copper easier to machine, harden lead and colour glass and ceramics. In the rubber industry it is used as an additive in vulcanisation.

Quick question

What is the atomic number of tellurium (Te)?

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