Indium – facts and properties

Position in the periodic table

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Occurrence and extraction

Indium is about as rare in the Earth's crust as silver. It forms no sizeable deposits of its own but occurs in traces in zinc ores, especially sphalerite, and also in lead, copper and tin ores. It is obtained as a by-product of zinc production, with China the leading producer. More indium is recovered from old displays and production waste. Because the amounts are small and demand is growing, indium counts as a critical raw material.

Its discovery is a classic example of spectral analysis. In 1863 the Freiberg chemists Ferdinand Reich and Hieronymus Theodor Richter were looking for the element thallium in a local sphalerite. Instead they saw a previously unknown indigo-blue line in the spectrum. Because Reich was colour-blind, Richter observed the spectrum. Together they then prepared the new metal and named it after the Latin indicum, indigo.

Properties and reactions

Indium is in period 5 and group 13, below gallium. Its electron configuration is [Kr] 4d¹⁰ 5s² 5p¹ and its electronegativity is 1.78. The metal has a density of 7.31 g/cm³, melts at 156.6 °C and boils at 2072 °C. The melting point is low, but the boiling point is very high. The element has 3 valence electrons, namely the two 5s and the one 5p electron.

Indium is so soft that it can be scratched with a fingernail. It deforms easily and stays ductile even at low temperatures. As with tin, bending it produces a creaking sound, the tin cry. It arises when, during bending, regions in the crystals suddenly flip into a mirror-image orientation (twinning).

Chemically indium resembles aluminium and gallium. In air it is covered by a thin oxide layer and is fairly stable. In compounds it usually has the oxidation number plus three, less often plus one. It reacts with acids to give hydrogen. Indium oxide is a yellowish solid that conducts electricity when mixed with a little tin oxide.

Uses in everyday life and technology

The most important use is indium tin oxide, ITO for short. It is a mixture of indium oxide and tin oxide that is transparent in a thin layer and yet conducts electricity. This combination is rare. That is why ITO serves as the electrode in touchscreens, flat-panel displays, OLED displays and solar cells. More than half of all indium is used for it.

Indium is also used for solders and seals. It sticks to glass, melts at a low temperature and stays soft. In semiconductors it forms compounds with phosphorus or arsenic, such as indium phosphide, found in fast electronic components, light-emitting diodes and lasers. Thin-film solar cells contain copper, indium, gallium and selenium.

Indium has a special use at low temperatures: it seals vacuum equipment because it stays soft even in severe cold. Some indium compounds are used in medicine as radioactive tracers. Metallic indium in small amounts is hardly toxic. In industry, however, dust of indium compounds should not be breathed in.

Quick question

What is the atomic number of indium (In)?

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