[13] It was discovered in 1863 by Ferdinand Reich and Hieronymous Theodor Richter by spectroscopic methods and named for the indigo blue line in its spectrum.

[15][16][17] Indium is also used in the semiconductor industry,[18] in low-melting-point metal alloys such as solders and soft-metal high-vacuum seals.

[19] Indium has no biological role and its compounds are toxic when inhaled or injected into the bloodstream, although they are poorly absorbed following ingestion.

[22] The word indicum means "Indian", as the naturally based dye indigo was originally exported to Europe from India.

[23] It is so soft (Mohs hardness 1.2) that it can be cut with a knife and leaves a visible line like a pencil when rubbed on paper.

[24] It is a member of group 13 on the periodic table and its properties are mostly intermediate between its vertical neighbors gallium and thallium.

As with tin, a high-pitched cry is heard when indium is bent – a crackling sound due to crystal twinning.

However it does have a size effect in bending and indentation, associated to a length-scale of order 50–100 μm,[29] significantly large when compared with other metals.

It does not form a boride, silicide, or carbide, and the hydride InH3 has at best a transitory existence in ethereal solutions at low temperatures, being unstable enough to spontaneously polymerize without coordination.

Indium is one of three known elements (the others being tellurium and rhenium) of which the stable isotope is less abundant in nature than the long-lived primordial radioisotopes.

Many of them slowly decompose in moist air, necessitating careful storage of semiconductor compounds to prevent contact with the atmosphere.

It was the first known organoindium(I) compound,[47] and is polymeric, consisting of zigzag chains of alternating indium atoms and cyclopentadienyl complexes.

[49][50] In 1863, German chemists Ferdinand Reich and Hieronymus Theodor Richter were testing ores from the mines around Freiberg, Saxony.

They dissolved the minerals pyrite, arsenopyrite, galena and sphalerite in hydrochloric acid and distilled raw zinc chloride.

They named the element indium, from the indigo color seen in its spectrum, after the Latin indicum, meaning 'of India'.

[56] The stable indium isotope, indium-113, is one of the p-nuclei, the origin of which is not fully understood; although indium-113 is known to be made directly in the s- and r-processes (rapid neutron capture), and also as the daughter of very long-lived cadmium-113, which has a half-life of about eight quadrillion years, this cannot account for all indium-113.

This is an aspect that is often forgotten in the current debate, e.g. by the Graedel group at Yale in their criticality assessments,[64] explaining the paradoxically low depletion times some studies cite.

[23][19] Its by-product status means that indium production is constrained by the amount of sulfidic zinc (and copper) ores extracted each year.

The supply potential of a by-product is defined as that amount which is economically extractable from its host materials per year under current market conditions (i.e. technology and price).

[70] China is a leading producer of indium (290 tonnes in 2016), followed by South Korea (195 t), Japan (70 t) and Canada (65 t).

Demand rose rapidly from the late 1990s to 2010 with the popularity of LCD computer monitors and television sets, which now account for 50% of indium consumption.

[72] The first large-scale application for indium was coating bearings in high-performance aircraft engines during World War II, to protect against damage and corrosion; this is no longer a major use of the element.

[79] Owing to its great plasticity and adhesion to metals, Indium sheets are sometimes used for cold-soldering in microwave circuits and waveguide joints, where direct soldering is complicated.

Indium is an ingredient in the gallium–indium–tin alloy galinstan, which is liquid at room temperature and replaces mercury in some thermometers.

[80] Other alloys of indium with bismuth, cadmium, lead, and tin, which have higher but still low melting points (between 50 and 100 °C), are used in fire sprinkler systems and heat regulators.

[67] Indium is one of many substitutes for mercury in alkaline batteries to prevent the zinc from corroding and releasing hydrogen gas.

[85] Radioactive indium-111 (in very small amounts) is used in nuclear medicine tests, as a radiotracer to follow the movement of labeled proteins and white blood cells to diagnose different types of infection.

In a similar way to aluminium salts, indium(III) ions can be toxic to the kidney when given by injection.

[20] The National Institute for Occupational Safety and Health has set a recommended exposure limit (REL) of 0.1 mg/m3 over an eight-hour workday.