Atomic number 49

Indium

In
+ phase
− phase

Shells Orbitals 4d 5s 5p

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

49 electrons in 5 shells: 2, 8, 18, 18, 3.

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

Category: Post-transition metal
Period: 5
Group: 13
Block: p
Standard atomic weight: 114.82(1) Da
Electron configuration: [Kr] 4d¹⁰ 5s² 5p¹
Phase at 25 °C: solid
Melting point: 156.6 °C
Boiling point: 2072 °C
Density: 7.31 g/cm³
Electronegativity (Pauling): 1.78
Discovered: 1863, Ferdinand Reich and Hieronymus Theodor Richter

Neighbours in the table

Cd Sn

Indium takes its name from a single stripe of colour. In 1863 Ferdinand Reich and Hieronymus Theodor Richter, both German, were searching zinc ore for traces of thallium. In that work the spectrum of their sample showed a brilliant indigo line, and that line announced an element nobody had isolated. CIAAW, IUPAC’s atomic-weights commission, records that the pair isolated the metal in 1867.

The metal they obtained is silvery-white and very soft, softer than lead in the NIOSH description. Bent as a pure bar, it gives out a high-pitched cry, and it wets glass, Los Alamos notes. Like other metals, indium is a lattice of positive ions in a mobile fluid of electrons. That bonding has no preferred direction, so the metal bends rather than cracks.

Indium’s shell counts, 2, 8, 18, 18 and 3, cover 49 electrons; its shorthand is [Kr] 4d¹⁰ 5s² 5p¹. Removing that lone 5p electron takes 5.786 eV, its first ionisation energy. Each group 13 element has a lower first ionisation energy than the element before it. The drop below cadmium is often attributed to a single p electron being easier to remove than electrons in filled s and d orbitals. Its usual oxidation state is +3, a bookkeeping charge as if all bonds were ionic, and it forms both In³⁺ and In⁺ ions.

Where it is found

Indium is thin in rock, 0.25 milligrams per kilogram of crust by Jefferson Lab’s estimate, and 2 × 10⁻² milligrams per litre of seawater. Los Alamos and Jefferson Lab put it about as abundant as silver. It turns up alongside zinc, iron, lead and copper in their ores, which Jefferson Lab says makes it easier to recover.

The US Geological Survey (USGS) names sphalerite, the zinc sulfide mineral, as its usual host. The zinc deposits worked for it hold from under 1 to 100 parts per million. Traces also occur in chalcopyrite and stannite, though recovering it from most of those deposits has not paid.

China refined 760 of the world’s estimated 1,080 metric tons in 2024, 70 per cent of the total, with South Korea at 180 and Japan at 60. The United States recovered none from ores that year. Recycled indium comes mostly from scrap indium tin oxide (ITO), the conducting film on flat-panel displays, and Japan and South Korea are where it is most commonly recovered. Until 1924, Los Alamos records, the world’s supply of the isolated metal was about a gram.

Why it matters

Most of the indium used worldwide goes into indium tin oxide, ITO, according to the USGS. Laid down as thin films, ITO conducts electricity across many kinds of flat-panel display, most often liquid crystal displays. American consumption reached an estimated 250 tons in 2024, worth about $85 million, and all of it was imported, since the country refined none.

Indium phosphide, InP, carries a newer demand. Lasers and receivers made from it send data along the optical fibres of fifth-generation (5G) networks, for lower latency (delay), less signal loss and higher speeds, the USGS says. ITO coats data-centre fibres and cables to cut losses, and indium-based solders join some of their electrical parts.

Older uses rest on the metal itself. Jefferson Lab describes indium coating the bearings of high-speed motors because it spreads lubricating oil evenly, and adding it to germanium for transistors. Plated onto metal or evaporated onto glass, it forms a mirror that Los Alamos rates as good as silver’s and slower to corrode in air. Galinstan, which blends gallium, indium and tin, has replaced mercury in traditional thermometers, the USGS reports.

Indium-111, whose half-life is 2.8048 days in the Atomic Mass Data Center’s table, labels white blood cells, according to IUPAC. Those cells gather at sites of infection, so the isotope helps doctors find where one lies.

Properties

Properties of indium 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	49
Standard atomic weight	114.82(1) Da	Da	IUPAC/CIAAW 2024
Melting point	156.6 °C	°C	Los Alamos: 156.6 °C
Boiling point	2072 °C	°C	Los Alamos: 2072 °C
Density	7.31 g/cm³	g/cm³
Electronegativity (Pauling)	1.78
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p¹
Phase at 25 °C	solid

Safety

Elemental indium carries the signal word Danger in company notifications to the European Chemicals Agency. Of the reports that give hazard statements, 63.3% say it damages organs through prolonged or repeated exposure. A solid piece shows no rapid reaction with air or water, and Los Alamos reports a low order of toxicity.

The concern is dust breathed at work: Haz-Map cites interstitial lung disease in indium-processing workers. NIOSH lists the eyes, skin, respiratory system, liver, kidneys, heart and blood as organs the metal can affect. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide card for indium set out the hazards for anyone working with indium dust.

Questions

Why is it called indium?

The name comes from indigo. CIAAW, IUPAC's commission on atomic weights, traces it to the indigo-blue line that the element gives in its spark spectrum. That coloured line was the first sign of the metal in 1863, when Reich and Richter were studying zinc blende. CIAAW dates their isolation of the element itself to 1867.

Is indium radioactive?

Very slightly. Natural indium is 95.719 per cent indium-115 by CIAAW's abundance figures, and the IAEA's nuclear data give that isotope a half-life of 441 trillion years, decaying entirely by beta-minus emission. Indium-113, the other 4.281 per cent, is stable, which makes it indium's only stable isotope.

Is indium safe to touch?

Touching a solid piece is a small concern: the metal shows no rapid reaction with air or water, according to CAMEO, though 15.4% of company reports to the European Chemicals Agency list skin irritation. Most reports that give hazard statements carry the signal word Danger for organ damage from repeated exposure, while Japan's 2016 classification is Not Classified. The real risk is dust breathed over time in industrial work.

Why did indium prices rise in 2024?

The US Geological Survey puts the average American warehouse price for 2024 at $340 a kilogram, 42 per cent above the 2023 average, with a peak of $420 in June. American prices tracked increases on China's Changzhou ZonglianJin trading platform. China refined 760 of the world's 1,080 tons that year, and the United States recovered none from its own ores.