Indium is the chemical element with symbol In and atomic number 49. It is a very soft, silvery metal in group 13. Indium is technologically important because indium tin oxide combines electrical conductivity with optical transparency, making it valuable in displays, touchscreens and photovoltaic devices.
Indium quick facts
| Property | Value |
| Symbol | In |
| Atomic number | 49 |
| Relative atomic mass | 114.818 |
| Group | 13 |
| Period | 5 |
| Block | p-block |
| Density | 7.31 g/cm³ |
| Melting point | 156.60°C |
| Boiling point | 2027°C |
| Electron configuration | [Kr] 4d¹⁰ 5s² 5p¹ |
| Common oxidation state | +3 |
| CAS number | 7440-74-6 |
| Key natural isotope | ¹¹⁵In |
Position and chemistry
Indium is in group 13, period 5 and the p-block. Its electron configuration is [Kr] 4d¹⁰ 5s² 5p¹. The +3 oxidation state is most common, although +1 compounds are also known.
See the complete CleverlySmart periodic table.
Physical properties
Indium is unusually soft: it can be cut with a knife and deformed easily. It melts at only 156.6°C and wets glass readily. When a bar of indium is bent it can emit a characteristic crackling sound known as the indium cry, caused by deformation of its crystal structure.
Important indium compounds
- Indium tin oxide (ITO): a transparent conductive oxide used in touchscreens, flat-panel displays, solar cells and heated windows.
- Indium phosphide (InP): a semiconductor used in high-speed electronics, photonics and telecommunications lasers.
- Indium arsenide (InAs) and indium antimonide (InSb): narrow-band-gap semiconductors useful in infrared detectors and high-speed devices.
- Indium oxide (In₂O₃): a conductive oxide and precursor to ITO.
Occurrence and production
Indium rarely occurs in concentrated minerals. Most commercial indium is recovered as a by-product of zinc refining, especially from sphalerite ores. Its supply therefore depends partly on zinc production and on the efficiency of recovering indium from processing residues.
Uses of indium
- Displays and touchscreens: ITO forms transparent electrodes.
- Solar cells: indium-containing materials are used in thin-film photovoltaic technologies such as CIGS.
- Semiconductors: InP, InGaAs and related compounds are essential in fibre-optic communications and high-speed electronics.
- Low-melting alloys: indium can lower melting temperatures in solders and fusible alloys.
- Coatings and seals: soft indium can form vacuum seals and specialised low-temperature joints.
Indium isotopes
Natural indium consists mainly of ¹¹⁵In, which is very weakly radioactive with an extremely long half-life, plus stable ¹¹³In. The radioisotope indium-111 is used in nuclear medicine for specialised diagnostic imaging and cell-labelling applications.
For wider isotope information, see the CleverlySmart isotope table.
Biological role and safety
Indium has no known essential biological role. Toxicity depends on the chemical form. Occupational inhalation of indium compounds used in electronics manufacturing can cause serious lung disease, so dust and process emissions require effective controls.
Discovery and name
German chemists Ferdinand Reich and Hieronymus Theodor Richter discovered indium spectroscopically in 1863. The element was named for the bright indigo spectral line that revealed its presence.
Frequently asked questions
Why is indium used in touchscreens?
Indium tin oxide can conduct electricity while remaining transparent, which is ideal for electrodes placed over a visible display.
Is indium radioactive?
Most natural indium is indium-115, which is technically radioactive but decays so slowly that its half-life is vastly longer than human timescales.
Deeper look at indium: physical properties and crystal structure
Indium is a very soft, silvery metal with a density of about 7.31 g/cm³. It melts at only about 156.6 °C but boils near 2,072 °C. Solid indium has a body-centred tetragonal crystal structure and can be cut easily with a knife. When bent, pure indium may produce a characteristic high-pitched “cry” as its crystal grains deform.
Why indium is important in modern electronics
The most familiar indium material is indium tin oxide (ITO), a transparent electrical conductor. Thin ITO coatings are used on touchscreens, flat-panel displays, solar cells and other devices that need a surface to conduct electricity while remaining transparent to visible light.
Indium also forms technologically important semiconductors. Indium phosphide (InP) is used in high-speed electronics, laser diodes and fibre-optic communications, while indium-containing compound semiconductors such as InGaAs are important in infrared detectors and specialised electronics.
Low-melting alloys and solders
Because indium is soft and has a low melting point, it is used in fusible alloys, specialist solders, thermal-interface materials and seals. Indium can wet glass and some ceramics, which makes it useful where conventional solders do not bond well.
Supply and recycling
Indium is not usually mined as a primary ore. Most commercial indium is recovered as a by-product of zinc processing. This makes supply dependent on the production of other metals. Recycling of display-manufacturing scrap and other indium-bearing materials is therefore increasingly important.
Isotopes and nuclear properties
Natural indium consists mainly of ¹¹⁵In, an isotope with an extremely long half-life, plus stable ¹¹³In. The unusual dominance of a very long-lived radioactive isotope is one of indium’s distinctive nuclear characteristics.
Frequently asked questions
Why is indium used in touchscreens?
ITO combines electrical conductivity with high transparency, allowing a display to detect electrical signals without blocking the image underneath.
Is indium rare?
Indium is not among the rarest elements in the crust, but economically concentrated deposits are uncommon. Most supply comes from by-product recovery rather than dedicated indium mines.
