- Indium (In) is a post-transition metal with atomic number 49, located in Group 13 of the periodic table alongside boron, aluminum, gallium, and thallium.
- It is a soft, malleable, silvery-white metal with a slightly bluish hue and a low melting point relative to most metals.
- Indium has three valence electrons—two in the 5s subshell and one in the 5p subshell—and it typically exhibits the +3 oxidation state, although the +1 state is also possible in some compounds. Its atomic structure consists of forty-nine protons, typically sixty-six neutrons, and forty-nine electrons arranged in five shells.
- Naturally occurring indium consists of two isotopes: indium-113 (¹¹³In), which is stable, and indium-115 (¹¹⁵In), which is weakly radioactive with an extremely long half-life of about 4.41×10¹⁴ years; ¹¹⁵In is the more abundant isotope, making up roughly 95.7% of natural indium.
- Indium is a relatively rare element, with an average abundance of about 0.05 parts per million in the Earth’s crust. It is never found in pure form but occurs as a trace component in zinc, lead, copper, and tin ores, most notably in sphalerite (ZnS). Commercial production of indium is almost entirely as a byproduct of zinc refining. Major producing countries include China, South Korea, Japan, and Canada.
- Indium was discovered in 1863 by German chemists Ferdinand Reich and Hieronymous Theodor Richter at the Freiberg School of Mines. They detected a brilliant indigo-blue spectral line while analyzing zinc ore samples, leading to the naming of the element from the Latin indicium, meaning “violet” or “indigo.” The metal was first isolated later that year by Richter.
- In modern applications, indium is most famous for its use in indium tin oxide (ITO), a transparent, conductive material essential for touchscreens, flat-panel displays, liquid crystal displays (LCDs), and photovoltaic cells. Indium is also used in solders and alloys, where it improves wetting properties and allows for low-temperature bonding, and in specialized coatings to prevent corrosion. Indium’s ability to wet glass, ceramics, and metals makes it valuable in sealing high-vacuum systems and cryogenic devices.
- Chemically, indium is relatively reactive for a post-transition metal. It forms compounds such as indium(III) oxide (In₂O₃), indium(III) chloride (InCl₃), and indium phosphide (InP), the latter being an important semiconductor in high-speed and optoelectronic devices. Indium’s chemistry closely resembles that of gallium, although it is less reactive and more metallic in character.
- Biologically, indium has no known essential function in living organisms. While metallic indium is considered to have low toxicity, some indium compounds—particularly indium phosphide and indium tin oxide—are toxic and potentially carcinogenic, especially through inhalation in occupational settings. Proper safety protocols are essential when handling fine indium powders or compounds.
- From an environmental perspective, indium mining and refining have relatively small global footprints compared to base metals, but improper disposal of indium-containing electronics can lead to localized environmental contamination. Recycling from end-of-life displays, solar panels, and electronics is increasingly important to secure supply and reduce waste, as indium is classified as a critical raw material due to its rarity and strategic importance.
