INDIUM

Name: Indium (from the coloration of the blue dye "Indigo")

Symbol: In

Group: 13

Period: 5

Block: p

Category: Post-transition metals

Atomic number: 51

Atomic mass: 114.818 u

Electrons per shell: 2, 8, 18, 18, 3

Electronegativity: 1.78

Density: 7.31 g/cc

Melting point: 156.6°C

Boiling point: 2072°C

Thermal conductivity: 82 W (m K)

Electrical conductivity: 1.2 × 10^7 S/m

Magnetic order: Diamagnetic

Ordinary state: Solid

Oxidation states: +3

Hardness Mohs: 1.2

Vickers Hardness: No data

Brinell Hardness: 8.83 MPa

Most stable isotopes: In-113 (4.28%), In-114 (95.72% - unstable)

Discoverer(s): Friedrich Rich and Hieronymus Richter, Germans (1863)

Indium (In), a chemical element with atomic number 49, is a post-transition metal of group 13, known for its softness, low melting point (156.6 °C), and the formation of eutectic alloys, some of which are liquid at room temperature, such as galinstan (with gallium, Ga, and tin, Sn). With an abundance of 0.05 ppm in the Earth's crust, indium is extremely rare, which drives up its cost (300–500 USD/kg in 2025) and distinguishes it from more common metals like tin (Sn) or zinc (Zn). Despite its proximity in the periodic table to cadmium (Cd), indium does not share its significant toxicity nor play a known biological role, making it safer for industrial applications. Its name, derived from the "indigo" blue color (from Latin indicium), reflects the bluish spectral lines observed during its discovery, and has no relation to the Asian subcontinent, despite possible confusion.

Indium was discovered in 1863 by German chemists Ferdinand Reich and Hieronymus Richter, who worked together at the University of Freiberg. While investigating zinc ores in search of thallium (Tl), Reich and Richter used flame spectroscopy to analyze samples and detected a new element that emitted a distinctive blue spectrum. Richter, who named the element "indium" due to its similarity to the indigo color, played a key role in the identification, as Reich was colorblind and could not distinguish the colors of the spectrum. This discovery, published in 1863, marked indium as one of the first elements identified using spectroscopy, a pioneering technique in 19th-century chemistry.

Initially, indium was a scientific curiosity due to its scarcity and lack of practical applications. It wasn't until the 20th century, with the rise of electronics, that its importance grew. In the 1940s, indium began to be used in alloys and coatings, leveraging its ability to "wet" glass and form intermetallic compounds. In the 1970s, the development of indium tin oxide (ITO) revolutionized the display and solar panel industry, while compounds like indium phosphide (InP) became essential in semiconductors. Today, global production (~850 tons annually in 2025), mainly as a byproduct of zinc mining in countries like China and Canada, reflects its growing demand. The history of indium, from its spectroscopic discovery to its critical role in modern technology, underscores its transformation from a rare element to an indispensable material in electronics and energy, despite its scarcity and high cost.

Indium (In), a chemical element with atomic number 49, is a post-transition metal of group 13, known for its softness, attractive luster, and specialized applications in electronics, alloys, and coatings. With a density of 7.31 g/cm³ and an extremely low abundance of 0.05 ppm in the Earth's crust, indium is one of the rarest metals, which contributes to its high cost (300–500 USD/kg in 2025). Its appearance, with a silver luster similar to that of platinum group metals (such as platinum, Pt, or palladium, Pd), makes it aesthetically attractive, although this luster can dull over time if not adequately protected. Its rarity and unique properties make it a valuable material, though less known than other metals.

Physically, indium is very soft (Mohs hardness of 1.5), comparable to talc, allowing it to be easily cut with a knife. It is ductile and malleable, facilitating its shaping into sheets or wires, and has a low melting point (156.6 °C), one of the lowest among metals, only surpassed by elements like gallium (Ga, 29.76 °C) or mercury (Hg). Its boiling point is 2,072 °C, and its electrical conductivity (1.2 × 10⁷ S/m) is good, though lower than copper (Cu). The thermal conductivity (82 W/(m·K)) makes it suitable for thermal applications. Indium forms low-melting-point alloys with metals such as lead (Pb), tin (Sn), bismuth (Bi), and gallium, some of which, like the gallium-indium-tin alloy (galinstan), are liquid at room temperature (~20 °C). These eutectic alloys are ideal for soldering and low-temperature applications.

Chemically, indium is moderately reactive but stable in dry air at room temperature, forming a thin layer of indium oxide (In₂O₃) that partially protects the metal from further oxidation by oxygen (O₂). It does not react with carbon (C), nitrogen (N₂), or silicon (Si), and therefore does not form stable carbides, nitrides, or silicides, a characteristic that distinguishes it from other transition metals. Its ability to "wet" glass, forming an intermetallic compound in a liquid state, makes it valuable as a sealant in glass parts, especially in electronics and optics. Indium is soluble in metals like lead, tin, bismuth, and gallium, but does not mix easily with transition metals like iron (Fe). Although it has been used in jewelry for its luster and malleability, its use is limited due to its cost and scarcity. In electronics, indium is crucial in compounds like indium tin oxide (ITO), used in touchscreens and solar panels, and in semiconductors like indium phosphide (InP). Despite its moderate toxicity, which is lower than that of cadmium (Cd), it requires careful handling. The combination of its rarity, physical and chemical properties, and versatility makes it a critical material in modern technologies, although its scarcity limits its mass adoption.

Indium's (In) corrosion resistance is a key characteristic of this post-transition metal from group 13. Indium is known for its softness (Mohs hardness of 1.5), low melting point (156.6 °C), and a bright silver luster that rivals that of noble metals. With a density of 7.31 g/cm³ and an abundance of 0.05 ppm in the Earth's crust, indium is extremely rare, which drives up its cost (300–500 USD/kg in 2025). Its corrosion resistance is comparable to that of tin (Sn), offering stability in moderate environments, but showing vulnerability to aggressive chemical agents. This combination of properties makes it useful in applications such as protective coatings and electronics, although its behavior against corrosion requires specific considerations.

Under standard conditions, indium is stable in dry and humid air, forming a thin layer of indium oxide (In₂O₃) that acts as a passivating barrier, protecting the metal from further oxidation by oxygen (O₂). In fresh water, indium effectively resists corrosion, maintaining its characteristic luster, although in saltwater (with sodium chloride, NaCl) it can degrade slowly due to the formation of indium chloride (InCl₃). However, indium is vulnerable to acids. Reducing acids, such as hydrochloric acid (HCl), react slowly, releasing hydrogen (H₂) and forming compounds like InCl₃. In contrast, oxidizing acids, such as nitric acid (HNO₃), cause a rapid reaction, dissolving the metal to form indium nitrate (In(NO₃)₃). Halogens, such as chlorine (Cl₂) or iodine (I₂), attack indium at any temperature, while strong bases, such as sodium hydroxide (NaOH), corrode it when hot, generating hydroxo-complexes like In(OH)₃.

Despite its reactivity in aggressive environments, indium is used as a coating in applications where its silver luster and reflective capacity are valued, such as in mirrors or electronic components. This layer, applied by electrodeposition, protects substrates like steel, although its luster can dull over time under unfavorable conditions, such as prolonged exposure to humidity or acids. Unlike cadmium (Cd), indium presents low toxicity, with no significant risks to human health or the environment, allowing for safer handling in industrial processes. Its relative stability in air and water, combined with its ability to "wet" glass (forming intermetallic compounds), makes it ideal for sealants in optics and electronics, such as in indium tin oxide (ITO) for touchscreens. Indium's moderate corrosion resistance, together with its aesthetic appeal and safety, positions it as a valuable material in modern technologies, although its scarcity and cost limit its widespread adoption.

Indium is the fourth metal on the scale of lowest melting points, after Mercury, Gallium, and Sodium (although Sodium is an alkaline metal with no structural use). It is mainly used as a component of low-melting-point eutectic alloys (e.g., Field's Metal). There are other slightly inferior but cheaper alloys such as Rose's Metal, Wood's Metal, etc. It should be noted that Field's Metal contains indium, a very expensive metal compared to typical ingredients (lead, tin, cadmium, bismuth, etc.).

As an alternative to metallic mercury in thermometers, the German alloy Galinstan was developed. Its exact chemical composition is not publicly known, although it is known to consist of a combination of Gallium, Indium, and Tin (hence the name: Gal-Gallium, In-Indium, Stan-Stannum, the Latin names of the metals, remember: stannum: tin). This alloy is superior to NaK (sodium-potassium alloy) because it is not as reactive. However, the presence of Gallium in the mixture makes it a very "sticky" liquid alloy. Just as mercury "absorbs" other metals, Galinstan "absorbs" metals such as Aluminum, Silver, etc., with the added disadvantage that it "wets" glass, which mercury does not.

Indium can "wet" glass, but only in a liquid state. It can be used as an alloying agent in some special grades of pewter (tin-based alloys) to increase corrosion resistance and fusibility, etc. Whether in its pure state or alloyed, it is an excellent soldering metal, especially for sealing vacuum parts, etc. Due to its ability to "wet" glass, it can be used in a liquid state to seal structural breaks or simply seal the piece in question. Alloyed with bismuth, it confers a certain ductility. Due to its compatibility with silver, it can be used as an alternative to traditional sterling silver compositions (92.5% silver, 7.5% copper) to increase its resistance to sulfation.