Indium

Indium
Atomic number	49
Symbol	In
Group	13 (boron group)
Boiling point	2072 °C
Electron configuration	[Kr] 4d10 5s2 5p1
Density	7.31 g/cm^3
Period	5
Discovery	1863 (F. Reich, T. Richter)
Melting point	156.6 °C
Block	p
Oxidation states	+1, +2, +3
Wikidata	Q1094

Indium (In) is a rare, post-transition metal in group 13 of the periodic table (the boron group). It has atomic number 49 and an atomic weight around 114.82. A silvery-white solid at room temperature, indium is extremely soft (softer than tin) and very malleable – it can be cut with a knife and will readily dent under a fingernail. The element melts at only 156.6 °C and boils around 2072 °C under standard pressure. In chemical compounds it most commonly adopts oxidation state +3, although a +1 state also occurs; it shows no negative oxidation states in stable compounds. At standard conditions indium is a solid metal. It is classified as a post-transition or “poor” metal because it lies to the right of the transition metals in the periodic table and has relatively low melting point and lower electrical conductivity than the true transition metals.

Indium is quite scarce: it makes up only about 0.2 parts per million of the Earth’s crust. It is never found as a free element in nature but occurs in trace amounts combined in zinc, tin, copper and lead ores (especially in sphalerite, a zinc sulfide). Industrial indium is obtained as a byproduct of zinc refining. Historically discovered by spectroscopy in 1863, indium today is best known for high-technology applications. For example, its oxide with tin (indium tin oxide or ITO) is an opaque coating form that becomes transparent and electrically conductive once deposited as a thin film. ITO is used in nearly every flat-panel display and touch screen. Indium’s softness and unique low-melting alloys also make it useful in specialized solders and vacuum seals. The element’s unusual combination of electronic and physical properties places it firmly in modern technology markets.

Atomic Structure and Electron Configuration

The indium atom has 49 electrons. Its electron configuration is [Kr] 4d^10 5s^2 5p^1, meaning the inner shells match the noble gas krypton and a filled 4d^10 subshell, with three electrons in the outer (5s and 5p) shell. These three outer electrons (two 5s electrons and one 5p electron) are the valence electrons that participate in bonding. Because it sits in the 5th period and p-block of the periodic table, indium’s valence shell is n=5. In compounds, indium typically loses all three valence electrons to form In³⁺ (a 5s^0 5p^0 core). Under milder conditions it may lose only the 5p^1 electron, giving the +1 state (In⁺) while retaining the 5s^2 “inert pair.”

Moving down group 13, atomic size grows and ionization energy falls. Indium’s atomic radius is on the order of 155 pm (empirical) or about 142 pm (covalent radius), significantly larger than its lighter congeners gallium and aluminum. The element’s Pauling electronegativity is about 1.78 – lower than gallium’s ~1.81 – reflecting its more metallic character. Its first ionization energy is about 559 kJ/mol (5.79 eV), which is lower than that of lighter group-13 elements. In short, indium’s larger size and lower ionization energy make it more willing to donate electrons than aluminum or gallium, consistent with its metallic bonding and chemistry.

Isotopes and Nuclear Properties

Natural indium consists of two isotopes. Roughly 95.7% of indium is ^115In and 4.3% is ^113In. Indium-113 is nonradioactive (“stable”). Indium-115 is technically radioactive but with an extremely long half-life (~4.4×10^14 years) longer than the age of the universe. It decays by β^– emission to tin-115, but its decay is so slow that ^115In is often treated as effectively stable. Both ^113In and ^115In have nuclear spin I=9/2, making them useful in nuclear magnetic resonance (NMR) studies (although their high spin also makes NMR measurements complex). In total, indium has no other naturally occurring isotopes, though many short-lived radioisotopes are known.

Important artificial indium isotopes include ^111In (half-life 2.8 days) and ^114mIn (metastable, 49.5 days). ^111In decays by electron capture and emits two gamma rays (171 and 245 keV); this isotope is widely used in nuclear medicine for diagnostic imaging (for example, labeling white blood cells or radiopharmaceuticals to detect infection or tumors). Another metastable state, ^115mIn (half-life 4.5 h), decays mainly by gamma emission. These isotopes are made in reactors or cyclotrons. Indium does not play a role in geologic or radiometric dating because its natural radioactivity is so feeble and its half-life far too long.

In summary: indium’s stable nuclei are ^113In and ^115In, with ^115In (95.7%) slowly beta-decaying to tin. Other radioisotopes exist only synthetically. The nuclear properties (very long half-life, spin-9/2) mean indium is not a practical chronometer but finds niche uses in science (e.g. NMR and medical imaging).

Allotropes and Typical Compounds

Elemental indium exists only in a single allotrope (no distinct forms like carbon’s graphite/diamond). At room conditions indium metal adopts a body-centered tetragonal crystal structure. Under extreme pressures or temperatures it can transform to other metal structures (like many metals do), but no alternative allotropes are used or isolated at normal conditions.

Indium forms a variety of compounds, most commonly with it in oxidation state +3 (In³⁺). Major inorganic compounds include:

• Oxides and hydroxides: Indium(III) oxide, In₂O₃, is a white solid with a cubic lattice. It is an n-type semiconductor (band gap ~2.9 eV) and forms the basis of indium tin oxide (ITO) when doped with a bit of tin. Indium hydroxide, In(OH)₃, often precipitates from solution of In³⁺; it is amphoteric (reacting with both acids and strong bases). Under alkaline conditions it forms soluble indate ions (e.g. [In(OH)₄]⁻).

• Halides: Indium(III) fluorine and chlorine (InF₃, InCl₃) are white solids, with InCl₃ being a polymeric lattice (similar to AlCl₃). Indium also forms bromide (InBr₃) and iodide (InI₃, bright yellow) analogously. In these trihalides, indium is +3. Indium(I) halides also exist as polymeric solids: for example, InCl (yellow) and InBr (dark) behave like salts of In⁺ and often contain In–In bonds or chains.

• Chalcogenides and pnictides: Indium sulfide (In₂S₃) is used in thin-film solar cells (CuInS₂ and related compounds). Indium phosphide (InP) and indium arsenide (InAs) are important III-V semiconductors for diodes and lasers (bandgaps ~1.35 eV and 0.36 eV respectively). InN (indium nitride) and InSb (small bandgap 0.17 eV) are also known semiconductor compounds.

• Organometallics: Indium forms alkyls and aryls. Trimethylindium (In(CH₃)₃) and triethylindium (In(C₂H₅)₃) are volatile colorless liquids used as metal-organic precursors. These compounds are crucial for metal-organic chemical vapor deposition (MOCVD) of indium-containing semiconductors (e.g. InGaN for LEDs, or InGaAs). These organoindium compounds are typically monomeric (In bonded to three carbon groups).

• Other compounds: Indium can form complexes with nitrate (In(NO₃)₃), sulfate, acetate, etc., usually in the +3 state. Hydrides of indium (like InH₃) are synthetically unstable; any In–H species exist only transiently under extreme conditions. Indium metal also alloys easily with many other metals (e.g. gallium, tin, bismuth, lead) to form low-melting alloys.

In solvents, In³⁺ behaves as a strong Lewis acid, often forming six-coordinate complexes (octahedral). Indium compounds are typically colorless or pale except where charge-transfer or metal-metal bonds occur (for example, In₂O₃ is white, InI₃ is yellow).

Physical Properties

Indium metal is a lustrous, silvery-white metal that is very soft (softer than lead). A small piece of indium feels metallic and cool to the touch. Like many metals, indium is an excellent conductor of heat and electricity, although not as conductive as copper or silver. Its density is about 7.31 g/cm³ at 20 °C (nearly the same as tin). Indium crystallizes in a body-centered tetragonal structure (confirmed by X-ray studies), meaning each atom is at the center of a distorted cube of other atoms.

Indium remains solid until heated to 156.6 °C, above which it liquefies. This low melting point – higher than mercury’s (–39 °C) but lower than tin’s (232 °C) – means liquid indium can exist readily at moderate heat. For comparison, indium melts in the palm of a warm hand (human body ~37 °C) if insulated, but it solidifies at room temperature once cooled. The boiling point of indium is about 2072 °C (around 2345 K). Indium also becomes superconducting when cooled below about 3.4 K (–269.8 °C), with zero electrical resistance experimentally observed.

Its thermal conductivity is roughly 82 W/m·K, which is lower than for copper or aluminum but reasonably high. Indium’s molar specific heat capacity is about 27–28 J/(mol·K) (around 0.23 J/g·K), so it warms up fairly quickly with heat. In spectroscopy, indium atoms produce characteristic emission lines (for example, in flame tests or gas discharges), notably in the violet/blue part of the visible spectrum. The discovery of indium itself was due to a bright indigo (deep-blue) emission line in its spectrum.

Overall, indium’s physical behavior is typical of a heavy metal: it is dense, malleable, and a metal at STP with good—but not exceptional—conductivity. Its unique traits are its softness, low melting point, and the ability to develop a self-passivating oxide film in air.

Chemical Reactivity and Trends

Indium is moderately reactive as a metal. It does not react with water at ordinary temperatures, so it will not corrode quickly in moist air. However, indium does oxidize: a thin transparent film of indium oxide (In₂O₃) forms on the metal surface in air, which usually protects the underlying metal from further rapid corrosion (much like the oxide layer on aluminum or tin).

Indium dissolves in acids. For example, hot concentrated hydrochloric acid will convert indium metal into indium(III) chloride while releasing hydrogen gas: `2 In (s) + 6 HCl (aq) → 2 InCl₃ (aq) + 3 H₂ (g)`. In oxidizing acids (nitric acid, sulfuric acid), indium behaves similarly to other active metals, giving In³⁺ ions in solution (and sometimes oxidizing to In₂O₃ or complex salts). Indium also reacts with halogens: heated indium metal combines with chlorine or bromine to form InCl₃ or InBr₃, white solids.

Under strong alkaline conditions, indium can form soluble indium(III) complexes. For instance, powdered indium dissolves in sodium hydroxide to give a solution of sodium indate, often written Na[In(OH)₄] or similar. In other words, indium exhibits a degree of amphoterism: In₂O₃ dissolves in both acids and bases. (Indium(I) compounds, by contrast, usually precipitate as In₂O or In₂S with +1 ions if over-reduced.)

In the activity series of metals, indium is less reactive than aluminum or zinc but more reactive than lead or tin. It can displace copper from copper salts, for example, by acting as a reducing agent. However, it is below alkali and alkaline-earth metals; it won’t react, say, with cold water or steam. Indium’s reactivity is reduced by that protective oxide, so indium articles are not highly flammable but will burn with difficulty in a strong flame.

Indium(III) compounds are somewhat analogous to aluminum(III). InCl₃ and In(NO₃)₃ in water produce strongly acidic solutions (In³⁺ behaves as a Lewis acid). Complexation is also important: indium forms chloro-complexes (InCl₄⁻, etc.) and chelates (with EDTA or citrate, for instance) when in solution, reflecting its +3 charge. Indium(I) chemistry is more limited: solid InI or InCl exist but easily disproportionate in water to In metal and In³⁺.

Overall, indium’s chemical trends reflect a “heavier aluminum.” It favors the +3 state and forms coordination compounds, oxides, sulfides and halides with typical ionic/covalent bonding patterns. Its inert pair effect is moderate: unlike gallium or aluminum, metallic indium can exhibit the +1 state under some conditions.

Occurrence and Production

Indium is relatively rare in the Earth’s crust (about 0.2 parts per million by mass). It is most commonly found as a trace element in zinc ore (sphalerite, ZnS), and to lesser extent in tin ore (cassiterite, SnO₂) and copper-lead deposits. It never forms a large ore body by itself, so it is economically mined only as a byproduct of others.

Commercial indium is obtained almost entirely as a byproduct of zinc and, to a lesser extent, lead and copper refining. For example, during zinc ore processing the zinc is often leached from the ore in sulfuric acid. Indium ions accumulate in the acidic solution (since InO(OH) and ZnO behave differently in solution) and can be later recovered by adding a base to precipitate indium hydroxide or by organic solvent extraction methods. In another common route, dusts from zinc smelting furnaces, which contain indium-enriched residues, are treated chemically to extract indium. The refined product is usually indium metal, produced by reducing purified indium(III) salts with hydrogen or metallic zinc.

World indium production is modest. In recent years about 800–1000 metric tons of indium are produced yearly worldwide. China is by far the largest producer (on the order of 60–70% of global output), with other contributions from South Korea, Japan, Canada, and several European countries. Because indium supply depends on the fortunes of the zinc industry, it can fluctuate.

Because of its critical role in electronics, recycling of indium from spent products is important. A significant amount of indium is recovered from electronic scrap (for example, reclaiming ITO from discarded LCD panels and solar cells). Research into extracting indium from waste streams and improving recovery processes is ongoing, reflecting the metal’s strategic importance.

Applications and Technology

Indium’s distinctive properties have led to several key applications, especially in electronics and materials science. The most famous application is indium tin oxide (ITO). In₂O₃ doped with a few percent of tin oxide forms a thin, transparent, electrically conducting coating. ITO coatings are used on glass or plastic to make touchscreens, flat-panel displays (LCDs, OLEDs), and solar cells electrically conductive without losing transparency. It is essentially the default transparent electrode material in modern displays and many photovoltaic devices.

Indium plays a vital role in semiconductor technology. Compounds of indium are used to engineer band gaps in semiconductors:

• Indium phosphide (InP) is used for high-speed and optoelectronic devices (lasers and photodetectors for fiber-optic communications).
• Indium gallium arsenide (InGaAs) and indium arsenide (InAs) are used in infrared detectors and high-frequency transistors.
• Indium gallium nitride (InGaN) alloys are the basis of blue and green LEDs and laser diodes (the key materials behind white LED lamps and Blu-ray DVDs).
• Copper indium gallium diselenide (CIGS) thin-film solar cells use indium to achieve high efficiency in sunlight conversion.

Indium’s low melting point and ductility are exploited in specialty alloys. For example, indium alloys with bismuth and tin to produce fusible alloys that melt at low temperatures (often below 100 °C) for use in automatic fire sprinklers and safety devices. Indium–tin and indium–bismuth tarn-free solders are used where precise low-temperature bonding is needed (e.g. cryogenic pumps or heat-sensitive electronics). Indium plating (thin layers of indium metal on parts) is used to improve solderability and corrosion resistance in certain electronics and aerospace components.

In cryogenic and vacuum technology, indium is valued as a sealing material. A thin ring or foil of indium can be cold-welded between metal flanges to form an airtight, flexible vacuum seal; it remains malleable down to liquid-helium temperatures and does not become brittle. Similarly, indium is used in superconducting magnet junctions and other low-temperature hardware because it accommodates thermal contraction without fracturing.

Medical and scientific applications include radionuclides of indium. ^111In (indium-111) is used in nuclear medicine: for instance, white blood cells labeled with ^111In allow doctors to image sites of infection in the body. Indium-based compounds also appear in research with neutron detectors and advanced optics (e.g. certain mirror coatings).

In summary, indium’s main uses hinge on its electrical properties and low-temperature behavior. It is integral to display technology (ITO screens), semiconductor devices (laser diodes, LEDs, solar cells), special solders and alloys, and scientific instrumentation. Its importance has grown with modern electronics – it is often called a technology-critical element.

Biology, Environment, and Safety

Indium has no known biological role. It is not an essential element for any organism. In trace amounts, indium can enter the food chain (plants can take it up from contaminated soil), but typical background exposures to indium in the environment are extremely low and not of common concern to human health.

However, indium compounds, especially in dust or fume form, can be hazardous. The biggest health issue is "indium lung" – a form of interstitial lung disease observed in some workers exposed to indium tin oxide (ITO) and indium compounds. Inhaling fine indium oxide or hydroxide particles (for example from grinding or processing ITO) has led to lung inflammation, fibrosis and impaired lung function in factory workers. Soluble indium salts (like indium chloride or nitrate) are moderately toxic as well: they can cause irritation of eyes, skin and the respiratory tract. Animal studies show that indium compounds can accumulate in organs (kidney, lung) and cause damage at high doses.

Safety guidelines reflect these concerns. There are no federally mandated limits specifically for indium in most jurisdictions, but organizations like NIOSH and ACGIH recommend that workplace air concentrations of indium metal or soluble compounds not exceed about 0.1 mg/m³ (over an 8-hour work shift). Indium metal itself (solid ingots) is of low toxicity by skin contact, but powder or dust must be handled with care (respirators, gloves).

Environmentally, indium behaves like a heavy metal. It doesn’t readily biodegrade or volatilize. Once released (e.g. from mining waste or electronic scrap), it tends to bind to particles and sediments. Plants can uptake indium and it may concentrate in certain tissues, but it is not known to biomagnify strongly up the food chain. As use of indium in electronics grows, there is some concern about e-waste: spent displays and solar panels must be recycled or landfilled in a way that prevents indium from leaching into groundwater.

In brief, general public risk from indium is minimal under normal conditions. The primary concern is occupational exposure in industrial settings that process indium compounds. Good industrial hygiene (dust control, ventilation, protective equipment) is recommended. At present indium is not classified as a major environmental pollutant (like lead or mercury), but it is monitored in workplaces and new guidelines are under review as its use expands.

History and Etymology

Indium was discovered in 1863 by two German chemists, Ferdinand Reich and Hieronymous Theodor Richter, at the Freiberg Mining Academy. They were analyzing zinc ore by flame spectroscopy and observed a bright blue-violet spectral line that did not match any known element. Because the new line’s color resembled the indigo dye, they named the element “indium” (from the Latin indicum, meaning indigo). Reich was color-blind and relied on Richter for the spectral work, so both are credited with the discovery.

The team isolated tiny amounts of indium metal a year later (1864) by chemical reduction techniques. They initially measured an atomic weight of about 75, which was puzzling chemically. Dmitri Mendeleev later noted that indium should have an atomic weight near 115 to fit in the periodic table, and subsequent experiments (accounting for errors) confirmed its weight to be ~115.

For many decades, indium had few practical uses beyond academic curiosity. In the late 19th and early 20th centuries it was mainly a laboratory curiosity, though researchers noted its softness and unique properties. Interest grew only late in the 20th century with the rise of semiconductors and LCD technology. Indium’s name and symbol have remained unchanged since its discovery.

There are no mythological or ancient tales about indium (it is too rare and was unknown to pre-modern cultures). Its “claim to fame” is its spectral color: pigment industries and astronomers recognize the indigo line of indium. In recent decades, indium has become a strategic material – sometimes called a “technology metal” – reflecting its key role in flat-panel displays, mobile devices and renewable energy.

Data Table

Property	Value
Chemical symbol	In
Atomic number (Z)	49
Atomic weight (g/mol)	114.818
Period / Group / Block	Period 5, Group 13 (IIIA), p-block
Category	Post-transition (poor) metal
Electron configuration	[Kr] 4d^10 5s^2 5p^1
Common oxidation states	+3 (main), +1
Density (20 °C)	7.31 g/cm³
Melting point	156.6 °C (430 K)
Boiling point	2072 °C (2345 K)
Electronegativity (Pauling)	1.78
1st Ionization energy	559 kJ/mol (5.79 eV)
Crystal structure (25 °C)	Body-centered tetragonal
Thermal conductivity	~82 W/(m·K)
Electrical resistivity (20 °C)	~2.6×10^–6 Ω·cm (metallic)
Stable isotopes (natural)	^113In (4.3%), ^115In (95.7%)
Nuclear spin (I)	9/2 for both ^113In and ^115In
Discovery	1863 by Reich & Richter, Germany
Named after	Indigo (indicum), dark-blue spectral line
Earth’s crust abundance	~0.2 ppm

This table summarizes indium’s fundamental identifiers and constants relevant to its chemistry and physics (values are typical or recommended values). All data are given in SI units except where noted.