Indium
Soft, silvery metal used in displays and semiconductors.
Indium is a chemical element with the symbol In and atomic number 49, classified as a silvery-white post-transition metal and one of the softest known elements. Its chemical behavior is largely intermediate between gallium and thallium, its vertical neighbors on the periodic table. Discovered in 1863 by Ferdinand Reich and Hieronymous Theodor Richter using spectroscopic methods, it was named for the prominent indigo blue line visible in its spectrum, with the name derived from the Latin *indicum*, meaning violet or indigo. Physically, indium is highly ductile and so soft (Mohs hardness 1.2) that it can be cut with a knife or marked on paper like a pencil. It wets glass, produces a crackling sound when bent due to crystal twinning, and has a low melting point of 156.60 °C, higher than gallium but lower than thallium. Its boiling point is 2072 °C, and it becomes a superconductor below 3.41 K. Indium crystallizes in a body-centered tetragonal structure and is highly soluble in liquid mercury. Chemically, indium has 49 electrons with a configuration of [Kr]4d¹⁰5s²5p¹. It most commonly forms the +3 oxidation state, but the +1 state also occurs due to the inert pair effect, with indium(I) compounds acting as strong reducing agents. Indium has 39 known isotopes; only two occur naturally: stable indium-113 and primordial indium-115, which has an extraordinarily long half-life of 4.41×10¹⁴ years and makes up 95.7% of natural indium. It is one of three elements where the stable isotope is less abundant than a long-lived radioisotope. Indium has no biological role, and its compounds are toxic when inhaled or injected, though poorly absorbed if ingested. It is produced exclusively as a by-product of processing other metal ores, chiefly sphalerite and zinc sulfide ores.
- discoverers
- Ferdinand Reich and Hieronymus Theodor Richter
Lore & Background
Indium is a silvery-white post-transition metal with a bright luster and is one of the softest elements, so soft it can be cut with a knife or even bitten by human teeth, registering a Mohs hardness of just 1.2. It is highly ductile and, when rubbed on paper, leaves a visible line similar to a pencil. A distinctive high-pitched cry, caused by crystal twinning, is heard when the metal is bent, much like tin. Its melting point is 156.60 °C, higher than gallium but lower than thallium and tin, while its boiling point is 2072 °C, which is higher than thallium but lower than gallium. The density of indium is 7.31 g/cm³, and it becomes a superconductor below 3.41 K. It crystallizes in a body-centered tetragonal system, a slightly distorted face-centered cubic structure. Indium is notable for its ability to wet glass, similar to gallium, and it has greater solubility in liquid mercury than any other metal, exceeding 50% by mass at 0 °C. Chemically, indium is intermediate between gallium and thallium, and it commonly exhibits the +3 oxidation state, though the +1 state also occurs due to the inert pair effect. Indium has no biological role, and its compounds are toxic when inhaled or injected, though poorly absorbed if ingested. It is produced exclusively as a by-product from processing other metal ores, primarily sphalerite and other zinc sulfide ores.
Reader's Guide
Indium’s industrial significance stems from its unique physical and chemical properties. It is a silvery-white, extremely soft post-transition metal (Mohs hardness 1.2) that can be cut with a knife and leaves a mark like a pencil on paper. It wets glass, has a low melting point of 156.60 °C, and emits a crackling sound when bent due to crystal twinning. Below 3.41 K, it becomes a superconductor. Its primary use is as indium tin oxide (ITO), a transparent conductive coating for flat-panel displays. It is also employed in low-melting-point alloys, solders, and high-vacuum seals, and in manufacturing blue and white LED circuits through indium gallium nitride p-type semiconductor substrates. Indium is produced exclusively as a by-product of processing other metal ores, chiefly sphalerite and other zinc sulfide ores. Chemically, it is intermediate between gallium and thallium, exhibiting both +1 and +3 oxidation states; the +3 state is more common, while +1 compounds are powerful reducing agents due to the inert pair effect. Indium has no biological role, and its compounds are toxic when inhaled or injected, though poorly absorbed orally. It has 39 known isotopes; only two occur naturally: stable indium-113 and long-lived indium-115 (half-life 4.41 × 10¹⁴ years), which constitutes 95.7% of natural indium. The element was discovered in 1863 via spectroscopy, named for the indigo blue line in its spectrum.
Did You Know?
- Indium is so soft (Mohs hardness 1.2) that it can be cut with a knife or bitten by human teeth.
- Indium leaves a visible line like a pencil when rubbed on paper.
- Indium dissolves in liquid mercury up to about 40–45% by mass at 0 °C, though some other metals like sodium or potassium are even more soluble.
Frequently Asked Questions
Who is Indium?
Indium is element number 49 on the periodic table, a silvery-white post-transition metal noted for being one of the softest elements in nature. It occupies Group 13, Period 5, sitting directly between gallium and thallium and sharing chemical traits with both neighbors.
What are Indium's powers/role?
Its most celebrated role is as a core component of indium tin oxide (ITO), the transparent conductive coating that makes touchscreens and flat-panel displays functional. It also appears in semiconductor devices, LED circuitry, and low-melting-point alloys used in niche engineering applications.
Why is Indium important?
Contemporary display technology is essentially inseparable from it, since the ITO layers on monitors, televisions, and smartphone screens rely on indium to carry electrical current while staying optically clear. It also supports semiconductor fabrication and specialized alloy work where a very low melting point is required.
Where does Indium sit in the periodic table?
It holds position 49 in Group 13, Period 5, which places it squarely between gallium above and thallium below. As a post-transition metal, it blends certain properties of both its vertical neighbors rather than fitting neatly into the classic transition-metal or alkali-metal categories.
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