Indium is a chemical element with the symbol In and atomic number 49. It is a soft, malleable metal that is silvery-white in color and resembles tin. Chemically, it behaves like its homologues on the periodic table: gallium and thallium.
Discovery and Naming
Indium was discovered in 1863 by the German chemists Ferdinand Reich and Hieronymous Theodor Richter. They named the element after the indigo blue line in its spectrum. The name “indium” is derived from the Latin word “indicum,” meaning “indigo.” Reich was colorblind and had Richter interpret colored spectral lines. Reich isolated the pure element in 1864 and present an ingot of the pure metal at the 1867 World Fair.
Appearance and Properties
Indium is a shiny, silvery-white metal. While not very reactive, it eventually oxidizes to a darker color after exposure in air. It is a soft solid at room temperature that is highly ductile and malleable. It is a good conductor of heat and electricity.
Indium has two naturally occurring isotopes: 113In and 115In. Of these, 115In is the most abundant, making up about 95.7% of naturally occurring indium. The remaining 4.3% consists of 113In. Indium-113 is stable. Technically, indium-115 is a radioactive isotope, but its half-life is 4.41×1014 years, which is over four times longer than the age of the universe. There are several (37) other synthetic isotopes of indium, but they have short half-lives and are not commonly found in nature. The most stable is indium-111, which has a half-life of 2.8 days.
Abundance and Sources
Indium is a rare element that does not occur in its pure form in nature. It is the 68th most abundant element in the Earth’s curst, comparable in abundance to bismuth and mercury. It is a primordial element that comes from the slow neutron capture process in stars with low to medium mass. Indium also forms from radioactive decay of cadmium-115. The metal mainly occurs in association with zinc ores. Canada, China, and South Korea are some of the primary producers of indium.
Uses of Indium
Indium stabilizes non-iron metals, so its first significant application was as a coating on bearings to protect against damage and corrosion. Today, the element serves many uses:
- Touchscreens and LCDs: Indium tin oxide is a transparent conductor in touchscreens and LCD displays.
- Soldering: Because of its low melting point, indium is an important solder.
- Sealants: Indium wire is a vacuum seal and a gasket in cryogenics.
- Thermometers: Because of its low melting point, the indium allow galinstan replaces mercury in some thermometers.
- Photovoltaic cells: Indium is a component of thin-film solar cells.
- Batteries: Indium replaces mercury as a less-toxic option for preventing zinc corrosion in alkaline batteries.
- Electronics: Indium phosphide finds use in high-frequency and high-power electronics.
- Nuclear Reactors: Because of its high neutron capture cross-section, indium finds use in nuclear reactor control rods.
- Pigments: YInMn blue is an intense blue pigment that is non-toxic and fade-resistant.
- Mirrors: Because it wets glass, it readily forms a corrosion-resistant mirror.
Indium exhibit several oxidation states (-5, -2, -1, 0, +1, +2, +3), but the most common is +3. The element is most stable in this state.
Biological Role, Health Effects, and Toxicity
Indium has no known biological role in the human body or any other organism. Prolonged exposure to indium compounds causes health problems, especially in the respiratory system. “Indium lung” is a condition involving pulmonary alveolar proteinosis and pulmonary fibrosis. Indium tin oxide, in particular, is toxic if ingested or inhaled in large amounts. Tiny quantities of radioactive indium find use in nuclear medicine and as tracers for white blood cells and proteins.
Key Indium Facts for Chemists
Here is the atomic data and a collection of indium physical properties:
|Group||13 (boron group)|
|Electron Configuration||[Kr] 4d10 5s2 5p1|
|Electrons per Shell||2, 8, 18, 18, 3|
|State at Room Temperature||Solid|
|Density (near room temp.)||7.31 g/cm3|
|Heat of Vaporization||231.8 kJ/mol|
|Heat of Fusion||3.28 kJ/mol|
|Ionization Energies||558.3 kJ/mol, 1820.7 kJ/mol, 2704 kJ/mol|
|Atomic Radius||167 pm|
|Covalent Radius||142 pm|
|van der Waals Radius||193 pm|
|Crystal Structure||body-centered tetragonal (BCC)|
|Young’s Modulus||11 GPa|
Interesting Indium Facts
Here are more interesting indium facts:
- Superconductivity: When cooled to cryogenic temperatures, indium becomes superconductive.
- No Biological Need: Although indium occurs in trace amounts in all living organisms, there’s no evidence to suggest that it serves any biological purpose or is essential for life.
- Sound of Freshness: When bent, pure indium produces a characteristic “cry” sound, similar to the cry of tin. This sound results from the metal’s crystal structure reforming.
- Flame Test: Indium turns flames an indigo to violet color.
- Low Abundance: Indium is rarer in Earth’s crust than silver, gold, or mercury.
- Radioactive Isotope: Activating the isotope 115In produces the metastable isotope 116mIn, which is a radioisotope used in some types of nuclear medicine imaging.
- Indium Beads: Molten indium droplets form a pattern of beads when they solidify that resemble water droplets on a surface.
- Compounds with Colors: Some compounds of indium are colorful. For instance, indium(III) chloride is yellow, while indium(III) sulfide is red.
- Wetting Properties: Indium has the unique ability to “wet” many non-metallic surfaces, including glass and ceramics. This makes it particularly valuable in sealing applications.
- Indium vs. Corrosion: Indium resists corrosion when exposed to water, but it does not resist acidic and alkaline solutions.
- Gallium-Indium Alloy: An alloy of gallium and indium remains liquid at room temperature, making it a non-toxic replacement for mercury.
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