Indium is a chemical element with symbol In and atomic number 49. It is a post-transition metallic element that is rare in Earth's crust. The metal is very soft, malleable and easily fusible, with a melting point higher than sodium, but lower than lithium or tin. Chemically, indium is similar to gallium and thallium, and it is largely intermediate between the two in terms of its properties.[5] It has no obvious role in biological processes and common compounds are not toxic. It is most notably used in low melting point metal alloys such as solders, in soft metal high vacuum seals, and in the production of transparent conductive coatings of indium tin oxide (ITO) on glass.
Indium is a very soft, silvery-white, highly ductile, relatively rare post-transition metal with a bright luster.[6] It is so soft (Mohs hardness 1.2) that the metal can be cut with a knife, as can sodium. It also leaves a visible line on paper.[7] Like tin, when it is bent indium emits a high-pitched "cry".[6] Like gallium, indium is able to wet glass. Like both, indium has a low melting point, 156.60 °C (313.88 °F); higher than its lighter homologue, gallium, but lower than its heavier homologue, thallium, and lower than tin. Only mercury, gallium, and most of the alkali metals have lower melting points.[8] Its boiling point is 2072 °C (3762 °F), higher than that of thallium, but lower than that of gallium, showing opposition to the melting points' trend. Indium thus has a very large liquid range of around 2000 °C. The density of indium, 7.31 g·cm−3, is also higher than that of gallium, but lower than that of thallium. Below its critical temperature of 3.41 K, indium becomes a superconductor. At standard temperature and pressure, indium crystallizes in the tetragonal crystal system in the space group I4/mmm (lattice parameters: a = 325 pm, c = 495 pm).[8]
Indium metal does not react with water, but it is oxidized by stronger oxidizing agents, such as halogens to give indium(III) compounds. It does not form a boride, silicide or carbide, and the hydride InH3 has only a transitory existence, even at low temperatures, before reverting to the elements.[9]
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