Indium is a chemical element with symbol In and atomic number 49. It is a post-transition metal that makes up 0.21 parts per million of the Earth's crust. Indium is very soft, malleable and easily fusible, with a melting point higher than sodium and gallium, but lower than lithium or tin. It was discovered using the spectroscope in 1863 by Ferdinand Reich and Hieronymous Theodor Richter, and it was named from the indigo blue line in its spectrum. It was isolated the next year.
Chemically, indium is similar to gallium and thallium, and it is largely intermediate between the two in terms of its propertiesIt is a minor component in zinc sulfide ores and hence it is made as a byproduct of zinc production. It is most notably used in the semiconductor industry, 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 has no biological role and its compounds are somewhat toxic when injected; most occupational exposure is through ingestion, in which indium compounds are not absorbed well, and inhalation, where they are moderately absorbed.