Thallium (indium's heavier homolog) shows an even stronger effect, causing oxidation to thallium(I) to be more probable than to thallium(III), whereas gallium (indium's lighter homolog) commonly shows only the +3 oxidation state. The stabilization of the monovalent state is attributed to the inert pair effect, in which relativistic effects stabilize the 5s-orbital, observed in heavier elements.

In some cases, the pair of 5s-electrons are not donated, resulting in indium(I), In +. In compounds, indium most commonly donates the three outermost electrons to become indium(III), In 3+. Indium has 49 electrons, with an electronic configuration of 4d 105s 25p 1. However it does have a size effect in bending and indentation, associated to a length-scale of order 50–100 µm, significantly large when compared with other metals. Indium displays a ductile viscoplastic response, found to be size-independent in tension and compression.

Indium has greater solubility in liquid mercury than any other metal (more than 50 mass percent of indium at 0 ☌). Indium crystallizes in the body-centered tetragonal crystal system in the space group I4/ mmm ( lattice parameters: a = 325 pm, c = 495 pm): this is a slightly distorted face-centered cubic structure, where each indium atom has four neighbours at 324 pm distance and eight neighbours slightly further (336 pm). Below the critical temperature, 3.41 K, indium becomes a superconductor. The density of indium, 7.31 g/cm 3, is also greater than gallium, but lower than thallium. The boiling point is 2072 ☌ (3762 ☏), higher than that of thallium, but lower than gallium, conversely to the general trend of melting points, but similarly to the trends down the other post-transition metal groups because of the weakness of the metallic bonding with few electrons delocalized. Like both, indium has a low melting point, 156.60 ☌ (313.88 ☏) higher than its lighter homologue, gallium, but lower than its heavier homologue, thallium, and lower than tin. Like gallium, indium is able to wet glass. Like tin, a high-pitched cry is heard when indium is bent – a crackling sound due to crystal twinning. It is a member of group 13 on the periodic table and its properties are mostly intermediate between its vertical neighbours gallium and thallium. It is so soft ( Mohs hardness 1.2) that like sodium, it can be cut with a knife. Indium is a silvery-white, highly ductile post-transition metal with a bright luster. Properties Physical Indium wetting the glass surface of a test tube Most occupational exposure is through ingestion, from which indium compounds are not absorbed well, and inhalation, from which they are moderately absorbed. Its compounds are toxic when injected into the bloodstream. Indium is considered a technology-critical element. 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 is a minor component in zinc sulfide ores and is produced as a byproduct of zinc refinement. They named it for the indigo blue line in its spectrum. Indium was discovered in 1863 by Ferdinand Reich and Hieronymous Theodor Richter by spectroscopic methods. Chemically, indium is similar to gallium and thallium, and it is largely intermediate between the two in terms of its properties.

Indium has a melting point higher than sodium and gallium, but lower than lithium and tin.

It is a post-transition metal that makes up 0.21 parts per million of the Earth's crust. It is a silvery-white metal that resembles tin in appearance. Indium is the softest metal that is not an alkali metal. Indium is a chemical element with the symbol In and atomic number 49.

Chemical element sn