Indium (In)

The ground-state electron configuration of indium is [Kr] 4d¹⁰ 5s² 5p¹. The single 5p electron makes it chemically similar to other Group 13 elements like aluminum and gallium, typically forming the +3 oxidation state.

Indium most commonly exhibits oxidation states of +1 and +3:

• +3 state (In³⁺) is more stable and found in compounds like \(\mathrm{In_2O_3}\) and \(\mathrm{InCl_3}\).
• +1 state (In⁺) appears in compounds such as \(\mathrm{InCl}\), but it is less stable and often disproportionates into In(0) and In(III).

Indium is a major component of indium tin oxide (ITO), a transparent and conductive material. ITO layers are deposited on glass or plastic to form electrodes in touch screens, LCDs, and solar panels. It combines high optical transparency with good electrical conductivity, properties that are rare in other materials.

Indium is a soft, malleable, silvery metal with a low melting point of about 156.6 °C. It can be easily cut with a knife and leaves a mark like lead. It also exhibits a characteristic high-pitched 'cry' when bent due to crystal twinning.

Indium is primarily a by-product of zinc ore processing (mainly from sphalerite, \(\mathrm{ZnS}\)). After zinc extraction, indium is recovered from smelter residues through leaching and electrolysis. Smaller amounts come from lead and copper ores.

ITO is an n-type semiconductor composed of indium(III) oxide (\(\mathrm{In_2O_3}\)) and tin(IV) oxide (\(\mathrm{SnO_2}\)) in a typical ratio of about 90:10 by weight. It is produced by mixing oxides and sintering them at high temperatures. The material is optically transparent in the visible range and conductive due to oxygen vacancies.

Indium’s electron configuration ([Kr] 4d¹⁰ 5s² 5p¹) allows the 5p electron to be easily removed, leading to the +1 state. However, removal of all three valence electrons (5s and 5p) results in a more stable +3 state. The inert pair effect becomes noticeable but is weaker than in heavier Group 13 elements like thallium.

Metallic indium is relatively non-toxic, but some indium compounds—particularly indium tin oxide (ITO) dust and indium halides—can be harmful if inhaled or ingested. Long-term exposure can cause lung or kidney damage. Therefore, handling indium compounds requires protective equipment and fume ventilation.

Indium is used in:

• Low-melting alloys and solders for electronics (e.g., fusible alloys).
• Semiconductors: indium phosphide (\(\mathrm{InP}\)) and indium arsenide (\(\mathrm{InAs}\)) in high-speed electronics and lasers.
• Coatings: indium films on bearings for reduced friction.
• Nuclear control rods due to neutron absorption capacity.

Yes. Indium readily forms compounds with many nonmetals:

• With oxygen: \(\mathrm{In_2O_3}\) – a basic oxide used in ITO.
• With chlorine: \(\mathrm{InCl_3}\) – a deliquescent salt, soluble in water.
• With sulfur: \(\mathrm{In_2S_3}\) – used in thin-film solar cells.