Indium (In): Properties, Reactions & Uses

Introduction: Why Indium Matters

Indium (In) is a soft, silvery-white, malleable metal belonging to Group 13 of the periodic table. Its unique properties, particularly its transparency and conductivity in compound form, make it indispensable in modern technology. From touchscreens and liquid crystal displays (LCDs) to advanced solar cells and LED lighting, Indium plays a critical role in the functionality of many everyday electronic devices. Its scarcity contributes to its economic value and continuous research into its applications and sustainable sourcing.

CBSE/JEE Quick Revision Notes

• Atomic Number (Z): 49
• Atomic Mass: 114.818 u
• Group: 13 (Boron family, p-block element)
• Period: 5
• Block: p-block
• Common Oxidation States: +3 (most stable), +1 (less stable due to inert pair effect)
• Nature: Soft, silvery-white, ductile, malleable metal
• Melting Point: 156.6 °C (relatively low)
• Boiling Point: 2072 °C
• Density: 7.31 g/cm³
• Electronegativity (Pauling scale): 1.78

Electron Configuration & Bonding Behavior

Ground State Electron Configuration: [Kr] 4d¹⁰ 5s² 5p¹

Valence Electrons: Indium has three valence electrons (5s² 5p¹).

Bonding Behavior: Indium typically exhibits metallic bonding in its elemental state. In compounds, it primarily forms ionic bonds with highly electronegative elements and covalent bonds with others.

• Oxidation States:
  • +3 Oxidation State: This is the most common and stable oxidation state, arising from the loss of all three valence electrons (5s² 5p¹).
  • +1 Oxidation State: The +1 oxidation state is also observed but is less stable than +3. This is attributed to the inert pair effect, where the 5s² electrons are reluctant to participate in bonding due to poor shielding by the intervening d-electrons, leading to increased effective nuclear charge and stronger attraction of the 5s electrons to the nucleus.

Crucial Chemical Reactions

Indium is moderately reactive. Its chemical behavior is similar to gallium and thallium, with the +3 oxidation state being more stable than +1, though the stability of +1 increases down the group (Ga < In < Tl).

1. Reaction with Air/Oxygen

Indium is stable in dry air at room temperature. When heated in air or oxygen, it forms indium(III) oxide.

• 4In(s) + 3O₂(g) → 2In₂O₃(s)

2. Reaction with Halogens

Indium reacts with halogens (X₂) to form trihalides (InX₃).

• 2In(s) + 3Cl₂(g) → 2InCl₃(s) (Indium(III) chloride)
• 2In(s) + 3Br₂(l) → 2InBr₃(s) (Indium(III) bromide)
• 2In(s) + 3I₂(s) → 2InI₃(s) (Indium(III) iodide)

Monohalides (InX) and dihalides (InX₂) also exist, but are often mixed-valence compounds, e.g., InBr is polymeric. InCl and InBr can be formed by heating InX₃ with In metal.

• 2InCl₃(s) + 4In(s) → 6InCl(s) (Formation of Indium(I) chloride)

3. Reaction with Acids

Indium reacts with most acids.

• Dilute Acids (non-oxidizing): Indium reacts slowly with dilute non-oxidizing acids like HCl and H₂SO₄ to liberate hydrogen gas, forming In(III) salts.

  • 2In(s) + 6HCl(aq) → 2InCl₃(aq) + 3H₂(g)
  • 2In(s) + 3H₂SO₄(dilute) → In₂(SO₄)₃(aq) + 3H₂(g)

• Oxidizing Acids (e.g., HNO₃): Indium reacts with oxidizing acids to form indium(III) salts, often without the evolution of hydrogen.

  • In(s) + 4HNO₃(conc.) → In(NO₃)₃(aq) + NO(g) + 2H₂O(l) (Example with concentrated HNO₃)

4. Reaction with Bases

Indium(III) hydroxide, In(OH)₃, is amphoteric, meaning it can react with both acids and strong bases. However, its acidic character is weaker than that of Al(OH)₃.

• Reaction with Acids:

  • In(OH)₃(s) + 3H⁺(aq) → In³⁺(aq) + 3H₂O(l)

• Reaction with Strong Bases:

  • In(OH)₃(s) + OH⁻(aq) → [In(OH)₄]⁻(aq) (Tetrahydroxoindate(III) ion)

Industrial and Biological Importance

Industrial Importance

1. Indium Tin Oxide (ITO): This is the most significant application. ITO is a transparent electrical conductor, critical for:
   • LCDs and Touchscreens: Used in smartphones, tablets, and computer monitors.
   • Solar Panels: As a transparent electrode.
   • OLEDs and Plasma Displays: For electrodes.
2. Low-Melting Point Alloys: Indium forms alloys with low melting points, used in:
   • Solders: Especially for delicate electronic components that cannot withstand high temperatures.
   • Thermal Fuses and Sprinkler Systems: As fusible links.
3. LEDs (Light Emitting Diodes): Indium gallium nitride (InGaN) is a key semiconductor material used in blue and white LEDs.
4. Nuclear Applications: Indium is used in control rods for nuclear reactors due to its high neutron absorption cross-section.
5. Coatings: Used as a corrosion-resistant coating for various metals, especially in aerospace applications.

Biological Importance

• Toxicity: Indium and its compounds are generally considered to have low acute toxicity, but prolonged exposure or high doses can be harmful, primarily affecting the kidneys and liver. Indium dust or fumes can cause respiratory irritation.
• Medical Isotopes: Radioactive isotopes like Indium-111 (¹¹¹In) are used in nuclear medicine for diagnostic imaging (e.g., tumor detection, cerebrospinal fluid studies).