Why is +3 the most common oxidation state for indium?

Indium has the electron configuration [Kr] 4d¹⁰ 5s² 5p¹. Losing all three valence electrons (5s² and 5p¹) to give In³⁺ is energetically favorable because it produces a stable [Kr] 4d¹⁰ pseudo-noble-gas core. This +3 state dominates in the vast majority of indium compounds including InCl₃, In₂O₃, InP, and ITO.

What is the +1 oxidation state of indium?

In the +1 state, indium loses only the 5p¹ electron, retaining the 5s² pair. This reflects the inert pair effect — a relativistic contraction of the 5s orbital that makes the s² electrons increasingly stable and less willing to bond as you descend Group 13 (Al→Ga→In→Tl). In⁺ appears in compounds like InCl and In₂O, and becomes more stable than Tl⁺ in the heavier group analogue thallium.

What is ITO and why does indium matter in it?

ITO (indium tin oxide) is a transparent conducting oxide composed of In₂O₃ doped with SnO₂. Indium is in the +3 state. ITO is the standard transparent electrode in touchscreens, LCD displays, organic solar cells, and OLEDs. Indium's importance in electronics comes almost entirely from ITO demand.

What is the electron configuration of In and In³⁺?

Neutral indium: [Kr] 4d¹⁰ 5s² 5p¹. In³⁺ ion: [Kr] 4d¹⁰ — all three valence electrons removed, leaving a fully filled 4d shell. The In⁺ ion would be [Kr] 4d¹⁰ 5s² (5p electron removed only).

How does indium compare to aluminum and thallium in Group 13?

All Group 13 elements have a ground-state ns² np¹ configuration and show +3 as the main state. However, the stability of the +1 (inert pair) state increases down the group: Al shows almost no +1 chemistry; Ga has rare +1 compounds; In has accessible +1 (InCl, In₂O); Tl has +1 as its most stable state (Tl⁺ is more stable than Tl³⁺). This trend is driven by relativistic stabilization of the ns² electrons.

Is indium toxic or safe to handle?

Elemental indium and most indium compounds are considered low-to-moderate toxicity compared to many heavy metals, but should still be handled with care. Inhalation of indium compounds (particularly in industrial settings) has been linked to indium lung disease, a serious pulmonary condition. Standard lab precautions — gloves, eye protection, and good ventilation — are recommended.

Indium Oxidation States

Name: Indium Oxidation States
Availability: InStock
Author: Nham Vu

Reference guide for indium (In, Z=49) oxidation states (+3, +1, 0), with electron configuration, inert pair effect explanation, and example compounds.

Atomic # 49 In Indium

Atomic Mass

114.818 u

Group

13 (IIIA)

Period

Block

p-block

Electronegativity

1.78 (Pauling)

Oxidation States

+3 (dominant), +1, 0

Indium exhibits three oxidation states: +3, +1, and 0, with +3 overwhelmingly dominant in all ordinary chemistry. Indium's ground-state configuration is [Kr] 4d¹⁰ 5s² 5p¹. Losing all three valence electrons gives In³⁺ with a stable [Kr] 4d¹⁰ core. The +1 state reflects the inert pair effect: relativistic contraction of the 5s orbital makes the s² pair increasingly reluctant to ionize as you descend Group 13, so In⁺ ([Kr] 4d¹⁰ 5s²) is accessible in a handful of compounds.

Oxidation State	Stability	Notes
+3	Stable — dominant	Universal state in common compounds: InCl₃, In₂O₃, InP, ITO. Loss of 5p¹ and 5s² leaves a stable [Kr] 4d¹⁰ configuration. Colorless In³⁺ ion in aqueous solution.
+1	Accessible — inert pair	Inert pair effect: 5s² electrons are retained; only 5p¹ electron is lost. Gives In⁺ with configuration [Kr] 4d¹⁰ 5s². Compounds: InCl, In₂O. Less stable than Tl⁺ (heavier analogue where inert pair dominates).
0	Elemental only	Pure indium metal. Soft, silvery-white, malleable. Melting point 156.6 °C. Standard reduction potential In³⁺/In = −0.34 V, indicating moderate reducing character.

Ionization Energies
IE₁ = 558.3 kJ/mol | IE₂ = 1820.7 kJ/mol | IE₃ = 2704 kJ/mol
IE₁ removes the lone 5p electron; IE₂ and IE₃ break into the 5s² pair. The relatively modest jump between IE₂ and IE₃ (vs the dramatic IE₁→IE₂ step in silver) means losing all three valence electrons to reach +3 is accessible with typical bond energies.

The overwhelming majority of indium chemistry involves In³⁺ (+3 state). Indium's commercial importance comes largely from its role in ITO (indium tin oxide) transparent conductors and III-V semiconductor compounds like InP and InAs. The handful of +1 compounds are laboratory or specialized materials.

Compound	Formula	In State	Notes
Indium(III) Chloride	InCl₃	+3	White hygroscopic solid; the most common indium starting material for synthesis. Soluble in water; forms the hexaaqua complex [In(H₂O)₆]³⁺ in solution.
Indium(III) Oxide	In₂O₃	+3	Pale yellow solid; the base oxide for ITO production. The most stable indium oxide; used in gas sensors and as a transparent conducting oxide precursor.
Indium Tin Oxide (ITO)	In₂O₃:SnO₂	+3	Transparent conducting oxide; ~90% In₂O₃ with ~10% SnO₂. The standard electrode material for LCDs, touchscreens, OLEDs, and thin-film solar cells. Indium demand is dominated by ITO production.
Indium Phosphide	InP	+3	III-V semiconductor; direct bandgap 1.35 eV. Used in high-frequency transistors, solar cells, and fiber-optic laser diodes. In³⁺ bonded to P³⁻ in a zinc-blende lattice.
Indium Arsenide	InAs	+3	III-V semiconductor with narrow bandgap (0.36 eV); used in infrared detectors and terahertz electronics.
Indium Antimonide	InSb	+3	Narrowest bandgap III-V semiconductor (0.17 eV at 300 K); high electron mobility. Used in far-infrared detectors and magnetoresistance sensors.
Indium(III) Nitrate	In(NO₃)₃	+3	Soluble white solid; used as a chemical vapor deposition (CVD) precursor and in synthesis of indium oxide nanoparticles.
Indium(III) Sulfate	In₂(SO₄)₃	+3	Water-soluble; used in electroplating baths to deposit thin indium layers onto metals for corrosion resistance and lubrication.
Indium(I) Chloride	InCl	+1	Yellow solid; inert pair effect compound. Disproportionates on heating: 3 InCl → InCl₃ + 2 In. Primarily of academic interest.
Indium(I) Oxide	In₂O	+1	Black solid; less stable than In₂O₃. Formed at low oxygen partial pressures; reverts to In₂O₃ and In metal under standard conditions.

Ground State Configuration

Filled d-shell — spectator in all oxidation states

Inert pair — retained in +1 state; lost in +3

Lone valence p electron — first to be lost

Notation

Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p¹

Noble gas: [Kr] 4d¹⁰ 5s² 5p¹

In⁺ ion: [Kr] 4d¹⁰ 5s²

In³⁺ ion: [Kr] 4d¹⁰

Ionization Steps

[Kr] 4d¹⁰ 5s² 5p¹

Neutral atom — standard p-block ground state

In⁺

[Kr] 4d¹⁰ 5s²

Remove 5p¹ electron (IE₁ 558.3 kJ/mol) — exposes the inert 5s² pair

In²⁺

[Kr] 4d¹⁰ 5s¹

Remove first 5s electron (IE₂ 1820.7 kJ/mol) — transient; not isolable

In³⁺

[Kr] 4d¹⁰

Remove second 5s electron (IE₃ 2704 kJ/mol) — stable closed-shell cation; dominant in all common chemistry

The Inert Pair Effect
The 5s² electrons contract relativistically as nuclear charge increases down Group 13. This makes them harder to ionize, stabilizing the +1 state relative to +3. The effect is minor for Al (almost no +1 chemistry), moderate for In (+1 is accessible), and dominant for Tl (Tl⁺ is more stable than Tl³⁺).

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Summary

Reference guide for indium (In, Z=49) oxidation states (+3, +1, 0), with electron configuration, inert pair effect explanation, and example compounds.

How it works

Click a tab — Oxidation States, Compounds, or Electron Config — to explore each area.
The Oxidation States panel explains +3 stability vs the inert pair effect behind +1, with a comparison table.
The Compounds panel lists common indium compounds with formulas and oxidation state assignments.
The Electron Config panel shows the orbital filling for neutral In and the In³⁺ ion.
Click any monospace table cell to copy its value to your clipboard.

Use cases

Students studying Group 13 trends and the inert pair effect down the p-block.
Chemistry teachers explaining post-transition metal oxidation states.
Engineers working with ITO (indium tin oxide) transparent conductors or InP semiconductors.
Researchers needing quick atomic data or compound lists for indium.
Anyone preparing for exams covering Period 5 or p-block element chemistry.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu