Indium (In)
Indium (In) is a post-transition metal with atomic number 49 and a standard atomic weight of 114.82. At room temperature it is a solid that melts at 157 °C and boils at 2,027 °C.
Indium key facts
Indium at a glance
Identity
| Symbol | In |
|---|---|
| Atomic number (Z) | 49 |
| Atomic weight | 114.82 ± 0.01 (CIAAW 2024) |
| Category | Post-transition metal |
| Group | 13 — boron group |
| Period | 5 |
| Block | p-block |
| CAS number | 7440-74-6 |
Atomic properties
| Electron configuration | [Kr] 4d10 5s2 5p1 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p1 |
|---|---|
| Electrons per shell | 2, 8, 18, 18, 3 |
| Oxidation states | +3 Less common: -5, -2, -1, 0, +1, +2 |
| Electronegativity (Pauling) | 1.78 |
| First ionisation energy | 5.786 eV (558.3 kJ/mol) 2nd: 18.87 eV · 3rd: 28.044 eV |
| Electron affinity | 0.3 eV |
| Atomic radius (empirical) | 155 pm |
| Covalent radius | 142 pm |
| Van der Waals radius | 193 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 156.6 °C · 313.9 °F · 429.75 K |
| Boiling point | 2,027 °C · 3,680.6 °F · 2,300.15 K |
| Density | 7.31 g/cm³ |
| Specific heat capacity | 0.233 J/(g·K) |
| Thermal conductivity | 81.8 W/(m·K) |
| Crystal structure | Tetragonal |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 115In 95.719% (radioactive), 113In 4.281% |
| Stable isotopes | 1 |
| Known isotopes | 42 |
| Abundance in Earth's crust | 0.25 mg/kg |
| Abundance in seawater | 0.02 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Indium electron configuration
Indium has 49 electrons. Written in short form, its ground-state configuration is [Kr] 4d10 5s2 5p1 — the electrons of the noble gas krypton, plus 4d10 5s2 5p1.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p1.
As a member of group 13 it has 3 valence electrons in its outer shell (shell 5), which largely decide its chemistry.
Discovery and name
1863 — Ferdinand Reich and Hieronymous Theodor Richter · Germany.
Its indigo spectral line.
Known for
Indium tin oxide is the transparent conductor on touchscreens and flat-panel displays. Pure indium is soft enough to leave a mark on paper.
Isotopes of indium
Naturally occurring indium is a mix of 2 isotopes. Each has 49 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 115In | 49 | 66 | 95.719% | radioactive · half-life 441 trillion years |
| 113In | 49 | 64 | 4.281% | stable |
42 isotopes of indium are known in our data set (half-lives as published in nuclear data tables).
Indium compared with group 13
The other elements in group 13 (boron group), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Boron | 5 | 10.81 | 2,077 °C | 4,000 °C | 2.34 g/cm³ | 2.04 |
| Aluminum | 13 | 26.982 | 660 °C | 2,519 °C | 2.7 g/cm³ | 1.61 |
| Gallium | 31 | 69.723 | 30 °C | 2,229 °C | 5.91 g/cm³ | 1.81 |
| Indium | 49 | 114.82 | 157 °C | 2,027 °C | 7.31 g/cm³ | 1.78 |
| Thallium | 81 | 204.38 | 304 °C | 1,473 °C | 11.8 g/cm³ | 1.62 |
| Nihonium | 113 | [286] | — | — | 16 g/cm³ (pred.) | — |