Iodine (I)
Iodine (I) is a halogen with atomic number 53 and a standard atomic weight of 126.90. At room temperature it is a solid that melts at 114 °C and boils at 184 °C.
Iodine key facts
Iodine at a glance
Identity
| Symbol | I |
|---|---|
| Atomic number (Z) | 53 |
| Atomic weight | 126.90 ± 0.01 (CIAAW 2024) |
| Category | Halogen |
| Group | 17 — halogens |
| Period | 5 |
| Block | p-block |
| CAS number | 7553-56-2 |
Atomic properties
| Electron configuration | [Kr] 4d10 5s2 5p5 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5 |
|---|---|
| Electrons per shell | 2, 8, 18, 18, 7 |
| Oxidation states | -1, +1, +3, +5, +7 Less common: +2, +4, +6 |
| Electronegativity (Pauling) | 2.66 |
| First ionisation energy | 10.451 eV (1,008.4 kJ/mol) 2nd: 19.131 eV · 3rd: 29.57 eV |
| Electron affinity | 3.059 eV |
| Atomic radius (empirical) | 140 pm |
| Covalent radius | 133 pm |
| Van der Waals radius | 198 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 113.7 °C · 236.7 °F · 386.85 K |
| Boiling point | 184.4 °C · 363.9 °F · 457.55 K |
| Density | 4.933 g/cm³ |
| Specific heat capacity | 0.214 J/(g·K) |
| Crystal structure | Orthorhombic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 127I 100% |
| Stable isotopes | 2 |
| Known isotopes | 42 |
| Abundance in Earth's crust | 0.45 mg/kg |
| Abundance in seawater | 0.06 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Iodine electron configuration
Iodine has 53 electrons. Written in short form, its ground-state configuration is [Kr] 4d10 5s2 5p5 — the electrons of the noble gas krypton, plus 4d10 5s2 5p5.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5.
As a member of group 17 it has 7 valence electrons in its outer shell (shell 5), which largely decide its chemistry.
Discovery and name
1811 — Bernard Courtois · France.
Greek iōdēs, violet — the colour of its vapour.
Known for
Essential for thyroid hormones — iodised salt prevents deficiency. Iodine tinctures disinfect wounds; iodine-131 treats thyroid cancer.
Isotopes of iodine
Naturally occurring iodine is a mix of 1 isotope. Each has 53 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 127I | 53 | 74 | 100% | stable |
42 isotopes of iodine are known in our data set (half-lives as published in nuclear data tables).
Iodine compared with group 17
The other elements in group 17 (halogens), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Fluorine | 9 | 18.998 | -220 °C | -188 °C | 1.696 g/L | 3.98 |
| Chlorine | 17 | 35.45 | -101 °C | -34 °C | 3.2 g/L | 3.16 |
| Bromine | 35 | 79.904 | -7.2 °C | 59 °C | 3.103 g/cm³ | 2.96 |
| Iodine | 53 | 126.90 | 114 °C | 184 °C | 4.933 g/cm³ | 2.66 |
| Astatine | 85 | [210] | 302 °C (est.) | — | 7 g/cm³ (pred.) | 2.2 |
| Tennessine | 117 | [294] | — | — | 7.2 g/cm³ (pred.) | — |