Periodic table › Iodine
1. Identity and classification
Colored, pungent, reactive diatomic nonmetal (gas, volatile liquid, or crystalline solid)
- Name and symbol
- Iodine (Iodine) · I
- Atomic number
- 53
- Relative atomic weight / mass number
- 126.90 ± 0.01
- Group · period · block
- 17 · 5 · p
- Category
- Halogen
- State near room temperature
- Solid
- Stable isotopes
- At least one stable isotope
- CAS number
- 7553-56-2
Square brackets [A] indicate a representative isotope mass number, not an average atomic weight. Neutron counts differ between isotopes.
2. Atomic structure and electron configuration
- Protons · electrons in a neutral atom
- 53 · 53
- Neutrons in reference isotope
- 74 (127I)
- Full electron configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁵- Abbreviated configuration
[Kr]5s² 4d¹⁰ 5p⁵- Electrons per shell
- 2, 8, 18, 18, 7
- Listed oxidation states
- +7, +5, +1, -1
- Electronegativity
- 2.66 (Pauling)
- First ionization energy
- 1008.368 kJ/mol · 10.451 eV
- Electron affinity
- 295.149 kJ/mol
- Covalent radius
- 139 pm
- van der Waals radius
- 198 pm
Configurations of superheavy elements may include theoretical predictions. Different definitions of atomic radius are not interchangeable.
3. Physical and thermal properties
- Density
- 4.93 g/cm³
- Melting point
- 386.85 K · 113.7 °C · 236.66 °F
- Boiling point
- 457.55 K · 184.4 °C · 363.92 °F
- Crystal structure
- Orthorhombic
- Specific heat
- 0.214 J/(g·K)
- Thermal conductivity
- 0.449 W/(m·K)
- Enthalpy of fusion
- 15.52 kJ/mol
- Enthalpy of vaporization
- 41.57 kJ/mol
- Magnetic ordering
- Diamagnetic
- Mohs hardness
- Not established
- Speed of sound
- Not established
Density and structure depend on temperature, pressure and allotrope. 25°C is a room-temperature reference, not STP. Unestablished values are not zero.
4. Chemical behavior and compounds
Iodine forms diatomic I₂ molecules and both positive and negative oxidation states.
Selected compounds: IO / I₂O₃ (Oxide) · IClₙ (Chloride) · IFₙ (Fluoride)
Reaction reference
Iodine is less reactive than chlorine or bromine.
Iodine has low solubility in pure water; iodide helps dissolve it as triiodide.
5. Isotopes
| Isotope | Status | Natural abundance | Half-life |
|---|---|---|---|
| 127I | Stable | 100% | Stable |
| 128I | Radioactive | — | 24.99 m |
Half-life units: s seconds, m minutes, h hours, d days, Y years. A dash means no natural-abundance value is given, not zero. Listed records are selected ground states; metastable states are distinct.
6. Discovery and occurrence
- Discovery
- 1811 CE
- Scientists associated with discovery
- Bernard Courtois
- Location
- France (Dijon)
- Origin
- Natural
- Listed crustal abundance
- Not established
Minerals and naming reference
Caliche nitrate deposits (Chile), subsurface brines, seaweed kelp
Named from scientific/historical roots associated with Iodine (I), discovered or characterized by Bernard Courtois.
7. Applications
Thyroid hormones (T₃ triiodothyronine & T₄ thyroxine), iodized table salt, Povidone-iodine surgical antiseptic, X-ray CT contrast agents
Detailed applications by field
- everyday
- Thyroid hormones (T₃ triiodothyronine & T₄ thyroxine), iodized table salt, Povidone-iodine surgical antiseptic, X-ray CT contrast agents
- industry
- Used in halogen processing, specialized alloys, and chemical manufacturing (Thyroid hormones (T₃ triiodothyronine & T₄ thyroxine), iodized table salt, Povidone-iodine surgical antiseptic, X-ray CT contrast agents).
- electronics
- Applied in electronic components, semiconductors, or functional coatings where Iodine's electronic structure ([Kr] 4d¹⁰ 5s² 5p⁵) provides target conductivity.
- medicine
- Diagnostic imaging, biomedical instrumentation, or pharmaceutical synthesis research.
- energy
- Utilized in power generation, battery materials, catalysts, or nuclear technology involving Iodine.
- aerospace
- High-performance materials, structural alloys, or space instrumentation utilizing Iodine.
- agriculture
- Soil geochemistry analysis, trace micronutrient research, or analytical environmental monitoring.
- science
- Spectroscopic standard, crystallography, and quantum chemistry studies of Period 5 p-block elements.
8. Biological role and safety
Hazard depends on the isotope, chemical form, dose and exposure route.
Elemental substances, ions, compounds and radioisotopes can have different health effects. This is an educational reference; laboratory work requires the material safety data sheet and trained supervision.
Biological role reference
Classified as essential in human and biological systems.