Iodine
[Kr] 4d¹⁰ 5s² 5p⁵ · 2 · 8 · 18 · 18 · 7
The only halogen solid at room temperature, and the only element whose deficiency is still the leading preventable cause of intellectual disability worldwide. Its violet vapour gave it its name.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 53
- Atomic mass
- 126.904 а.е.м.
- Electron configuration
- [Kr] 4d¹⁰ 5s² 5p⁵
- Electrons per shell
- 2 · 8 · 18 · 18 · 7
- Block
- p-block
- Group
- Halogens
- Period
- 5
- Electronegativity (Pauling)
- 2.66
- Electronegativity (Allen)
- 2.36
- Atomic radius
- 115 pm
- Covalent radius
- 139 pm
- Van der Waals radius
- 198 pm
- Ionisation energy 1
- 1 008.4 kJ/mol
- Ionisation energy 2
- 1 845.9 kJ/mol
- Ionisation energy 3
- 3 180 kJ/mol
- Electron affinity
- 295.2 kJ/mol
- Common oxidation states
- -1, +1, +5, +7
- Oxidation states
- -1, +1, +3, +4, +5, +6, +7
Physical properties
- State at 25 °C
- solid
- Density
- 4.933 g/cm³
- Melting point
- 386.85 K · 113.7 °C
- Boiling point
- 457.4 K · 184.3 °C
- Speed of sound
- no data
Thermal properties
- Heat of fusion
- 15.52 kJ/mol
- Heat of vaporisation
- 41.57 kJ/mol
- Specific heat capacity
- 0.214 J/(g·K)
- Thermal conductivity
- 0.449 W/(m·K)
Mechanical properties
- Young's modulus
- no data
- Shear modulus
- no data
- Bulk modulus
- 7.7 GPa
- Poisson ratio
- no data
- Mohs hardness
- no data
- Brinell hardness
- no data
Electrical and magnetic properties
- Electrical resistivity
- 1.3·10¹³ nΩ·m
- Magnetic ordering
- diamagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- no data
Crystal structure
- Crystal structure
- orthorhombic
- Lattice constants
- a = 727.24 pm · b = 479.79 pm · c = 979.28 pm
Abundance
- In the crust
- 0.45 mg/kg
- In the ocean
- 0.06 mg/L
- In the universe
- 0.001 mg/kg
- In the human body
- 0.2 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 127I | 100 % | stable | — |
| 129I | — | 15.69 Myr | β− |
| 131I | — | 8.02 d | β− |
Iodine-131, half-life 8 days, is the main source of early dose after nuclear accidents and simultaneously a treatment for hyperthyroidism and thyroid cancer.
Discovery
- Year of discovery
- 1811
- Discovered by
- Bernard Courtois
- Where
- France
- Origin of the name
- from Greek iodes (violet), the colour of its vapour
History
Bernard Courtois was treating seaweed ash with sulfuric acid while making saltpetre in 1811 when violet vapour rose off it. Gay-Lussac proved in 1813 that it was a new element and named it for the colour.
Where it occurs
0.45 mg/kg of the crust, but seaweed concentrates iodine thirty thousandfold. The industrial sources are Chilean nitrate and Japanese subsurface brines.
How it is produced
From oilfield brines and nitrate ores, by chlorine displacement or iodate reduction.
Role in living things
Iodine's only known function is making the thyroid hormones thyroxine and triiodothyronine, which set metabolic rate and brain development. An adult needs 150 µg a day.
Safety
Deficiency causes goitre and, in a foetus, irreversible impairment of brain development. Excess is harmful too and suppresses thyroid function. Iodine-131 from fallout concentrates in the thyroid and causes cancer, which is why stable potassium iodide is issued after accidents.
Uses
- Iodised table salt — the largest micronutrient deficiency programme in history
- X-ray contrast media for angiography and CT
- Antiseptics: tincture of iodine, povidone-iodine
- Catalysts for acetic acid production
- Halogen lamps and polarising film
Curiosities
- Iodine sublimes straight from solid to violet vapour at ordinary pressure, skipping the liquid.
- Taking potassium iodide tablets after a radiation accident saturates the thyroid so it cannot take up the radioactive isotope.
- Salt iodisation raised average IQ by several points in iodine-deficient regions.
- Iodine is the heaviest element that living organisms require.