Quick Facts
| Property | Value |
|---|---|
| Symbol | I |
| Atomic Number | 53 |
| Atomic Mass | 126.90 u |
| Category | Halogen |
| Period | 5 |
| Group | 17 |
| Block | p |
| Electron Configuration | [Kr] 4d¹⁰ 5s² 5p⁵ |
Physical Properties
| Property | Value |
|---|---|
| State at 20°C | Solid |
| Density | 4.933 g/cm³ |
| Melting Point | 113.7°C (386.85 K) |
| Boiling Point | 184.3°C (457.45 K) |
| Appearance | Lustrous purple-black crystalline solid |
Atomic Properties
| Property | Value |
|---|---|
| Electron Configuration | [Kr] 4d¹⁰ 5s² 5p⁵ |
| Electronegativity | 2.66 (Pauling scale) |
| First Ionization Energy | 1008.4 kJ/mol |
| Atomic Radius | 140 pm |
| Covalent Radius | 139 pm |
| Van der Waals Radius | 198 pm |
History and Discovery
Discovered by: Bernard Courtois Year of Discovery: 1811 Location: Paris, France
Etymology
Named from the Greek "iodes" meaning "violet," describing the color of its vapor.
Discovery Story
Bernard Courtois discovered iodine in 1811 while extracting sodium and potassium compounds from seaweed ash (kelp). He noticed violet vapors when treating the ash with sulfuric acid. Humphry Davy and Joseph Louis Gay-Lussac confirmed it as a new element in 1813-1814.
Isotopes
| Isotope | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|
| ¹²⁷I | 100% | Stable | - |
| ¹²⁹I | Trace | 15.7 million years | β⁻ |
| ¹³¹I | Synthetic | 8.02 days | β⁻ |
Occurrence
Natural Abundance
Iodine comprises about 0.45 ppm of Earth's crust. It is concentrated in seawater (about 0.05 ppm), seaweed, and brines. Major producers are Chile, Japan, and the United States.
Extraction and Production
- Brine Extraction: From oil field brines and natural brines
- Nitrate Deposits: Chilean caliche deposits
- Seaweed: Historical source, minor today
- Global production: about 30,000 tons annually
Applications and Uses
Nutrition and Medicine
- Iodized salt (preventing goiter)
- Thyroid hormone synthesis
- Antiseptics (povidone-iodine, tincture of iodine)
- Contrast agents for X-rays
Industrial Applications
- Catalysts
- Animal feed supplements
- Stabilizers for nylon
- Printing inks and dyes
Chemical Applications
- Organic synthesis
- Polarizing films for LCD screens
- Pharmaceuticals
Other Uses
- Cloud seeding (silver iodide)
- Photography (historical)
- Water purification
Biological Role
Iodine is an essential trace element for humans and animals. It is required for thyroid hormone synthesis.
In the Human Body
- About 15-20 mg in adult body
- Concentrated in thyroid gland
- Essential for thyroid hormones (T3, T4)
- Required for metabolism and development
- Daily requirement: 150 μg
Deficiency Effects
Iodine deficiency causes goiter, hypothyroidism, and developmental problems. It is a major global health issue.
Dietary Sources
Seafood, seaweed, dairy products, and iodized salt are primary sources.
Safety and Hazards
Toxicity
Iodine is essential but toxic in excess. Elemental iodine is corrosive. High doses can cause thyroid dysfunction.
Handling Precautions
- Iodine vapor irritates eyes and respiratory tract
- Causes skin stains and burns
- Keep away from heat (sublimes easily)
- Radioactive I-131 requires special handling
Environmental Impact
Iodine is naturally occurring and essential. Radioactive I-131 from nuclear accidents (Chernobyl, Fukushima) is a significant hazard to the thyroid.
Interesting Facts
- About 2 billion people worldwide are at risk of iodine deficiency
- Iodine sublimes directly from solid to purple vapor
- Iodized salt has nearly eliminated goiter in developed countries
- Iodine-131 from nuclear accidents accumulates in the thyroid
- Seaweed can contain over 1,000 times the iodine concentration of seawater