Iodine
Element 53 · Halogen
Iodine is the heaviest of the common halogens and the only one that is a solid at room temperature — lustrous purple-black crystals that release a striking violet vapor when warmed, subliming without melting. Like the other halogens it is a strong oxidizing agent, though milder than chlorine or bromine. Iodine is biologically essential: the thyroid gland needs it to make the hormones that regulate metabolism, which is why table salt is fortified with iodide in many countries. In the lab, iodine tincture disinfects wounds, silver iodide seeds clouds, and iodine-starch gives the classic deep-blue test for starch.
Key facts
| Atomic number | 53 |
|---|---|
| Atomic weight | 126.90 |
| Category | Halogen |
| Group / Period | 17 / 5 |
| Block | p |
| Electron configuration | [Kr] 4d10 5s2 5p5 |
| Common oxidation states | -1, +1, +3, +5, +7 |
| State at STP | Solid |
| Electronegativity | 2.66 |
| Atomic radius | 133 pm (covalent) |
| Density | 4.933 g/cm³ |
| Melting point | 113.7 °C |
| Boiling point | 184.3 °C |
Occurrence
Iodine is found dissolved in seawater and concentrated in underground brines and in Chilean caliche deposits. Seaweed accumulates iodine from seawater and was historically a commercial source.
Applications
Antiseptics and disinfectants
Iodized table salt
LCD polarizing filters
Contrast agents for X-ray imaging
Common compounds
- Potassium iodide
- Sodium iodide
- Silver iodide
- Iodine tincture
Iodine is the only halogen that is a solid at room temperature — heat it gently and it sublimes directly into a deep violet gas.
Data source: IUPAC standard atomic weights 2021; NIST physical property data