Iodine
halogenProperties
| Property | Value |
|---|---|
| Atomic Mass | 126.9 amu |
| Category | halogen |
| Group | 17 |
| Period | 5 |
| Electron Configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5 |
| Electronegativity | 2.66 (Pauling) |
| Oxidation States | -1, 1, 3, 5, 7 |
| Melting Point | 386.85 K (113.7 °C) |
| Boiling Point | 457.4 K (184.3 °C) |
| Density | 4.933 g/cm³ |
| Discovered By | Bernard Courtois (1811) |
About Iodine
Open a bottle of iodine crystals on a warm bench and you'll see the violet vapor before you see the solid — the vapor pressure at room temperature is high enough that I₂ sublimes visibly without ever melting. That's the property that gave it its name (ioeides, violet) and the property that lets you do iodine staining of TLC plates with nothing more than a closed jar. Chemically, iodine is the gentlest of the halogens: still a strong enough oxidizer to drive iodometric titrations against thiosulfate, but mild enough that the I⁻ ion is a useful nucleophile in SN2 reactions and a good leaving group when displaced. Biologically the entire human use of iodine comes down to thyroxine — T4 carries four iodine atoms per molecule, and the thyroid concentrates dietary I⁻ by a factor of about 30. Most commercial iodine is pulled from caliche brines in Chile or natural-gas formation waters in Japan.
Fun Fact
Add a drop of starch to an iodometric titration and you can see the endpoint at I₂ concentrations down around 2 × 10⁻⁵ M — the deep blue-black color is a charge-transfer complex of triiodide threaded inside the amylose helix.
Common Uses
- Iodine-starch indicator and iodometric titration of oxidizers
- Povidone-iodine and tincture-of-iodine topical antiseptics
- Iodinated contrast agents for CT angiography and urography
- Silver iodide nuclei for cloud seeding and photographic emulsions
- Catalyst in the Cativa process for acetic acid manufacture