Iodine (I)
“A violet vapour that rose from seaweed ash — now it guards every thyroid in Bangladesh through a pinch of iodized salt.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
The notes
Characteristics
- A halogen of Group 17, Period 5, p-block: configuration [Kr] 4d¹⁰ 5s² 5p⁵ leaves it one electron short of an octet, so it seizes electrons eagerly.
- Solid at room temperature yet sublimes with ease — the violet cloud of drifting I₂ molecules is one of chemistry’s loveliest sights.
- Almost insoluble in pure water but dissolves happily in potassium iodide solution (Lugol’s iodine), the basis of its medical preparations.
- An essential trace element: the thyroid gland traps iodine to build the hormones T₃ and T₄, which set the pace of metabolism; shortage swells the gland into a goitre.
A lustrous, blue-black crystalline solid that gleams almost like a metal — the only halogen that is solid at room temperature. Warmed gently, it skips the liquid stage entirely, subliming straight into a beautiful violet vapour.
Where you meet it
✦ Uses
- Iodized salt — Bangladesh’s national salt-iodization programme doses every packet with potassium iodate, driving back the goitre that once troubled millions.
- Wound antiseptics: tincture of iodine and povidone-iodine — the orange “Betadine” stocked in every pharmacy — disinfect cuts and scrapes.
- Potassium iodide tablets that flood the thyroid with stable iodine during nuclear emergencies, shutting out radioactive iodine-131.
- Iodine-based contrast dyes that light up blood vessels in X-ray and CT scans, plus silver iodide crystals for seeding rain clouds.
◍ Everyday examples
- The salt tin in a Bangladeshi kitchen is stamped “iodized” — a tiny daily dose of iodine rides along with every meal.
- A drop of orange Betadine on a scraped knee is iodine working as an antiseptic.
- Sea fish, shrimp and seaweed concentrate iodine from seawater — the richest natural sources in the diet.
History & name
In 1811 the Parisian saltpetre-maker Bernard Courtois was burning seaweed to obtain soda for gunpowder in the middle of the Napoleonic wars. One day he tipped too much concentrated sulfuric acid into the mother liquor — violet fumes billowed out and settled on cold glass as dark, glittering crystals. Samples reached the chemists Clément and Desormes, and by 1813 both Gay-Lussac and Davy had shown the crystals were a new element; Gay-Lussac named it iodine, from the Greek iōdēs, “violet-coloured”, in honour of the vapour Courtois first saw.