Heat dark violet crystals and watch them vanish into a thick purple vapor — no liquid in between. This is iodine, the only halogen you can hold in your hand. French chemist Bernard Courtois discovered it in 1811 while processing seaweed ash, stunned by the mysterious violet cloud rising from his experiment.
But iodine's real superpower lies inside us. Without it, the thyroid gland cannot produce hormones. Without those hormones, metabolism stalls, the brain suffers, and children's development slows. That is why table salt is iodized in most countries around the world.
In pure form, iodine is a lustrous dark-violet crystalline solid with a sharp, distinctive smell. It barely dissolves in water but dissolves readily in alcohol — hence the familiar brown tincture from the medicine cabinet. In nature, iodine is found in seawater, seaweed, Chilean saltpeter deposits, and underground brines.
The human body needs just 150 micrograms of iodine per day — less than a grain of sand. Yet without this tiny amount, the thyroid gland cannot function properly, and it controls the metabolism of the entire body.
Crystalline iodine leaves stubborn brown stains on skin and clothing. Concentrated solutions irritate skin and mucous membranes. Iodine vapor is harmful to eyes and the respiratory tract — work should be done in a fume hood. Excess iodine intake is dangerous: it can cause hyperthyroidism, rashes, and thyroid problems. The recommended daily intake for adults is 150 micrograms, and toxicity begins at about 1 mg per day with prolonged use.
The name 'iodine' comes from Greek 'iodes' meaning violet. When Bernard Courtois first saw purple vapor rising from seaweed ash, he could hardly believe his eyes.
Iodine is a sublimation champion. It transforms from crystals directly into gas, skipping the liquid phase entirely. The beautiful purple vapor is a classic chemistry demonstration.
Iodine deficiency is the world's most common cause of preventable intellectual disability. Nearly 2 billion people are at risk, mostly in mountainous and landlocked regions far from the sea.
During nuclear accidents, potassium iodide tablets are distributed to the public. Stable iodine fills up the thyroid gland, blocking radioactive iodine-131 from accumulating.
Forensic investigators use iodine vapor to reveal fingerprints on paper. The iodine adsorbs onto oily residues left by skin, making invisible prints clearly visible.
Seaweed can concentrate iodine from water by a factor of 30,000. Kelp contains up to 0.45% iodine — an absolute record among living organisms.
| Isotope | Mass (u) | Abundance | Half-life | Decay |
|---|---|---|---|---|
127I | 126.904473 | 100.00% | stable | — |
129I☢ | 128.904988 | synthetic | 1.57×10⁷ years | β− |
131I☢ | 130.906125 | synthetic | 8.02 days | β− |
Treatment of seaweed ash