All practice pages/Interatomic theory/Diatomics
The molecule
Two iodine atoms sharing a single bond: shiny dark crystals that turn into violet vapor, and the antiseptic in the brown bottle.
The least reactive of the four common halogens, but still a halogen. It combines with aluminum in a dramatic, violet-smoking reaction started by a drop of water, and with hot metals generally. It is displaced from iodides by both chlorine and bromine. With starch it forms an intense blue-black complex, sensitive enough to detect a trace of either, which is why iodine solution is used to test for starch and starch is used to signal the end of iodine titrations. Sodium thiosulfate turns it back into colorless iodide.
From natural brines, mostly in Japan: chlorine oxidizes the dissolved iodide to iodine, which is blown out with air and collected. In Chile it is recovered from caliche ore, where it occurs as iodate that is reduced with sodium bisulfite. Historically it came from burning seaweed.
Antiseptics: tincture of iodine, and the brown povidone-iodine scrub used before surgery. Potassium iodide or iodate added to table salt prevents goiter and the developmental damage that iodine deficiency causes, one of the cheapest public-health measures ever. Iodine compounds are the contrast agents that make blood vessels show up in X-ray scans, radioactive iodine-131 treats overactive thyroids, iodine is a catalyst in making acetic acid, and silver iodide seeds clouds.
The vapor irritates eyes and lungs, and the solid stains skin and clothing brown. A few grams swallowed is dangerous. Potassium iodide tablets are stockpiled near nuclear plants to saturate the thyroid so radioactive iodine cannot lodge there.