All practice pages/Interatomic theory/Elements
Element 53, a halogen
A shiny violet-black solid that turns straight into purple vapor, and the one halogen your body cannot do without.
Period 5, group 17: the bottom of the 7. The other halogens are tinted, and every cell opens its own article.
Seven valence electrons, one short of eight. Two iodine atoms share one pair, a single bond, the longest and weakest of the seven.
The rarest of the stable halogens, about 0.00005% of the crust. Seawater has only 0.05 mg per liter, but seaweed concentrates it thousands of times over. Today most comes from natural brines in Japan and from caliche ore in Chile, where it occurs as iodate.
Bernard Courtois found it in 1811 in the ash of seaweed he was processing into saltpeter for Napoleon's gunpowder: too much sulfuric acid gave off a violet vapor that condensed into dark crystals. Joseph Gay-Lussac named it from the Greek iodes, violet-colored.
Iodized salt prevents iodine deficiency, iodine solutions and povidone-iodine disinfect skin before surgery, and iodine compounds make X-ray contrast agents, industrial catalysts and cloud-seeding silver iodide. Radioactive iodine-131 treats thyroid disease.