![Iodine element card: symbol I, atomic number 53, atomic mass 126.90, electron configuration [Kr] 4d¹⁰ 5s² 5p⁵, oxidation states +7 +5 +3 +1 -1, Halogen, group 17, period 5, solid at room temperature.](https://stage.chemistry-dictionary.com/wp-content/uploads/2026/09/iodine-chemical-element.webp)
Iodine belongs to the halogen family in group 17 on the periodic table. It is the least reactive and most electropositive of the stable halogens. At room temperature, pure iodine is a shiny, dark crystal.
When heated, the solid undergoes sublimation, turning directly into a thick violet gas. This gas has a sharp, irritating smell that is similar to chlorine. In nature, iodine mostly exists as the iodide ion, which has the formula I-.
We find this dissolved ion in oceans and deep underground brine pools. A common student misconception is that solid iodine cannot melt at normal pressure. If you heat the crystals slowly, they will melt into a runny liquid.
This liquid forms at 113.7 degrees Celsius before boiling into a gas. Another mistake is thinking iodized salt contains pure iodine. In reality, salt makers mix in small amounts of potassium iodide, or KI.
Iodine forms colorful mixtures called coordination complexes with other substances. For example, mixing iodine with starch produces a very dark blue-black color. This reaction is a classic test for detecting starch in food.
Iodine also dissolves easily in nonpolar liquids like hexane. This dissolving process produces a beautiful purple solution that is easy to identify.