Iodine
Definition and meaning of Iodine in chemistry.
![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.](/assets/images/terms/iodine-chemical-element.webp)
Iodine is a nonmetallic, dark-gray solid element with the symbol I and atomic number 53. It is the heaviest of the stable halogen elements on the periodic table. Humans need this essential nutrient to make thyroid hormones that control growth.
In more detail
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.
Key facts
| Field | General Chemistry |
|---|---|
| Symbol | I |
| Atomic number | 53 |
| Atomic mass | 126.90 u |
| Category | Halogen |
| Melting point | 113.7 °C |
Doctors use iodine solutions as a strong antiseptic to kill germs on skin. This liquid is called a tincture of iodine. It contains elemental iodine and sodium iodide dissolved in alcohol and water. Applying it to a cut prevents infections before major operations.
Frequently asked questions
Why does solid iodine turn directly into a gas?
This process is called sublimation and happens because the solid has weak forces between molecules. Gentle heating easily breaks these bonds.
Why is potassium iodide added to table salt?
Our bodies need iodine to make thyroid hormones. Salt makers add potassium iodide to prevent thyroid diseases like goiter.
How does the starch test with iodine work?
Iodine molecules fit inside the coiled structure of starch molecules. This change in shape creates a dark blue coordination complex.
Related terms
Sources & references
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