Iodine
Definition and meaning of Iodine in chemistry.
Iodine is a nonmetallic, dark-gray to purple-black solid element with the symbol I and atomic number 53. It is the heaviest of the stable halogens and is an absolute essential trace nutrient for proper thyroid gland function and hormone synthesis in humans.
In more detail
Located in group 17 of the periodic table, iodine is the least reactive and the most electropositive of the stable halogens, displaying a unique blend of nonmetallic and weak metallic properties. At room temperature, it exists as a lustrous, dense crystalline solid that slowly sublimes to form a striking violet gas with a harsh, irritating odor. Iodine primarily occurs as a highly water-soluble iodide ion (I-) in the oceans and in underground brine pools, making it relatively rare in the Earth's crust compared to other halogens like chlorine and fluorine. It readily forms compounds with many elements to yield iodides, but it generally requires higher temperatures to react than its lighter group members. Iodine is particularly notable for forming deeply colored coordination complexes, such as its reaction with starch to produce an intense blue-black color, and for its ability to dissolve in nonpolar organic solvents like hexane or chloroform to produce vibrant purple solutions.
Key facts
| Field | General Chemistry |
|---|---|
| Symbol | I |
| Atomic number | 53 |
| Atomic mass | 126.90 u |
| Category | Halogen |
| State at room temperature | Solid |
| Melting point | 113.7 °C |
| Boiling point | 184.3 °C |
| Year discovered | 1811 |
Iodine is widely used in global medicine as a powerful broad-spectrum antiseptic, typically formulated as a solution of elemental iodine and sodium iodide in a mixture of water and alcohol known as tincture of iodine, which is used to thoroughly clean wounds and prepare patient skin for surgical procedures.
Frequently asked questions
Why is iodine routinely added to commercial table salt?
Iodine is added to table salt in the form of potassium iodide to prevent widespread iodine deficiency, which can lead to severe thyroid gland problems such as goiter, metabolic imbalances, and significant developmental delays in children.
Does solid iodine melt under normal atmospheric conditions or just sublime?
While it is commonly taught in basic chemistry that iodine sublimes directly from a solid to a gas at standard atmospheric pressure, it actually melts into a liquid at 113.7 °C before boiling at 184.3 °C if it is heated gently and in a contained environment.