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17.4B: Dichlorine, Dibromine, and Diiodine

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  • Iodine

    Iodine was discovered by French chemist Bernard Courtois in 1811. His father was a manufacturer of saltpeter (a vital part of gunpowder) and at the time of the French Napoleonic Wars, saltpeter was in great demand. Saltpeter produced from French niter beds required sodium carbonate, which could be isolated from seaweed collected on the coasts of Normandy and Brittany. To isolate the sodium carbonate, the seaweed was burned and the ash washed with water. The remaining waste was destroyed by adding sulfuric acid. Courtois once added excessive sulfuric acid and a cloud of purple vapour rose. He noted that the vapour crystallized on cold surfaces, making dark crystals. Courtois suspected that this was a new element but lacked the funds to pursue it further.

    Samples of the material reached Humphry Davy and Joseph Louis Gay-Lussac and in early December 1813 both claimed that they had identified a new element. Arguments erupted between them over who had identified iodine first, but both scientists acknowledged Courtois as the first to isolate the element.

    Iodine is found on Earth mainly as the highly water-soluble iodide ion I-, concentrated in oceans and brine pools. Like the other halogens, free iodine occurs mainly as a diatomic molecule I2. In the universe and on Earth, iodine's high atomic number makes it a relatively rare element. However, its presence in ocean water has given it a role in biology. It is the heaviest essential element widely utilized by life in biological functions.

    Under standard conditions, iodine is a bluish-black solid that sublimes to form a noxious violet-pink gas. It melts at 113.7 °C (386.85 K) and forms compounds with many elements but is less reactive than the other halogens, and has some metallic light reflectance.

    Elemental iodine is slightly soluble in water, with one gram dissolving in 3450 ml at 20 °C and 1280 ml at 50 °C; potassium iodide may be added to increase solubility via formation of triiodide ions (I3-). Nonpolar solvents such as hexane and carbon tetrachloride provide a higher solubility.

    Iodine normally exists as a diatomic molecule with an I-I bond length of 270 pm, one of the longest single bonds known. The I2 molecules tend to interact via weak London dispersion forces, and this interaction is responsible for the higher melting point compared to more compact halogens, which are also diatomic. Since the atomic size of iodine is larger, its melting point is higher.

    The I-I bond is relatively weak, with a bond dissociation energy of 151 kJmol-1, and most bonds to iodine are weaker than for the lighter halides. One consequence of this weak bonding is the relatively high tendency of I2 molecules to dissociate into atomic iodine.

    orthorhombic structure of I2
    a= 0.72701, b= 0.97934, c= 0.47900 nm

    The halogens, Cl2, Br2, and I2 adopt similar orthorhombic structures in which diatomic molecules lie in layers:
    Cl a= 0.624 b= 0.826 c= 0.448 nm
    Br a= 0.667 b= 0.872 c= 0.448 nm
    I a= 0.72701, b= 0.97934, c= 0.47900 nm
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