It is used as a convenient gamma-emitting tag for proteins in biological assays, and a few nuclear medicine imaging tests where a longer half-life is required. The next-longest-lived radioisotope, iodine-125, has a half-life of 59 days. This nuclide is also newly made by cosmic rays and as a byproduct of artificial nuclear fission, which it is used to monitor as a very long-lived environmental contaminant. Instead, iodine-129 is an extinct radionuclide, and its presence in the early Solar System is inferred from the observation of an excess of its daughter xenon-129. This is long enough to make it a permanent fixture of the environment on human time scales, but far too short for it to exist as a primordial isotope today. ![]() The longest-lived radioisotope, 129I, has a half-life of 15.7 million years. Of the 37 known (characterized) isotopes of iodine, only one, 127I, is stable. One consequence of this weak bonding is the relatively high tendency of I 2 molecules to dissociate into atomic iodine. The I-I bond is relatively weak, with a bond dissociation energy of 36 kcal/mol, and most bonds to iodine are weaker than for the lighter halides. The crystal motif in the Hermann–Mauguin notation is Cmca (No 64), Pearson symbol oS8. The solid crystallizes as orthorhombic crystals. Since the atomic size of iodine is larger, its melting point is higher. The I 2 molecules tend to interact via the weak van der Waals forces called the London dispersion forces, and this interaction is responsible for the higher melting point compared to more compact halogens, which are also diatomic. Iodine normally exists as a diatomic molecule with an I-I bond length of 270 pm, one of the longest single bonds known. A metal ion may replace the solvent, in which case the two species exchange electrons, the ion undergoing π backbonding. Charge-transfer complexes form when iodine is dissolved in polar solvents, modifying the energy distribution of iodine's molecular orbitals, hence changing the colour. Polar solutions are brown, reflecting the role of these solvents as Lewis bases, while nonpolar solutions are violet, the color of iodine vapor. Nonpolar solvents such as hexane and carbon tetrachloride provide a higher solubility. ![]() Melting at 113.7 C, it forms compounds with many elements but is less reactive than the other members of its group, the halogens, and has some metallic light reflectance.Įlemental iodine is slightly soluble in water, with one gram dissolving in 3450 ml at 20 ☌ and 1280 ml at 50 ☌ potassium iodide may be added to increase solubility via formation of triiodide ions. Under standard conditions, iodine is a bluish-black solid with a metallic lustre, appearing to sublimate into a noxious violet-pink gas, the colour due to absorption of visible light by electronic transitions between the highest occupied and lowest unoccupied molecular orbitals. Iodine deficiency affects about two billion people and is the leading preventable cause of intellectual disabilities. Iodine's rarity in many soils, due to initial low abundance as a crust-element, and also leaching of soluble iodide by rainwater, has led to many deficiency problems in land animals and inland human populations. It is the heaviest essential element utilized widely by life in biological functions (only tungsten, employed in enzymes by a few species of bacteria, is heavier). However, its presence in ocean water has given it a role in biology. In the universe and on Earth, iodine's high atomic number makes it a relatively rare element. Like the other halogens, free iodine occurs mainly as a diatomic molecule I 2, and then only momentarily after being oxidized from iodide by an oxidant like free oxygen. I −, which concentrates it in oceans and brine pools. Iodine is found on Earth mainly as the highly water-soluble iodide ion A number of iodine radioisotopes, such as 131I, are also used in medical applications. Iodine's relatively high atomic number, low toxicity, and ease of attachment to organic compounds have made it a part of many X-ray contrast materials in modern medicine. ![]() Iodine and its compounds are primarily used in nutrition, and industrially in the production of acetic acid and certain polymers. The name is from Greek ioeidēs, meaning violet or purple, due to the color of elemental iodine vapor. Iodine is a chemical element with symbol I and atomic number 53.
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