Iodine


 * Iodine** is a chemical element with the symbol **I** and atomic number 53. The name is from Greek ἰοειδής //ioeidēs//, meaning violet or purple, due to the color of elemental iodine vapor. Iodine and its compounds are primarily used in nutrition, and industrially in the production of acetic acid and certain polymers. 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. It can also be injected into the body to watch for the correct flow of blood through veins. Iodine is found on Earth mainly as the highly water-soluble iodide I-, which concentrates it in oceans and brine pools. 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. Iodine is required by higher animals, which use it to synthesize thyroid hormones, which contain the element. Because of this function, radioisotopes of iodine are concentrated in the thyroid gland along with nonradioactive iodine.

a nonmetallic halogen element occurring at ordinary temperatures as a grayish-black crystalline solid that sublimes toa dense violet vapor when heated: used in medicine as an antiseptic. Iodine is in group 17, halogen, and period 5 on the periodic table.


 * Iodine** (from the Gr. //Iodes//, meaning "violet"), is a __[|chemical element] __ in the __[|periodic table] __that has the symbol **I** and __[|atomic number] __ 53. This is an insoluble element that is required as a __[|trace element] __ for living __[|organisms] __. Chemically, iodine is the least reactive of the __[|halogens] __, and the most electropositive metallic halogen. Iodine is primarily used in__[|medicine] __, __[|photography] __ and in dyes.



Solid iodine is a blue-black color and is shiny. Iodine is a halogen which means it is a non-metal. Your thyroid gland uses iodine to produce the hormones thyroxine and triiodotyronine. There are a number of isotopes for iodine. Almost all are radioactive. The thyroid gland uses iodine to make the hormones thyroxine and triiodotyronine.



Structure and bonding
Iodine normally exists as a diatomic molecule with a I-I bond length of 270 pm, [|[10]] one of the longest single bonds known. The I2 molecules tend to interact via the weak [|van der Waals] force called the [|London 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 solid crystallizes as [|orthorhombic] crystals. The crystal motif in the [|Hermann–Mauguin notation] is Cmca (No 64), [|Pearson symbol] oS8. 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. One consequence of this weak bonding is the relatively high tendency of I2 molecules to dissociate into atomic iodine.



Isotopes and their applications
Of the 37 known (characterized) [|isotopes] of iodine, only one, 127I, is stable. The longest-lived radioisotope, 129I, has a half-life of 15.7 million years. 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. 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 nuclide is also newly-made by cosmic rays and as a byproduct of human nuclear fission, which it is used to monitor as a very long-lived environmental contaminant. The next-longest-lived radioisotope, [|iodine-125], has a half-life of 59 days. 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. It is also commonly used in [|brachytherapy] implanted capsules, which kill tumors by local short-range gamma radiation (but where the isotope is never released into the body). [|Iodine-123] (half-life 13 hours) is the isotope of choice for nuclear medicine imaging of the thyroid gland, which naturally accumulates all iodine isotopes. [|Iodine-131] (half-life 8 days) is a beta-emitting isotope, which is a common nuclear fission product. It is preferably administered to humans only in very high doses which destroy all tissues that accumulate it (usually the thyroid), which in turn prevents these tissues from developing cancer from a lower dose (paradoxically, a high dose of this isotope appears safer for the thyroid than a low dose). Like other radioiodines, I-131 accumulates in the thyroid gland, but unlike the others, in small amounts it is highly carcinogenic there, it seems, owing to the high local cell mutation due to damage from [|beta decay]. Because of this tendency of 131I to cause high damage to cells that accumulate it and other cells near them (0.6 to 2 mm away, the range of the beta rays), it is the only iodine radioisotope used as direct therapy, to kill tissues such as cancers that take up artificially iodinated molecules (example, the compound [|iobenguane], also known as MIBG). For the same reason, only the iodine isotope I-131 is used to treat [|Grave's disease] and those types of thyroid cancers (sometimes in metastatic form) where the tissue that requires destruction, still functions to naturally accumulate iodide. Nonradioactive ordinary [|potassium iodide] (iodine-127), in a number of convenient forms (tablets or solution) may be used to saturate the thyroid gland's ability to take up further iodine, and thus protect against accidental contamination from iodine-131 generated by [|nuclear fission] accidents, such as the [|Chernobyl disaster] and more recently the [|Fukushima I nuclear accidents], as well as from contamination from this isotope in [|nuclear fallout] from [|nuclear weapons].



__** Biological role **__ Iodine is an essential trace element for life, the heaviest element commonly needed by living organisms. Only tungsten, a component of a few bacterial enzymes, has a higher atomic number and atomic weight. Thyroxines are iodine-containing hormones that justify the widespread use of iodised salt. Iodine's main role in animal biology is as a constituent of the thyroid hormones //thyroxine// (T4) and //triiodothyronine// (T3). These are made from addition condensation products of the amino acid tyrosine, and are stored prior to release in an iodine-containing protein called thyroglobulin. T4 and T3 contain four and three atoms of iodine per molecule, respectively. The thyroid gland actively absorbs iodide from the blood to make and release these hormones into the blood, actions that are regulated by a second hormone TSH from the pituitary. Thyroid hormones are phylogenetically very old molecules that are synthesized by most multicellular organisms, and that even have some effect on unicellular organisms. Thyroid hormones play a basic role in biology, acting on gene transcription to regulate the basal metabolic rate. The total deficiency of thyroid hormones can reduce basal metabolic rate up to 50%, while in excessive production of thyroid hormones the basal metabolic rate can be increased by 100%. T4 acts largely as a precursor to T3, which is (with minor exceptions) the biologically active hormone. Iodine has a nutritional relationship with selenium. A family of selenium-dependent enzymes called deiodinases converts T4 to T3 (the active hormone) by removing an iodine atom from the outer tyrosine ring. These enzymes also convert T4 to reverse T3 (rT3) by removing an inner ring iodine atom, and convert T3 to 3,3'-diiodothyronine (T2) also by removing an inner ring atom. Both of the latter are inactivated hormones that are ready for disposal and have, in essence, no biological effects. A family of non-selenium-dependent enzymes then further deiodinates the products of these reactions. Iodine accounts for 65% of the molecular weight of T4 and 59% of the T3. Fifteen to 20 mg of iodine is concentrated in thyroid tissue and hormones, but 70% of the body's iodine is distributed in other tissues, including mammary glands, eyes, gastric mucosa, the cervix, and salivary glands. In the cells of these tissues, iodide enters directly by sodium-iodide symporter (NIS). Its role in mammary tissue is related to fetal and neonatal development, but its role in the other tissues is unknown. Dietary intake The daily Dietary Reference Intake recommended by the United States Institute of Medicine is between 110 and 130 µg for infants up to 12 months, 90 µg for children up to eight years, 130 µg for children up to 13 years, 150 µg for adults, 220 µg for pregnant women and 290 µg for lactating mothers. The Tolerable Upper Intake Level (UL) for adults is 1,100 μg/day (1.1 mg/day). The tolerable upper limit was assessed by analyzing the effect of supplementation on thyroid-stimulating hormone An iodine deficency can result in a condition called goiters. Goiters are when the thyroid gland enlarges and creates a tumor-like bulge. Goiters can only be removed surgically. Iodine tablets and crystals are used as a water purification system for backpacking.