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Iodine

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                53               tellurium ← iodine → xenon
                Br
                ↑
                I
                ↓
                At

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                        Name, Symbol, Number iodine, I, 53
                                                  Chemical series halogens
                                             Group, Period, Block 17, 5, p
                                     Appearance violet-dark gray, lustrous
                                           Atomic mass 126.90447 (3) g/mol
                               Electron configuration [Kr] 4d^10 5s^2 5p^5
                                       Electrons per shell 2, 8, 18, 18, 7
                                                       Physical properties
                                                               Phase solid
                                      Density (near r.t.) 4.933 g·cm^−3
                                                   Melting point 386.85  K
                                                 (113.7 ° C, 236.66 ° F)
                                                     Boiling point 457.4 K
                                                  (184.3 ° C, 363.7 ° F)
                                            Critical point 819 K, 11.7 MPa
                                  Heat of fusion (I[2]) 15.52 kJ·mol^−1
                            Heat of vaporization (I[2]) 41.57 kJ·mol^−1
                   Heat capacity (25 °C) (I[2]) 54.44 J·mol^−1·K^−1

   CAPTION: Vapor pressure (rhombic)

                                          P/Pa   1  10  100 1 k 10 k 100 k
                                         at T/K 260 282 309 342 381   457

                                                         Atomic properties
                                            Crystal structure orthorhombic
                                                Oxidation states ±1, 5, 7
                                                   (strongly acidic oxide)
                                    Electronegativity 2.66 (Pauling scale)
                                    Ionization energies 1st: 1008.4 kJ/mol
                                                        2nd: 1845.9 kJ/mol
                                                          3rd: 3180 kJ/mol
                                                      Atomic radius 140 pm
                                              Atomic radius (calc.) 115 pm
                                                    Covalent radius 133 pm
                                               Van der Waals radius 198 pm
                                                             Miscellaneous
                                             Magnetic ordering nonmagnetic
                            Electrical resistivity (0 °C) 1.3×10^7 Ω·m
                      Thermal conductivity (300 K) 0.449 W·m^−1·K^−1
                                                      Bulk modulus 7.7 GPa
                                             CAS registry number 7553-56-2
                                                         Selected isotopes

                  CAPTION: Main article: Isotopes of iodine

                             iso   NA   half-life    DM   DE ( MeV)   DP
                            ^127I 100% I is stable with 74 neutrons
                            ^129I syn  1.57×10^7 y Beta^- 0.194     ^129Xe
                            ^131I syn  8.02070 d   Beta^- 0.971     ^131Xe

                                                                References

   Iodine ( IPA: /ˈʌɪə(ʊ)ˌdiːn/, Greek: iodes, meaning "violet"), is a
   chemical element in the periodic table that has the symbol I and atomic
   number 53. Chemically, iodine is the least reactive of the halogens,
   and the most electropositive halogen after astatine. Iodine is
   primarily used in medicine, photography and dyes. It is required in
   trace amounts by most living organisms.

   As with all other halogens (members of Group VII in the Periodic
   Table), iodine forms diatomic molecules, and hence, has the molecular
   formula of I[2].

Occurrence on earth

   Iodine naturally occurs in the environment chiefly as dissolved iodide
   in seawater, although it is also found in some minerals and soils. The
   element may be prepared in an ultrapure form through the reaction of
   potassium iodide with copper(II) sulfate. There are also several other
   methods of isolating this element. Although the element is actually
   quite rare, kelp and certain other plants have the ability to
   concentrate iodine, which helps introduce the element into the food
   chain as well as keeping its cost down.

Uses

   Iodine is used in pharmaceuticals, antiseptics, medicine, food
   supplements, dyes, catalysts and photography.

Isotopes

   There are 37 isotopes of iodine and only one, ^127I, is stable.

   In many ways, ^129I is similar to ^36Cl. It is a soluble halogen,
   fairly non-reactive, exists mainly as a non-sorbing anion, and is
   produced by cosmogenic, thermonuclear, and in-situ reactions. In
   hydrologic studies, ^129I concentrations are usually reported as the
   ratio of ^129I to total I (which is virtually all ^127I). As is the
   case with ^36Cl/Cl, ^129I/I ratios in nature are quite small, 10^−14 to
   10^−10 (peak thermonuclear ^129I/I during the 1960s and 1970s reached
   about 10^−7). ^129I differs from ^36Cl in that its half-life is longer
   (15.7 vs. 0.301 million years), it is highly biophilic, and occurs in
   multiple ionic forms (commonly, I^− and IO[3]^−) which have different
   chemical behaviors. This makes it fairly easy for ^129I to enter the
   biosphere as it becomes incorporated into vegetation, soil, milk,
   animal tissue, etc.

   Excesses of stable ^129Xe in meteorites have been shown to result from
   decay of "primordial" ^129I produced newly by the supernovas which
   created the dust and gas from which the solar system formed. ^129I was
   the first extinct radionuclide to be identified as present in the early
   solar system. Its decay is the basis of the I-Xe radiometric dating
   scheme, which covers the first 50 million years of solar system
   evolution.

   Effects of various radioiodine isotopes in biology are discussed below.

Notable characteristics

   Iodine is a dark-gray/purple-black solid that sublimes at standard
   temperatures into a purple-pink gas that has an irritating odour. This
   halogen forms compounds with many elements, but is less active than the
   other members of its Group VII (halogens) and has some metallic-like
   properties. Iodine dissolves easily in chloroform, carbon
   tetrachloride, or carbon disulphide to form purple solutions (It is
   only slightly soluble in water, giving a yellow solution). The deep
   blue colour of starch-iodine complexes is produced only by the free
   element.

   Many students who have seen the classroom demonstration where iodine
   crystals are gently heated in a test tube come away with the impression
   that liquid iodine cannot exist at atmospheric pressure. This
   misconception arises because sublimation occurs without the
   intermediacy of liquid. The truth is that if iodine crystals are heated
   carefully to their melting point of 113.7 °C, the crystals will fuse
   into a liquid, which will be present under a dense blanket of the
   vapour.

Descriptive Chemistry

   Elemental iodine is poorly soluble in water, with one gram dissolving
   in 3450 ml at 20 °C and 1280 ml at 50 °C. By contrast with chlorine,
   the formation of the hypohalite ion (IO^–) in neutral aqueous solutions
   of iodine is negligible.

                I[2]+ H[2]0 ⇋ H^+ + I^– + HIO  (K = 2.0×10^-13)

   Solubility in water is greatly improved if the solution contains
   dissolved iodides such as hydroiodic acid, potassium iodide, or sodium
   iodide. Dissolved bromides also improve water solubility of iodine.
   Iodine is soluble in a number of organic solvents, including ethanol
   (20.5 g/100 ml at 15 °C, 21.43 g/100 ml at 25 °C), diethyl ether (20.6
   g/100 ml at 17 °C, 25.20 g/100 ml at 25 °C), chloroform, acetic acid,
   glycerol, benzene (14.09 g/100 ml at 25 °C), carbon tetrachloride
   (2.603 g/100 ml at 35 °C), and carbon disulfide (16.47 g/100 ml at 25
   °C). Aqueous and ethanol solutions are brown. Solutions in chloroform,
   carbon tetrachloride, and carbon disulfide are violet.

   Elemental iodine can be prepared by oxidizing iodides with chlorine:

                2I^– + Cl[2] → I[2] + 2Cl^–

   or with manganese dioxide in acid solution:

                2I^– + 4H^+ + MnO[2] → I[2] + 2H[2]O + Mn^++

   Iodine is reduced to hydroiodic acid by hydrogen sulfide:

                I[2] + H[2]S → 2HI + S↓

   or by hydrazine:

                2I[2] + N[2]H[4] → 4HI + N[2]

   Iodine is oxidized to iodate by nitric acid:

                I[2] + 10HNO[3] → 2HIO[3] + 10NO[2] + 4H[2]O

   or by chlorates:

                I[2] + 2ClO[3]^– → 2IO[3]^– + Cl[2]

   Iodine is converted in a two stage reaction to iodide and iodate in
   solutions of alkali hydroxides (such as sodium hydroxide):

                I[2] + 2OH^– → I^– + IO^– + H[2]O (K = 30)
                3IO^– → 2I^– + IO[3]^–            (K = 10^20)

History

   Iodine was discovered by Bernard Courtois in 1811. He was born to a
   manufacturer of saltpeter (potassium nitrate, a vital part of
   gunpowder). At the time France was at war, saltpeter, a component of
   gunpowder, was in great demand. Saltpeter produced from French niter
   beds required sodium carbonate, which could be isolated from seaweed
   washed up on the coasts of Normandy and Brittany. To isolate the sodium
   carbonate, seaweed was burned and the ash then washed with water. The
   remaining waste was destroyed by adding sulfuric acid. One day Courtois
   added too much sulfuric acid and a cloud of purple vapor rose. Courtois
   noted that the vapor crystallized on cold surfaces making dark
   crystals. Courtois suspected that this was a new element but lacked the
   money to pursue his observations.

   However he gave samples to his friends, Charles Bernard Desormes (1777
   - 1862) and Nicolas Clément (1779 - 1841) to continue research. He also
   gave some of the substance to Joseph Louis Gay-Lussac (1778 - 1850), a
   well-known chemist at that time, and to André-Marie Ampère (1775 -
   1836). On 29 November 1813 Dersormes and Clément made public Courtois’
   discovery. They described the substance to a meeting of the Imperial
   Institute of France. On December 6 Gay-Lussac announced that the new
   substance was either an element or a compound of oxygen. Ampère had
   given some of his sample to Humphry Davy (1778 - 1829). Davy did some
   experiments on the substance and noted its similarity to chlorine. Davy
   sent a letter dated December 10 to the Royal Society of London stating
   that he had identified a new element. A large argument erupted between
   Davy and Gay-Lussac over who identified iodine first but both
   scientists acknowledged Barnard Courtois as the first to isolate the
   chemical element.

Notable inorganic iodine compounds

     * Ammonium iodide (NH[4]I)
     * Caesium iodide (CsI)
     * Copper(I) iodide (CuI)
     * Hydroiodic acid (HI)
     * Iodic acid (HIO[3])
     * Iodine cyanide (ICN)
     * Iodine heptafluoride (IF[7])
     * Iodine pentafluoride (IF[5])
     * Lead(II) iodide (PbI[2])
     * Lithium iodide (LiI)
     * Nitrogen triiodide (NI[3])
     * Potassium iodide (KI)
     * Sodium iodide (NaI)

Stable iodine in biology

   Iodine is an essential trace element; its only known roles in biology
   are as constituents 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
   a protein-like molecule called thryroglobulin. T4 and T3 contain four
   and three atoms of iodine per molecule, respectively. The thyroid gland
   actively absorbs iodide ion from the blood to make and release these
   hormones into the blood, actions which are regulated by a second
   hormone TSH from the pituitary. Thyroid hormones are phylogenetically
   very old molecules which are sythesized by most multicellular
   organisms, and which even have some effect on unicellular organisms.

   Thyroid hormones play a very 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 some minor exceptions) the biologically active
   hormone.

Dietary intake

   The United States Food and Drug Administration recommends ( 21 CFR
   101.9 (c)(8)(iv)) 150 micrograms of iodine per day for both men and
   women. This is necessary for proper production of thyroid hormone.
   Natural sources of iodine include seaweed, such as kelp and seafood.
   Salt for human consumption is often enriched with iodine and is
   referred to as iodized salt.

Iodine deficiency

   In areas where there is little iodine in the diet—typically remote
   inland areas and semi-arid equatorial climates where no marine foods
   are eaten— iodine deficiency gives rise to goiter, so called endemic
   goiter. The mechanism is that low amounts of thyroid hormone in the
   blood due to lack of iodine to make them, give rise to high levels of
   the pituitary hormone TSH, which in turn stimulates abnormal growth of
   the thyroid gland. In some such areas, this is now combatted by the
   addition of small amounts of iodine to table salt in form of sodium
   iodide, potassium iodide, potassium iodate—this product is known as
   iodized salt. Iodine compounds have also been added to other
   foodstuffs, such as flour, in areas of deficiency.

   Iodine deficiency is the leading cause of preventable mental
   retardation, an effect which happens primarily when babies and small
   children are made hypothyroid by lack of the element (this condition in
   adults results in mental slowing, but by itself, almost never causes
   severe or irreversible mental problems). Iodine deficiency remains a
   serious public health problem in developing countries.

Toxicity of Iodine

   Excess iodine has symptoms similar to those of iodine deficiency.
   Commonly encountered symptoms are abnormal growth of the thyroid gland
   and disorders in functioning and growth of the organism as a whole.

   Elemental iodine, I[2], is deadly poison if taken in larger amounts; if
   2-3 grams of it is consumed, it is fatal to humans.

   Iodides are similar in toxicity to bromides.

Radioiodine and biology

Radioiodine and the thyroid

   The artificial radioisotope ^131I (a beta emitter), also known as
   radioiodine which has a half-life of 8.0207 days, has been used in
   treating cancer and other pathologies of the thyroid glands. ^123I is
   the radioisotope most often used in nuclear imaging of the kidney and
   thyroid as well as thyroid uptake scans (used for the evaluation of
   Grave's disease). The most common compounds of iodine are the iodides
   of sodium and potassium (KI) and the iodates (KIO[3]).

   ^129I ( half-life 15.7 million years) is a product of ^130Xe spallation
   in the atmosphere and uranium and plutonium fission, both in subsurface
   rocks and nuclear reactors. Nuclear processes, in particular nuclear
   fuel reprocessing and atmospheric nuclear weapons tests have now
   swamped the natural signal for this isotope. ^129I was used in
   rainwater studies following the Chernobyl accident. It also has been
   used as a ground-water tracer and as an indicator of nuclear waste
   dispersion into the natural environment.

   If humans are exposed to radioactive iodine, the thyroid gland will
   absorb it as if it were non-radioactive iodine, leading to elevated
   chances of thyroid cancer. Isotopes with shorter half-lifes such as
   ^131I present a greater risk than those with longer half-lives since
   they generate more radiation per unit of time. Taking large amounts of
   regular iodine will saturate the thyroid and prevent uptake. Iodine
   pills are sometimes distributed to persons living close to nuclear
   establishments, for use in case of accidents that could lead to
   releases of radioactive iodine.
     * Iodine-123 and iodine-125 are used in medicine as tracers for
       imaging and evaluating the function of the thyroid.
     * Iodine-131 is used in medicine for treatment of thyroid cancer and
       Grave's disease.
     * Uncombined (elemental) iodine is mildly toxic to all living things.
     * Potassium iodide (KI tablets, or "SSKI" = "Super-Saturated KI"
       liquid drops) can be given to people in a nuclear disaster area
       when fission has taken place, to flush out the radioactive
       iodine-131 fission product. The half-life of iodine-131 is only
       eight days, so the treatment would need to continue only a couple
       of weeks. In cases of leakage of certain nuclear materials without
       fission, or certain types of dirty bomb made with other than
       radioiodine, this precaution would be of no avail.

Radioiodine and the kidney

   In the 1970s imaging techniques were developed in California to utilize
   radioiodine in diagnostics for renal hypertension.

Non-hormone-related applications of iodine

     * Tincture of iodine (3% elemental iodine in water/ethanol base) is
       an essential component of any emergency survival kit, used both to
       disinfect wounds and to sanitize surface water for drinking (3
       drops per litre, let stand for 30 minutes). Alcohol-free iodine
       solutions such as Lugol's iodine, as well as other free
       iodine-providing antiseptics iodophors, are also available as
       effective elemental iodine sources for this purpose.
     * Iodine compounds are important in the field of organic chemistry
       and are very useful in medicine.
     * Silver iodide is used in photography.
     * Tungsten iodide is used to stabilize the filaments in light bulbs.

Precautions for stable iodine

   Direct contact with skin can cause lesions, so it should be handled
   with care. Iodine vapor is very irritating to the eye and to mucous
   membranes. Concentration of iodine in the air should not exceed 1 mg/
   m³ (eight-hour time-weighted average). When mixed with ammonia, it can
   form nitrogen triiodide which is extremely sensitive and can explode
   unexpectedly.

Clandestine Use

   In the United States, the Drug Enforcement Agency (DEA) regards iodine
   and compounds containing iodine (ionic iodides, iodoform, ethyl iodide,
   and so on) as reagents useful for the clandestine manufacture of
   methamphetamine. Persons who attempt to purchase significant quantities
   of such chemicals without establishing a legitimate use are likely to
   find themselves the target of a DEA investigation. Persons selling such
   compounds without doing due diligence to establish that the materials
   are not being diverted to clandestine use may be subject to stiff fines
   Retrieved from " http://en.wikipedia.org/wiki/Iodine"
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