|Name, Symbol, Number||iodine, I, 53|
|Group, Period, Block||17, 5, p|
|Appearance||violet-dark gray, lustrous
|Atomic mass||126.90447(3) g/mol|
|Electron configuration||[Kr] 4d10 5s2 5p5|
|Electrons per shell||2, 8, 18, 18, 7|
|Density (near r.t.)||4.933 g/cm³|
|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||(I2) 15.52 kJ/mol|
|Heat of vaporization||(I2) 41.57 kJ/mol|
|Heat capacity||(25 °C) (I2) 54.44 J/(mol·K)|
|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|
|Electrical resistivity||(0 °C) 1.3×107 Ω·m|
|Thermal conductivity||(300 K) 0.449 W/(m·K)|
|Bulk modulus||7.7 GPa|
|CAS registry number||7553-56-2|
Iodine (chemical symbol I, atomic number 53) is a nonmetal that belongs to a group of chemical elements known as halogens. At ordinary temperatures and pressures, it is a dark-gray/purple-black solid that readily sublimes—that is, it goes directly from the solid phase to the gas phase. The gas is purple-pink in color and has an irritating odor. The name iodine was coined from the Greek word iodes, meaning "violet."
Elemental iodine is corrosive on the skin and toxic if ingested. In the form of iodide ions, however, iodine is required as a trace element for most living organisms. In humans, the deficiency or excess of iodide ions can lead to swelling and malfunctioning of the thyroid gland.
Iodine and its compounds have a variety of applications. For instance, tincture of iodine is used to disinfect wounds and sanitize water for drinking. Silver iodide is used in photography, and tungsten iodide is used to stabilize filaments in light bulbs. A number of organic compounds containing iodine are useful in the preparation of pharmaceuticals and dyes. The radioactive isotopes iodine-123 and iodine-125 are used as probes for imaging the thyroid and evaluating its health. An artificial radioactive isotope, iodine-131, is used for the treatment of thyroid cancer.
Iodine occurs in nature in the form of iodide ions, chiefly in solution in seawater but also in some minerals and soils. Although the element is quite rare, it is concentrated in kelp and some other plants, which help introduce the element into the food chain and keep its cost down.
Iodine was discovered in 1811 by the Frenchman Bernard Courtois, when he was working with his father to manufacture saltpeter (potassium nitrate). At the time, Napoleon's army was engaged in war and saltpeter, a key component of gunpowder, was in great demand. The process of producing saltpeter from French niter beds required sodium carbonate, which could be isolated from the ashes of burned seaweed. The remaining waste was destroyed by adding sulfuric acid. One day, Courtois added too much sulfuric acid and a cloud of purple vapor arose. The vapor condensed on cold surfaces to produce dark crystals.
Courtois performed a few experiments that led him to suspect that this was a new element, but he lacked the funds to pursue his observations. He therefore gave samples of the material to his friends, Charles Bernard Désormes (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 November 29, 1813, Désormes and Clément made public Courtois’ discovery, describing 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). When Davy experimented with the substance, he 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 major argument erupted between Davy and Gay-Lussac over who identified iodine first, but both scientists acknowledged Courtois as the first to isolate the substance.
There are several methods of isolating iodine.
In the periodic table, iodine is located in group 17 (former group 7A), the halogen family, between bromine and astatine. In addition, it lies in period 5, between tellurium and xenon. The molecular formula of iodine is I2.
Chemically, iodine forms compounds with many elements, but it is the least reactive of the halogens. In addition, it is the most electropositive halogen after astatine and has some metallic properties.
It is only slightly soluble in water, giving a yellow solution, but it readily dissolves in chloroform, carbon tetrachloride, or carbon disulphide to form purple solutions. The free element forms a deep blue complex with starch.
Iodine does not react with oxygen or nitrogen, but with ozone it forms an unstable oxide, I4O9. When mixed with water, it reacts to produce hypoiodite ions (OI−). Under appropriate conditions, iodine reacts with other halogens—fluorine, chlorine, and bromine—to produce "interhalogen" compounds, including IF3, IF5, IF7, ICl, I2Cl6, and BrI. When mixed with ammonia, iodine can form nitrogen triiodide, which is extremely sensitive and can explode unexpectedly.
There are 37 isotopes of iodine, of which only one, 127I, is stable.
The isotope 129I, with a half-life 15.7 million years, is produced in the Earth's atmosphere when the nuclei of 130Xe are struck by high-energy cosmic rays. It is also produced by the fission of uranium and plutonium, in both subsurface rocks and nuclear reactors. Nuclear fuel reprocessing and atmospheric nuclear weapons tests have now swamped the natural signal for this isotope.
In hydrologic studies, 129I concentrations are usually reported as the ratio of 129I to total I (which is virtually all 127I). This ratio in nature is quite small, ranging from 10−14 to 10−10 (peak thermonuclear 129I/I during the 1960s and 1970s reached about 10−7). 129I occurs in multiple ionic forms (commonly, I− and IO3−) and readily enters the biosphere, becoming incorporated into vegetation, soil, milk, and animal tissue.
Excess quantities of stable 129Xe in meteorites appear to have resulted from the decay of "primordial" 129I produced by the supernovas that created the dust and gas from which the solar system formed. The decay of 129I is the basis for the iodine-xenon radiometric dating scheme, which covers the first 50 million years of development of the solar system.
A wide range of organic and inorganic compounds contain iodine. In the case of organic compounds, chemists can replace hydrogen atoms with iodine atoms, thus creating many new products.
A list of notable inorganic compounds of iodine is given below, in alphabetical order.
Iodine is an essential trace element in the human body. The thyroid hormones thyroxine (T4) and triiodothyronine (T3) contain four and three atoms of iodine per molecule, respectively. The thyroid actively absorbs elemental iodine from the blood to make and release these hormones into the blood, actions that are regulated by a second hormone (thyroid-stimulating hormone, TSH) from the pituitary.
Thyroid hormones are phylogenetically very old, as they are sythesized by most multicellular organisms and even have some effect on unicellular organisms. These hormones play a very basic role in biology, acting on mitochondria to regulate metabolism. T4 acts largely as a precursor to T3, which is (with some minor exceptions) the biologically active hormone.
Iodine deficiency is a serious problem in various parts of the globe. It particularly affects people in places 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 leads to goiter. It is also the leading cause of preventable mental retardation.
The U.S. Food and Drug Administration recommends an intake of 150 micrograms of iodine per day for both men and women. This is necessary for proper production and functioning of thyroid hormones. Natural sources of iodine include seaweed and seafood.
All links retrieved March 5, 2018.
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