Iodine in Periodic Table
Iodine (I), chemical element, Group 17 (Group VIIA) of the periodic table or a member of the halogen family has widely used in the manufacturing of various organic compounds like iodoform (CHI3) and methyl iodide (CH3I). It is the stable and heaviest member of the halogen family placed below fluorine, chlorine, and bromine or before radioactive element astatine. The essential element, iodine used in our body for making thyroid hormones to control metabolism and many other important biological functions. In 1811, iodine was discovered by the French chemist Bernard Courtois and the name of the element suggested by Joseph Louis Gay-Lussac from the Greek word ioeides meaning violet colored.
The non-metallic chemical element, iodine has chemical symbol I, chemical formula I2, atomic number 53, atomic weight 126.904, melting point 113.7 °C, boiling point 184.3 °C, density 4.933 gm/cm3, electron configuration [Kr] 4d10 5s2 5p5. Except for fluorine, halogen exhibits several positive oxidation number or state in their oxides, oxoacids, and interhalogen compounds. Polyatomic cations X2+ and X3+ are known for iodine. None of the halogens form a stable monoatomic cation through the ionization energy of halogens except fluorine is less than that of hydrogen.
Abundance and Isolation
Iodine is less abundant among the halogens group, only 0.46 ppm (61st order). Brains contain iodide ions up to 100 ppm. Ocean water contains iodine up to the extent of 0.05 ppm but certain marine life like Laminaria and Fucus can concentrate it up to 0.45 percent of their dry weight. Iodine has thirty-seven known isotopes but only one stable natural isotope like 127I. The other is radioactive isotopes with a very short half-life. Radioactive decay of iodine-131 is used in nuclear medicine and radiation therapy for the diagnosis and treatment of thyroid gland disorder.
Depending upon the sources, iodine can be prepared by different types of processes. In the United States and Japan, natural brines containing iodides may be chlorinated to evolve iodine gas. The gas was swept by air and purified by sublimation. In another method, the iodide is precipitated by the silver nitrate solution. The precipitated silver iodide (AgI) is treated with iron powder with the formation of metallic silver and FeI2. The free silver was treated with nitric acid for reuse in the process. The ferrous iodide (FeI2) solution is chlorinated to form iodine, absorbed by active charcoal or ion exchange resin.
Sea-weeds are collected, dried, and brunt. The ash treated with water and soluble iodide separated from other insoluble materials. The solution is concentrated to crystallize other soluble salts. The final filtrate is distilled with sulfuric acid and manganese dioxide in cast iron resort for the production of iodine.
Chemistry and Reactivity
Iodine has an electron configuration [Kr] 4d10 5s2 5p5, with seven electrons in the ultimate quantum shell. Like other halogens, it has also one electron short of the next noble gas. The noble gas configuration is achieved by gaining one electron from reactive surrounding or sharing electrons by covalent bonding. Iodine is the weakest oxidizing agent among all stable halogens due to low ionization energy and low electron affinity. Other hand, fluorine is the most chemically reactive element or halogen due to its electronegativity and low bond energy. Fluorine combines directly most of the metals and nonmetals. Some metals like iron, copper, nickel, and aluminum form a protective layer of fluoride ion which prevents further reaction.
Iodine atom has the tendency to form compounds in which it is present as a cation to shows basic properties. For example ICl, IBr, INO3, I2SO4, I(CH3COO)3, etc. Elemental iodine molecule is formed by covalent chemical bonding with the molecular formula I2. The color of the halogen shows interesting gradation from pale yellow (fluorine), greenish-yellow (chlorine), reddish-brown (bromine). The color of the gaseous diatomic halogen molecules arises due to the absorption of visible radiation for the π∗ → σ∗ transition. The separation between the two levels decreases from the F2 to I2 molecule which shifting the absorption to lower frequencies. Therefore, the iodine molecule transmits strongly in the violet region which is unchanged by dissolving many organic solvents like hydrocarbon and carbon tetrachloride.
Iodine is the least reactive and stable halogens but it is still reactive among the periodic table elements. The effect of increasing size among the halogens, iodine shows the tendency to attain higher covalency and coordination number. For example, it alone forms IF7 while chlorine and bromine do not go beyond ClF3 and BrF5 respectively.
All the halogen reacts with hydrogen to form volatile covalent hydrides. The reactivity towards hydrogen gradually falls from fluorine to iodine. The hydrogen iodide is obtained by treating a mixture of water and red phosphorus with iodine. The hydrides HCl, HBr, and HI are gases at ordinary temperature but HF is liquid because of the association through hydrogen bonding. All the hydrides ionize in an aqueous solution due to solvation. Weaker bond dissociation energy makes all the three hydracids like HCl, HBr, and HI, strong acids with a low pH scale.
Almost all elements of the periodic table except helium, neon, and argon form binary compounds with iodine to give a wide range of halides. The elements like oxygen, nitrogen, fluorine, chlorine, bromine have electronegativity higher than that of bromine. Therefore, the binary compounds of such elements are not called iodide, they are called oxides, nitrides, fluorides, chlorides, and bromides. Most metal iodides have ionic compounds but the partial covalent character is developed depending upon the charge and oxidation state of the metal. Nonmetal iodide tends to form by covalent bonding.
Oxides and Oxoacids
Oxides of halogen are unstable compounds and the higher oxides are more stable than the lower ones. Iodine oxides are the most stable among all halogen oxides. I2O5 is the most stable oxides but relatively less stable oxides like I2O4 and I4O9 also formed iodine molecules. I2O5 is a hygroscopic solid, it dissolved in water to form HIO3.
The stability of the oxoacids and their anions in an aqueous solution is determined by redox reactions, together with significant kinetic influences. Iodic acid (HIO3) is an important oxoacid of iodine formed by boiling iodine with ten times its weight of fuming nitric acid. Crystalline solid potassium iodate prepared by heating I2 with a concentrated solution of KClO3 in presence of little HNO3. It is a good oxidant used in many oxidimetric titrations. Like antimony and tellurium, iodine also exhibits a tendency to attain higher coordination numbers and forms H5IO6.
Uses of Iodine
World production of iodine lies around 16,000 tonnes per annum, Japan alone nearly one half this amount. One half of Iodine is used in the production of various organic compounds like iodoform (antiseptic) and methyl iodide. Iodine and its compounds are widely used in animal feed supplements, dyes, medicine, sanitation, an inhibitor of pollution like smog, for purification of metals like titanium, zirconium, and hafnium, and photography. Silver iodide is used in high-speed photography.
Several inorganic compounds containing iodine used as a chemical catalyst in the rubber industry, in dyestuff, and for many other analytical purposes. Iodine molecule is also used in secondary standard electromagnetic spectrum analysis due to its sharp spectral lines with the wavelength range 500–700 nm Sodium iodide (NaI) and NaIO3 is added to table salt to supplement iodine deficiency in our body which may hamper the production of thyroxin, a growth-regulating hormone.