Atomic mass | 126.9045 g.mol -1 |
Electronegativity according to Pauling | 2.5 |
Density | 4.93 g.cm-3 at 20°C |
Melting point | 114 °C |
Boiling point | 184 °C |
Vanderwaals radius | 0.177 nm |
Ionic radius | 0.216 nm (-1) ; 0.05 nm (+7) |
Isotopes | 15 |
Electronic shell | [ Kr ] 4d10 5s25p5 |
Energy of first ionisation | 1008.7 kJ.mol -1 |
Standardpotential | + 0.58 V ( I2/ I- ) |
Discovered | Bernard Courtois in 1811 |
Iodine under standard conditions is a bluish black solid. It can be seen apparently sublimating at standard temperatures into a violet-pink gas that has an irritating odor. This halogen forms compounds with many elements, but is less reactive than the other members of its Group VII (halogens) andhas some metallic light reflectance.
In the gas phase iodine shows its violet color.
Elemental iodine dissolves easily in most organic solvents such as hexane or chloroform due to its lack of polarity, but is only slightly soluble in water. However, the solubility of elemental iodine in water can be increased by the addition of potassium iodide. The molecular iodine reacts reversibly with thenegative ion, generating the triiodide anion I3− in equilibrium, which is soluble in water. This is also the formulation of some types of medicinal (antiseptic) iodine, although tincture of iodine classically dissolves the element in aqueous ethanol.
Solutions of elemental iodine have the unique property of exhibiting dramatically different colors depending on the polarity of the solvent. Whendissolved in nonpolar solvents like hexane, the solution appears deep violet; in moderately polar dichloromethane the solution is dark crimson, and in strongly polar solvents like acetone or ethanol, it appears dark orange or brown. This is due to ligand field interactions of solvent molecules with the d-orbitals of iodine, which is the only halogen with a sufficiently occupied electronicconfiguration to allow such interactions. This same property allows the formation of hypervalent iodine compounds, which have expanded bonding orbitals beyond the generally allowed octet rule.
Students who have seen the classroom demonstration in which iodine crystals are gently heated in a test tube to violet vapor may gain the impression that liquid iodine does not exist at atmospheric pressure. Thismisconception arises because the vapor produced has such a deep colour that the liquid appears not to form. In fact, if iodine crystals are heated carefully to just above their melting point of 113.7 °C, the crystals melt into a liquid which is present under a dense blanket of the vapor.
When iodine is encapsulated into carbon nanotubes it forms atomic chains, whose structure depends on the nanotubediameter.
Iodine naturally occurs in the environment chiefly as a dissolved iodide in seawater, although it is also found in some minerals and soils. This element also exists in small amounts in the mineral caliche, found in Chile, between the Andes and the sea. A type of seaweed, kelp, tends to be high in iodine as well.
Though deposits of elementaliodine do not occur on Earth, small amounts of elemental iodine vapor (I2) and solution occur in many environments, as a result of oxidation of iodides by elemental oxygen in the atmosphere, in the presense of carbon dioxide. This free iodine causes potassium iodide solutions and crystals to "yellow" after long contact with air. The oxidation of iodide to iodine in air is also responsible for theloss of iodide content in iodised salt if exposed to air: the alkali metal iodide, over time and exposure to excess oxygen and carbon dioxide, slowly oxidizes to metal carbonate and elemental iodine, which evaporates.
Organoiodine compounds are produced by marine life forms, the most notable being iodomethane (commonly called methyl iodide). The total iodomethane that is produced by the marine...