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Name, Symbol, Number iodine, I, 53
Series halogens
Group, Period, Block 17 (VIIA), 5, p
Density, Hardness 4940 kg/m3, no data
Appearance violet-dark grey,
Atomic properties
Atomic weight 126.90447 g/mol
Atomic radius (calc.) 140 (115) pm
Covalent radius 133 pm
van der Waals radius 198 pm
Electron configuration [Kr]4d10 5s2 5p5
e- 's per energy level 2, 8, 18, 18, 7
Oxidation states (Oxide)  ?1,5,7 (strong acid)
Crystal structure Orthorhombic
Physical properties
State of matter solid (nonmagnetic)
Melting point 386.85 K (236.66 ?F)
Boiling point 457.4 K (363.7 ?F)
Molar volume 25.72 cm3/mol
Heat of vaporization 20.752 kJ/mol
Heat of fusion 7.824 kJ/mol
Vapor pressure __ Pa at __ K
Speed of sound __ m/s at __ K
Electronegativity 2.66 (Pauling scale)
Specific heat capacity 145 J/(kg·K)
Electrical conductivity 80 nS/m
Thermal conductivity 0.449 W/(m?K)
1st ionization potential 1008.4 kJ/mol
2nd ionization potential 1845.9 kJ/mol
3rd ionization potential 3180 kJ/mol
Most stable isotopes
iso NA half-life
(not SI)
(not SI)
127I 100% I is stable with 74 neutrons
129I {syn.} 1.57E7y Beta- 0.194 129Xe
131I {syn.} 8.02070 d Beta- 0.971 131Xe
SI units & STP are used except where noted.

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.

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Notable characteristics

Iodine is a bluish-black, lustrous solid that sublimes at standard temperatures into a blue-violet gas that has an irritating odor. This halogen also forms compounds with many elements, but is less active than the other member of its series and has some metallic-like properties. Iodine dissolves easily in chloroform, carbon tetrachloride, or carbon disulfide to form purple solutions (It is only slightly soluble in water). The deep blue color with starch solution is characteristic of the free element.


In areas where there is little iodine in the diet—typically remote inland areas where no marine foods are eaten—iodine deficiency gives rise to goitre, so called endemic goitre. In many (but not all) such areas, this is now prevented 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 deficiency is the leading cause of mental retardation. Other uses:

  • One of the halogens, it is an essential trace element; the thyroid hormones, thyroxine and triiodotyronine contain 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 liter, let stand for 30 minutes)
  • Iodine compounds are important in the field of organic chemistry and are very useful in medicine.
  • Iodides and thyroxine which contains iodine, are both used in internal medicine and, in combination with alcohol (as tincture of iodine) are used externally to disinfect wounds
  • Silver iodide is used in photography
  • Potassium iodide (KI) tablets can be given to people in a nuclear disaster area. KI prevents the body from absorbing the radioactive iodine produced at the disaster area.
  • Tungsten iodide is used to stabilize the filaments in light bulbs
  • Nitrogen triiodide is an explosive, too unstable to be used commercially, but is commonly used in college pranks
  • Iodine-131 is used as a tracer in medicine


Iodine (Gr. iodes meaning violet) was discovered by Barnard Courtois in 1811. He was the son of a manufacturer of saltpeter (potassium nitrate, a vital part of gunpowder). At the time France was at war and gunpowder was in great demand. Saltpeter was isolated from seaweed washed up on the coasts of Normandy and Brittany. To isolate the potassium nitrate, seaweed was burned and the ash then washed with water. The remaining waste was destroyed by adding sulfuric acid. One day Curtois added too much sulfuric acid and cloud of purple vapor rose. Curtois noted that the vapor crystallized on cold surfaces making dark crystals. Curtois 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 Cl魥nt (1779-1841) to continue research. He also gave some of the substance to Joseph Louis Gay-Lussac (17781850), a well-known chemist at that time, and to Andr魍arie Amp貥 (1775-1836). On November 29 1813 Dersormes and Cl魥nt made public Curtois’ 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. Amp貥 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 Bernard Curtois as the first to isolate the element.


Iodine can 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.


There are thirty isotopes of iodine and only one, I-127, is stable. The artificial radioisotope I-131 (a beta emitter), also known as radioiodine which has a half-life of 8 days, has been used in treating cancer and other pathologies of the thyroid glands. The most common compounds of iodine are the iodides of sodium and potassium (KI) and the iodates (KIO3).

I-129 (half-life 15.7 million years) is a product of Xe-129 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. I-129 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.

In many ways, I-129 is similar to chlorine-36. 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, I-129 concentrations are usually reported as the ratio of I-129 to total I (which is virtually all I-127). As is the case with Cl-36/Cl, I-129/I ratios in nature are quite small, 10-14 to 10-10 (peak thermonuclear I-129/I during the 1960s and 1970s reached about 10-7). I-129 differs from Cl-36 in that its half-life is longer (15.7 vs. 0.3 million years), it is highly biophilic, and occurs in multiple ionic forms (commonly, I- and iodate) which have different chemical behaviors. This makes it fairly easy for I-129 to enter the biosphere as it becomes incorporated into vegetation, soil, milk, animal tissue, etc.

Excesses of Xe-129 in meteorites have been shown to result from decay of I-129. This 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.


Direct contact with skin can cause lesions so care needs to be taken in handling iodine. Iodine vapor is very irritating to eyes and mucous membranes. The maximum allowable concentration of iodine in air should not exceed 1 mg/m? (8-hour time-weighted average).


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