Name: Iodine
Symbol: I
Atomic Number: 53
Atomic Weight: 126.904470
Family: Halogens
CAS RN: 7553-56-2
Description: A violet-dark grey solid.
State (25C): Solid
Oxidation states: 1, +5, +7

Molar Volume: 25.74 cm3/mole
Valence Electrons: 5p5

Boiling Point:  458.55K, 185.4C, 365.7F
Melting Point:
386.65K, 113.5C, 236.3F
Electrons Energy Level: 2, 8, 18, 18, 7
Isotopes: 37 + 1 Stable
Heat of Vaporization: 20.752 kJ/mol
Heat of Fusion: 7.824 kJ/mol
Density: 4.93 g/cm3 @ 300K
Specific Heat: 0.214 J/gK
Atomic Radius: 1.32
Ionic Radius: 2.2
Electronegativity: 2.66 (Pauling); 2.21 (Allrod Rochow)
Elemental Iodine is a dark grey solid with a faint metallic luster. When heated at ordinary air pressures it sublimes to a violet gas. The name Iodine is taken from the Greek ioeides which means "violet colored".  Iodine was discovered by Bernard Courtois in 1811.

Commercially Iodine is recovered from seaweed and brines. It is an important trace element in the human diet, required for proper function of the thyroid gland. Thus Iodine is added to table salt ("iodized") to insure against iodine deficiencies. Radioactive isotopes of iodine are used in medical tracer work involving the thyroid and also to treat diseases of that gland.

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, KI,  with Copper (II) Sulfate, CuSO4.   There are also a few 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.






1s2 2s2p6 3s2p6d10 4s2p6d10 5s2p5


Iodine was discovered by Bernard Courtois in 1811.  He was born to a manufacturer of saltpeter (Potassium Nitrate, KNO3, 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 Na2CO3, 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, H2SO4.  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 Clement (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 Andre-Marie Ampere (1775 - 1836). On November 29, 1813, Dersormes and Clment made public Courtois’ discovery.  They described the substance to a meeting of the Imperial Institute of France.  On December 6, 1813, Gay-Lussac announced that the new substance was either an element or a compound of Oxygen.  Ampre had given some of his sample to Sir Humphry Davy (1778 - 1829).  Davy did some experiments on the substance and noted its similarity to Chlorine.  Davy sent a letter dated December 10, 1813 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.


Iodine is a dark-gray/purple-black solid that sublines at standard temperatures into a purple-pink gas that has an irritating odor. 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 color of starch-iodine complexes is produced only by the free element.

2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10
5s2 5p5

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.

Elemental Iodine is poorly soluble in water, with one gram dissolving in 3450 ml at 20 C and 1280 ml at 50C.  By contrast with Chlorine, the formation of the Hypohalite Ion (IO) in neutral aqueous solutions of Iodine is negligible.

I2+ H2O rarrow.gif (63 bytes) H+ + I + HIO  (K = 2.010-13)

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 + Cl2 rarrow.gif (63 bytes) I2 + 2Cl

or with Manganese Dioxide in acid solution:

2I + 4H+ + MnO2 rarrow.gif (63 bytes) I2 + 2H2O + Mn2+

Iodine is reduced to Hydroiodic Acid by Hydrogen Sulfide:

I2 + H2S rarrow.gif (63 bytes) 2HI + S

or by Hydrazine:

2I2 + N2H4 rarrow.gif (63 bytes) 4HI + N2

Iodine is oxidized to Iodate by Nitric Acid:

I2 + 10HNO3 rarrow.gif (63 bytes) 2HIO3 + 10NO2 + 4H2O

or by Chlorates:

I2 + 2ClO3 rarrow.gif (63 bytes) 2IO3 + Cl2

Iodine is converted in a two stage reaction to Iodide and Iodate in solutions of alkali hydroxides (such as Sodium Hydroxide):

I2 + 2OH rarrow.gif (63 bytes) I + IO + H2O (K = 30)
3IO rarrow.gif (63 bytes) 2I + IO3 (K = 1020)


Iodine can be found in the Mineral Caliche, found in Chile, between the Alps and the sea.  It can also be found in some seaweeds as well as extracted from seawater, however extracting Iodine from the mineral is the only economical way to extract the substance.


Today, Iodine is chiefly obtained from deposits of Sodium Iodate (NaIO3) and Sodium Periodate (NaIO4) in Chile and Bolivia.

Trace amounts of Iodine are required by the human body.  Iodine is part of Thyroxin, a hormone produced by the thyroid gland that controls the body's rate of physical and mental development.  A lack of Iodine can also cause a goiter, a swelling of the thyroid gland.  Iodine is added to salt (iodized salt) to prevent these diseases.

Iodine is used as a test for starch and turns a deep blue when it comes in contact with it.  Potassium Iodide (KI) is used to make photographic film and, when mixed with iodine in alcohol, as an antiseptic for external wounds.  A radioactive isotope of Iodine, Iodine-131, is used to treat some diseases of the thyroid gland.

Care should be taken in handling and using iodine. It can burn the skin and damage the eyes and mucous membranes.  Pure Iodine is poisonous if ingested.

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


Ammonium Iodide (NH4I) Caesium Iodide (CsI)
Copper (II) Iodide (CuI) Hydroiodic Acid (HI)
Iodic Acid (HIO3) Iodine Cyanide (ICN)
Iodine Heptafluoride (IF7) Iodine Pentafluoride (IF5)
Lead (II) Iodide (PbI2) Lithium Iodide(LiI)
Nitrogen Triiodide (NI3) Potassium Iodide (KI)
Sodium Iodide (NaI)


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 IO3-) 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 83 million years of solar system evolution.

atom.gif (700 bytes)

Isotope Atomic Mass Half-Life
I108 107.943 36 ms
I109 108.938 100 us
I110 109.935 0.65 seconds
I111 110.93 2.5 seconds
I112 111.928 3.42 seconds
I113 112.9236 6.6 seconds
I114 113.922 2.1 seconds
I115 114.918 1.3 minutes
I116 115.917 2.91 seconds
I117 116.9136 2.22 minutes
I118 117.9134 13.7 minutes
I119 118.9102 19.1 minutes
I120 119.91 81 minutes
I121 120.9074 2.12 hours
I122 121.9076 3.63 minutes
I123 122.9056 13.27 hours
I124 123.9062 4.1760 days
I125 124.9046 59.408 days
I126 125.9056 13.11 days
I127 126.9045 Stable
I128 127.9058 24.99 minutes
I129 128.905 1.57E 7 years
I130 129.9067 12.36 hours
I131 130.9061 8.02070 days
I132 131.908 2.295 hours
I133 132.9078 20.8 hours
I134 133.9099 52.5 minutes
I135 134.9101 6.57 hours
I136 135.9147 83.4 seconds
I137 136.9179 24.5 seconds
I138 137.9224 6.49 seconds
I139 138.9261 2.29 seconds
I140 139.931 0.86 seconds
I141 140.935 0.43 seconds
I142 141.94 ~0.2 seconds
I143 142.944 >150 ns
I144 143.95 >150 ns


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, NH3, it can form Nitrogen Triiodide, NI3, 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.

Biological Role

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 Throsine, 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 synthesized 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 sea life, such as kelp and certain 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 hypothyroidism, symptoms of which are extreme fatigue, goiter, mental slowing, depression, weight gain, and low basal body temperatures.

Iodine deficiency is also 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.  The addition of Iodine to table salt has largely eliminated this problem in the wealthier nations, but Iodine deficiency remains a serious public health problem in the developing world.

Toxicity of Iodine

40px-Skull_and_crossbones.svg.jpg (1420 bytes) 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, I2, 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 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 (NaI) and Potassium (KI) and the Iodates (NaIO3 & KIO3).

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.

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

atom.gif (700 bytes)

Iodine Data


Atomic Structure

Atomic Radius (): 1.32
Atomic Volume cm3/mol : 25.74cm3/mol
Covalent Radius: 1.33
Crystal Structure: Orthorhombic
Ionic Radius: 2.2

Chemical Properties

Electrochemical Equivalents: 4.7348 g/amp-hr
Electron Work Function: unknown
Electronegativity: 2.66 (Pauling); 2.21 (Allrod Rochow)
Heat of Fusion: 7.824 kJ/mol
Incompatibilities: Ammonia, acetylene, acetaldehyde, powdered aluminum, active metals, liquid chlorine
First Ionization Potential: 10.451
Second Ionization Potential: 19.131
Third Ionization Potential: 33
Valence Electron Potential: -6.55
Ionization Energy (eV): 10.451 eV

Physical Properties

Atomic Mass Average: 126.9045
Boiling Point: 458.55K, 185.4C, 365.7F
Melting Point: 386.65K, 113.5C, 236.3F
Heat of Vaporization: 20.752 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: N/A
Electrical Conductivity: 8.0E-16 106/cm
Thermal Conductivity: 0.00449 W/cmK
Density: 4.93 g/cm3 @ 300K
Enthalpy of Atomization: 106.7 kJ/mole @ 25C
Enthalpy of Fusion: 7.76 kJ/mole
Enthalpy of Vaporization: 20.88 kJ/mole
Flammability Class: Non-combustible solid
Molar Volume: 25.74 cm3/mole
Optical Refractive Index: unknown
Relative Gas Density (Air=1): unknown
Specific Heat: 0.214 J/gK
Vapor Pressure: unknown
Estimated Crustal Abundance: 4.510-1 milligrams per kilogram
Estimated Oceanic Abundance: 610-2 milligrams per liter

(Gr. iodes, violet) Discovered by Courtois in 1811, Iodine, a halogen, occurs sparingly in the form of iodides in sea water from which it is assimilated by seaweeds, in Chilean saltpeter and nitrate-bearing earth, known as caliche in brines from old sea deposits, and in brackish waters from oil and salt wells.   Ultra pure iodine can be obtained from the reaction of potassium iodide with copper sulfate.  Several other methods of isolating the element are known. Iodine is a bluish-black, lustrous solid, volatilizing at ordinary temperatures into a blue-violet gas with an irritating odor; it forms compounds with many elements, but is less active than the other halogens, which displace it from iodides.  Iodine exhibits some metallic-like properties.  It dissolves readily in chloroform, carbon tetrachloride, or carbon disulfide to form beautiful purple solutions.  It is only slightly soluble in water.  Iodine compounds are important in organic chemistry and very useful in medicine.  Thirty isotopes are recognized. Only one stable isotope,127I, is found in nature.   The artificial radioisotope 131I, with a half-life of 8 days, has been used in treating the thyroid gland.  The most common compounds are the iodides of sodium and potassium (KI) and the iodates (KIO3).  Lack of iodine is the cause of goiter.  Iodides, and thyroxin which contains iodine, are used internally in medicine, and as a solution of KI and iodine in alcohol is used for external wounds.   Potassium iodide finds use in photography.  The deep blue color with starch solution is characteristic of the free element.  Care should be taken in handling and using iodine, as contact with the skin can cause lesions; iodine vapor is intensely irritating to the eyes and mucous membranes.  The maximum allowable concentration of iodine in air should not exceed 1 mg/m3 (8-hour time-weighted average - 40-hour).

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond