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General Info
Atomic Symbol Atomic symbol::I
Atomic Number Atomic number::53
Atomic Weight Atomic weight::126.9 g/mol
Chemical series Halogens
Appearance Solid state of Iodine
Sample iodine.jpg
Group, Period, Block 17, 5, p
Electron configuration [Kr] 4d10, 5s2, 5p5
Electrons per shell 2, 8, 18, 18, 7
Electron shell iodine.png
CAS number CAS number::7553-56-2
Physical properties
Phase Solid
Density Density::4.933 g/ml
Melting point Melting point::113.7 °C
Boiling point Boiling point::184.3 °C
Isotopes of Iodine
iso NA half-life DT DE (MeV) DP
123I syn 13 h ε, γ 0.16 123Te
127I 100% 127I is stable with 74 neutrons.
129I trace 15.7x106 y β− 0.194 129Xe
131I syn 8.02070 d β−, γ 0.971 131Xe
All properties are for STP unless otherwise stated.

Iodine, a chemical element that is a solid nonmetal belonging to the Halogen group. On the Periodic Table of the Elements it is located in Group VIIA. It has a chemical symbol I and an atomic number of 53. In solid states, iodine has a dark grey/black appearance. When it is heated to its gaseous state, it transforms to a violet/purple vapor. The element was discovered in France in the early 1800's. Its name comes from the Greek word, "Iodes," meaning violet.

This element is one of the least reactive halogens, and one of the most electropositive halogens. Iodine can be found in seawater and some soils and minerals. It is also found in seaweed, kelp, and caliche. It can be used in the production of salts, medicine, photography, and dyes.[1][2]


Sublimed Iodine Crystals

The element Iodine is located in the group of halogens under the Group VIIA. In its solid state, the chemical shows a bluish-black color. When it is put into its gaseous state, it demonstrates its light purple-violet color. Although iodine is capable of creating compounds with most elements, it is one of the least reactive of the halogens. It is only slightly soluble in water. Although solubility of this element in water is very low, its solubility can increase by adding potassium iodide. Iodine will then react with the negative ion to generate the triiodide anion I3, which is soluble in water. Iodine easily dissolves in hexane, chloroform, carbon tetrachloride, or carbon disulphide. When this reaction occurs, its violet color is clearly visible. [3]

The color of solutions of iodine can mainly be determined by the polarity of the solvent. When iodine is dissolved in non-polar solvents, it shows its violet color. Characteristics of its color can widely vary though. The solvent dichloromethane can be dissolved with iodine to demonstrate a dark crimson color. Generally speaking, the higher the polarity of the solvent, the darker the color gets, leading to dark orange and brown. This property is caused by ligand field interactions of solvent molecules with the d-orbitals of iodine, which is the only halogen with a sufficiently occupied electronic configuration. These interactions could not happen without a sufficiently occupied electronic configuration.[4]

Iodine is rarely found in a liquid state. If the element is heated just above its melting point (236.66 °F), some liquid iodine can be found with the violet vapor.


Iodine for the most part is very rare within our atmosphere. The abundance of this element actually ranks as the sixty-second most abundant element on earth. [5] It is found as iodide ions. These can be found primarily in solutions of sea water and in some minerals and soils. It is also present in certain types of seaweed. Although the element is not found in many locations, kelp and other algae have some ability to concentrate iodine, which helps introduce the element into the food chain. Outside of these places, iodine can be found in a mineral by the name of caliche. This mineral is exclusive to the country of Chile.[6]

Some marine life forms produce what are called organoiodine compounds. The most commonly known of these is methyl iodide, or otherwise known as iodomethane. This iodomethane is broken up by oxidation reactions in the atmosphere and a global iodine cycle is created. The complete amount of iodomethane that the marine environment is able to produce is estimated to be 214 kilotons by tests such as microbial activity in rice paddies and by the burning of biological material.[7]


Dark Grey Crystals of Iodine

There are many various uses of the element iodine. Iodine compounds are used in organic chemistry and are essential for different areas in medicine. Iodine in the artificial radioisotope 131I is used in treating the thyroid gland. Iodides, and thyroxin which contains iodine, are used internally in several parts of medicine. Iodine that is found in alcohol and Kl can be used for treatment of external wounds on the body. Potassium iodide can actually be used in different places in photography.[8]

The mineral caliche is found in Chile between the Andes and the ocean. The high concentration of iodine in the caliche and the extensive mining made Chile the largest producer of iodine in 2007. The caliche contains sodium nitrate that can be used as the main product of the mining activities and small amounts of sodium iodate and sodium iodide. During production of sodium nitrate, the sodium iodate and iodide are extracted from the rest. Iodine can be produced from seawater through electrolysis but is not used due to the abundant iodine-rich brine. One more source of iodine is kelp. This process was used more during the 1800's, but now there are more sufficient ways of retrieving the rare iodine.

Some other uses of iodine can be traced through uses of non-radioactive iodine like in the silver iodide that is used in photography. Tungsten iodide can be used in the stabilization of the filaments in ordinary light bulbs. Table salt is also often filled with iodine. The sodium chloride can be added with small amounts of sodium iodide, potassium iodide, or potassium iodate. This is what is known as iodized salt. Flour also contributes to this use of iodine. Tincture of iodine is also a necessary part of emergency survival kits. It is used to disinfect cuts and sanitization of water. Some compounds containing iodine can be used in the preparation of pharmaceuticals and some dyes. Radioactive iodine also serves uses. Iodine-131 is a radioisotope that can be used for the treatment of thyroid cancer and other diseases. Iodine-125 and 123 are used in medicine to determine how will the thyroid gland is functioning, by serving as a tracer for imaging. Iodine-129 was used after the nuclear reactor accident at Chernobyl in rainwater studies. Iodine can be used as a tracer of groundwater and a tool to measure the amount of nuclear waste escape and dispersion into the natural environment.[9]

History of Iodine

The discovery of iodine is credited to a man named Bernard Courtois of France. This Frenchman produced potassium nitrate, otherwise known as saltpeter, with his father. During this time, the French were at war. This caused a high demand in his product, which was essential for gun powder. Sodium carbonate was used on French niter beds, and was found by isolation from washed up seaweed. To separate this sodium carbonate from the seaweed, the seaweed needed to be burned and then have its ashes washed with water. What was left was finally destroyed by the addition of sulfuric acid. One day in 1811, Bernard added too much sulfuric acid, which caused a cloud of purple-violet colored vapor to arise. Little did he know, this was a new, yet to be discovered element which runs by the name of iodine. He recorded that this vapor crystallized on cold surfaces that contacted the vapor. He ran a few experiments on this dark crystals, but these studies were out of his budget. He collected samples which he passed on to Charles Bernard Desormes, Nicolas Clément, and Joseph Louis Gay-Lussac, a well-known chemist at the time. Two years later, Courtois's two friends, Desormes and Clément publicly announced the discovery of this new element. They stated all evidences and knowledge of this element to the Imperial Institute of France. On December 6, 1813, Gay-Lussac announced that this was either a compound of oxygen or an element. Argument is still present over who proved the existence of this element, but the original discovery of iodine is credited to Frenchman Bernard Courtois.[10][11]