Periodic Table > Iodine


Some elements exist in several different structural forms, these are called allotropes.

For more information on Murray Robertson’s image see Uses and properties facts below.


Fact box terminology

Elements appear in columns or ‘groups’ in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

Elements are laid out into rows or ‘periods’ so that similar chemical behaviour is observed in columns.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp, principal, diffuse, and fundamental.

Atomic Number
The number of protons in the nucleus.

Atomic Radius/non -bonded (Å)
based on Van der Waals forces (where several isotopes exist, a value is presented for the most prevalent isotope). These values were calculated using a multitude of methods including crystallographic data, gas kinetic collision cross sections, critical densities, liquid state properties, for more details please refer to the CRC Handbook of Chemistry and Physics.

Electron Configuration
The arrangements of electrons above the last (closed shell) noble gas.

Elements are defined by the number of protons in its centre (nucleus), whilst the number of neutrons present can vary. The variations in the number of neutrons will create elements of different mass which are known as isotopes.

Melting Point (oC)
The temperature at which the solid-liquid phase change occurs.

Melting Point (K)
The temperature at which the solid-liquid phase change occurs.

Melting Point (oF)
The temperature at which the solid-liquid phase change occurs.

Boiling Point (oC)
The temperature at which the liquid-gas phase change occurs.

Boiling Point (K)
The temperature at which the liquid-gas phase change occurs.

Boiling Point (oF)
The temperature at which the liquid-gas phase change occurs.

Elements that do not possess a liquid phase at atmospheric pressure (1 atm) are described as going through a sublimation process.

Density (kgm-3)
Density is the weight of a substance that would fill 1 m3 (at 298 K unless otherwise stated).

Relative Atomic Mass
The mass of an atom relative to that of Carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists the value given is the abundance weighted average.

Key Isotopes (% abundance)
An element must by definition have a fixed number of protons in its nucleus, and as such has a fixed atomic number, however variants of an element can exist with differing numbers of neutrons, and hence a different atomic masses (e.g. 12C has 6 protons and 6 neutrons and 13C has 6 protons and 7 neutrons).

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems (where several isotopes exist, a value is presented for the most prevalent isotope).

Fact box

Group 17  Melting point 113.7 oC, 236.66 oF, 386.85 K 
Period Boiling point 184.4 oC, 363.92 oF, 457.55 K 
Block Density (kg m-3) 4953 
Atomic number 53  Relative atomic mass 126.904  
State at room temperature Solid  Key isotopes 127
Electron configuration [Kr] 4d105s25p5  CAS number 7553-56-2 
ChemSpider ID 4514549 ChemSpider is a free chemical structure database

Uses and properties terminology

Image Explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.

Natural Abundance

Where this element is most commonly found in nature.

Biological Roles

The elements role within the body of humans, animals and plants. Also functionality in medical advancements both today and years ago.


The description of the element in its natural form.

Uses and properties

Image explanation
The image is of seaweed. Many species of seaweed contain iodine.
A black, shiny, crystalline solid. When heated, iodine sublimes to form a purple vapour.
Photography was the first commercial use for iodine after Louis Daguerre, in 1839, invented a technique for producing images on a piece of metal. These images were called daguerreotypes.

Today, iodine has many commercial uses. Iodide salts are used in pharmaceuticals and disinfectants, printing inks and dyes, catalysts, animal feed supplements and photographic chemicals. Iodine is also used to make polarising filters for LCD displays.

Iodide is added in small amounts to table salt, in order to avoid iodine deficiency affecting the thyroid gland. The radioactive isotope iodine-131 is sometimes used to treat cancerous thyroid glands.
Biological role
Iodine is an essential element for humans, who need a daily intake of about 0.1 milligrams of iodide. Our bodies contain up to 20 milligrams, mainly in the thyroid gland. This gland helps to regulate growth and body temperature.

Normally we get enough iodine from the food we eat. A deficiency of iodine can cause the thyroid gland to swell up (known as goitre).
Natural abundance
Iodine is found in seawater, as iodide. It is only present in trace amounts (0.05 parts per million); however, it is assimilated by seaweeds. In the past iodine was obtained from seaweed.

Now the main sources of iodine are iodate minerals, natural brine deposits left by the evaporation of ancient seas and brackish (briny) waters from oil and salt wells.

Iodine is obtained commercially by releasing iodine from the iodate obtained from nitrate ores or extracting iodine vapour from the processed brine.
Atomic data terminology

Atomic radius/non -bonded (Å)
Based on Van der Waals forces (where several isotopes exist, a value is presented for the most prevalent isotope). These values were calculated using a multitude of methods including crystallographic data, gas kinetic collision cross sections, critical densities, liquid state properties,for more details please refer to the CRC Handbook of Chemistry and Physics.

Electron affinity (kJ mol-1)
The energy released when an additional electron is attached to the neutral atom and a negative ion is formed (where several isotopes exist, a value is presented for the most prevalent isotope). *

Electronegativity (Pauling scale)
The degree to which an atom attracts electrons towards itself, expressed on a relative scale as a function bond dissociation energies, Ed in eV. χA - χB =(eV)-1/2sqrt(Ed(AB)-[Ed(AA)+Ed(BB)]/2), with χH set as 2.2 (where several isotopes exist, a value is presented for the most prevalent isotope).

1st Ionisation energy (kJ mol-1)
The minimum energy required to remove an electron from a neutral atom in its ground state (where several isotopes exist, a value is presented for the most prevalent isotope).

Covalent radius (Å)
The size of the atom within a covalent bond, given for typical oxidation number and coordination (where several isotopes exist, a value is presented for the most prevalent isotope). ***

Atomic data

Atomic radius, non-bonded (Å) 1.980 Covalent radius (Å) 1.36
Electron affinity (kJ mol-1) 295.149 Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol-1)
Bond enthalpies terminology

Covalent Bonds
The strengths of several common covalent bonds.

Bond enthalpies

Covalent bonds
I–I  151.2  kJ mol -1 H–I  298.3  kJ mol -1 C–I  228  kJ mol -1
C–I  234  kJ mol -1  

Mining/Sourcing Information

Data for this section of the data page has been provided by the British Geological Survey. To review the full report please click here or please look at their website here.

Key for numbers generated

Governance indicators

1 (low) = 0 to 2

2 (medium-low) = 3 to 4

3 (medium) = 5 to 6

4 (medium-high) = 7 to 8

5 (high) = 9

Reserve base distribution

1 (low) = 0 to 30 %

2 (medium-low) = 30 to 45 %

3 (medium) = 45 to 60 %

4 (medium-high) = 60 to 75 %

5 (high) = 75 %

(Where data are unavailable an arbitrary score of 2 was allocated. For example, Be, As, Na, S, In, Cl, Ca and Ge are allocated a score of 2 since reserve base information is unavailable. Reserve base data are also unavailable for coal; however, reserve data for 2008 are available from the Energy Information Administration (EIA).)

Production Concentration

1 (low) = 0 to 30 %

2 (medium-low) = 30 to 45 %

3 (medium) = 45 to 60 %

4 (medium-high) = 60 to 75 %

5 (high) = 75 %

Crustal Abundance

1 (low) = 100 to 1000 ppm

2 (medium-low) =10 to 100 ppm

3 (medium) = 1 to 10 ppm

4 (medium-high) = 0.1 to 1 ppm

5 (high) = 0.1 ppm

(Where data are unavailable an arbitrary score of 2 was allocated. For example, He is allocated a score of 2 since crustal abundance data is unavailable.)

Explanations for terminology

Crustal Abundance (ppm)

The abundance of an element in the Earth's crust in parts-per-million (ppm) i.e. The number of atoms of this element per 1 million atoms of crust.


The country with the largest reserve base.

Reserve Base Distribution

This is a measure of the spread of future supplies, recording the percentage of a known resource likely to be available in the intermediate future (reserve base) located in the top three countries.

Production Concentrations

This reports the percentage of an element produced in the top three countries. The higher the value, the larger risk there is to supply.

Total Governance Factor

The World Bank produces a global percentile rank of political stability. The scoring system is given below, and the values for all three production countries were summed.

Relative Supply Risk Index

The Crustal Abundance, Reserve Base Distribution, Production Concentration and Governance Factor scores are summed and then divided by 2, to provide an overall Relative Supply Risk Index.

Supply risk

Scarcity factor 6.5
Country with largest reserve base Chile
Crustal abundance (ppm) 0.71
Leading producer Chile
Reserve base distribution (%) 66.70
Production concentration (%) 59.70
Total governance factor(production) 4
Top 3 countries (mined)
  • 1) Chile
  • 2) Japan
  • 3) USA
Top 3 countries (production)
  • 1) Chile
  • 2) Japan
  • 3) USA

Oxidation states and isotopes

Key for Isotopes

Half Life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture


Common Oxidation states
The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Free atoms have an oxidation state of 0, and the sum of oxidation numbers within a substance must equal the overall charge.

Important Oxidation states
The most common oxidation states of an element in its compounds.

Elements are defined by the number of protons in its centre (nucleus), whilst the number of neutrons present can vary. The variations in the number of neutrons will create elements of different mass which are known as isotopes.

Oxidation states and isotopes

Common oxidation states 7, 5, 1, -1
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  127I 126.904 100

Pressure and temperature - advanced terminology

Molar Heat Capacity (J mol-1 K-1)

Molar heat capacity is the energy required to heat a mole of a substance by 1 K.

Young's modulus (GPa)

Young's modulus is a measure of the stiffness of a substance, that is, it provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus (GPa)

The shear modulus of a material is a measure of how difficult it is to deform a material, and is given by the ratio of the shear stress to the shear strain.

Bulk modulus (GPa)

The bulk modulus is a measure of how difficult to compress a substance. Given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour Pressure (Pa)

Vapour pressure is the measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Molar heat capacity
(J mol-1 K-1)
54.43 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) 7.7
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - - - - - - - - - -
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In the early 1800s, Bernard Courtois of Paris manufactured saltpetre (potassium nitrate, KNO3) and used seaweed ash as his source of potassium. One day in 1811, he added sulfuric acid and saw purple fumes which condensed to form crystals with a metallic lustre. Courtois guessed this was a new element. He gave some to Charles-Bernard Desormes and to Nicolas Clément who carried out a systematic investigation and confirmed that it was. In November 1813, they exhibited iodine at the Imperial Institute in Paris. That it really was new was proved by Joseph Gay-Lussac and confirmed by the Humphry Davy who was visiting Paris. Davy sent a report to the Royal Institution in London where it was mistakenly assumed he was the discoverer, a belief that persisted for more than 50 years.

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Listen to Iodine Podcast
Transcript :

Chemistry in its Element - Iodine



You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry

(End promo)


Chris Smith


Hello, this week cretins, fire crackers and clean water.   The story starts in Italy,  and here's Andrea Sella. 


Andrea Sella


When I was a child, I used spend a couple of weeks each summer high in the Italian Alps in an idyllic little village called Cogne that nestles quietly between high ice-clad peaks. To most Italians the name is associated with a sensational murder. Others know that in winter the valley has some of the finest ice-climbing in the Alps. But to me, Cogne will always be connected with the element iodine. 

One afternoon, when I was around 10 years old, returning with my Dad from a long hike, we passed a dull grey building on the edge of the village. It was surrounded by a tall metal fence and had an institutional look about it. Sitting on the bench all on his own was a strange looking old man - he had rather shaggy hair, a vacant look, and a large, distended pouch of skin where his neck should have been.   I was utterly shocked by this strange being. I pestered my father with questions. Who was he? What was wrong with him? Why did he look so sad?

My father, whose patience in the face of a barrage of questions was almost infinite, explained that the poor man had grown up with insufficient iodine in his diet. Iodine, he went on was essential for the proper development of the thyroid gland in the neck, and that if one didn't eat the right kind of salt, especially as a child, one might develop goitre and one's mental development would also be affected. I would later read of English travellers passing through the Alps referring to The Valleys of the Cretins - travel books of the period often include lurid illustrations of these poor unfortunates. The numbers are staggering; the Napoleonic census of the canton of Valais in 1800 found 4000 cretins in a population of 70,000 - well over 50% would have had goiter.

The disease had been known to medical writers for centuries. Galen for example recommended treatment with marine sponges. In 1170 Roger of Salerno recommended seaweed. Similar suggestions were also made in China. 

Paracelsus, the great renaissance healer, alchemist, and writer was one of the first to spot the connexion between goiter and cretinism, and first suggested that minerals in drinking water might play a role in causing the condition. But what these mysterious minerals might be was a mystery.

In 1811 a young French chemist, Bernard Courtois, working in Paris stumbled across a new element. His family's firm produced the saltpetre needed to make gunpowder for Napoleon's wars. To do this they used wood ash. Wartime shortages of wood forced them instead to burn seaweed, which was plentiful on the coastlines of northern France. Adding concentrated sulphuric acid to the ash, Courtois, obtained an astonishing purple vapour that crystallized onto the sides of the container. Astonished by this discovery he bottled up the crystals and sent them to one of the foremost chemists of his day Joseph Gay-Lussac who confirmed that this was a new element and named it iode - iodine - after the greek word for purple. Courtois continued to play with the element and was rather shocked to discover that when mixed with ammonia it produced a chocolate-coloured solid that exploded violently at the least provocation. His contemporary, Pierre Dulong, was less fortunate, losing an eye and part of a hand while studying the material, the first in a long list of casualties from this nasty material.

The toxic qualities of iodine were soon realized, and the tincture, a yellowish brown solution began to be widely used as a disinfectant. Even today, the most common water purification tablets one can buy in travel shops are based on iodine.

It was only two years after its discovery, that a doctor in Geneva Francois Coindet began to wonder whether it wasn't the iodine in the seaweed that was the missing mineral responsible for goiter.   He therefore began administering tincture of iodine to his patients by mouth, an unpleasant business, but which, he reported, led to the disappearance of swelling in 6 to 10 weeks. His colleagues, however, accused him of poisoning his patients, and at one point he was said to be unable to go into the streets for fear of being attacked. 

But, while elemental iodine clearly was toxic, Coindet was on the right track, and during the 19th century by a process of one step forward two steps back the hypothesis gradually gained credence as experiments using the more palatable salt, potassium iodide, showed that goitres could be reversed. By the early 1920's Swiss cantons began to introduce iodized salt and over the following decades many countries that had been plagued by goitre followed suit, a policy so effective that many of us in the developed world are unaware of how serious a disease this had been and the word cretin has lost much of its meaning. 

When I returned to Cogne last summer, I tried to remember where the institute had been. All I could find was a summer holiday camp, with children playing happily behind the gates where I had seen the old man. I phoned my Dad to ask him, and we chatted about the old days - the bad old days of the cretins - and of ghosts banished by that unique purple element, iodine. 


Chris Smith


Ghosts that clearly live on amongst the British aristocracy.   That was UCL chemist Andrea Sella telling the tale of Iodine, element number 53.   Next week we're shining the spotlight on a substance that needs no illuminating at all and that's because it makes its own light.  


Brian Clegg


It was seen as a source of energy and brightness, it was included in toothpastes and patent medicines - it was even rubbed into the scalp as a hair restorer.

But the application of radium that would bring it notoriety was its use in glow-in-the-dark paint. Frequently used to provide luminous readouts on clocks and watches, aircraft switches and instrument dials, the eerie blue glow of radium was seen as a harmless, practical source of night time illumination. It was only when a number of the workers who painted the luminous dials began to suffer from sores, anaemia and cancers around the mouth that it was realized that something was horribly wrong. 


Chris Smith


And you can hear the story of Radium from Brian Clegg on next week's Chemistry in its Element, I hope you can join us.   I'm Chris Smith, thank you for listening and goodbye. 




Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists dot com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld dot org forward slash elements. 


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Description :
The Group 7 elements are called the halogens. This experiment involves some reactions of the halogens.
Description :
Studying the physical characteristics of the group 7 non-metals known as the halogens
Description :
Each of the halogens forms a monovalent (singly-charged) anion. In this experiment you will be looking at the similarities and differences in some of the properties of these halide ions.
Description :
The reaction between aluminium and iodine is catalysed by water. This is a spectacular demonstration as clouds of purple iodine vapour are produced.
Description :
The halogens are elements of Group 7 of the Periodic table. This experiment illustrates some of the trends and similarities within the compounds of this group.
Description :
This activity compares the colours of three halogens in aqueous solution and in a non-polar solvent. These halogens also react to a small extent with water, forming acidic solutions with bleaching pr...

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Images:  Visual Elements © Murray Robertson 2011
Mining and Sourcing data:  British Geological Survey – natural environment research council.
Text:  John Emsley Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, 2nd Edition, 2011.
Additional information for platinum, gold, neodymium and dysprosium obtained from Material Value Consultancy Ltd
Data: CRC Handbook of Chemistry and Physics, CRC Press, 92nd Edition, 2011.
G. W. C. Kaye and T. H. Laby Tables of Physical and Chemical Constants, Longman, 16th Edition, 1995.
Members of the RSC can access these books through our library.