Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

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 (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

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

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

Density (g cm−3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

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.

Atoms of the same element with different numbers of 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.

Fact box

Group 17  Melting point 113.7°C, 236.7°F, 386.9 K 
Period Boiling point 184.4°C, 363.9°F, 457.6 K 
Block Density (g cm−3) 4.933 
Atomic number 53  Relative atomic mass 126.904  
State at 20°C Solid  Key isotopes 127
Electron configuration [Kr] 4d105s25p5  CAS number 7553-56-2 
ChemSpider ID 4514549 ChemSpider is a free chemical structure database


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.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

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.
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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.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic radius, non-bonded (Å) 1.98 Covalent radius (Å) 1.36
Electron affinity (kJ mol−1) 295.152 Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)

Bond enthalpy (kJ mol−1)
A measure of how much energy is needed to break all of the bonds of the same type in one mole of gaseous molecules.

Bond enthalpies

Covalent bond Enthalpy (kJ mol−1) Found in
I–I 150.9 I2
C–I 218 general
C–I 213 CH3I
H–I 298.7 HI


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. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

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

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


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.

Supply risk

Relative supply risk 6.5
Crustal abundance (ppm) 0.71
Recycling rate (%) Unknown
Substitutability Unknown
Production concentration (%) 59.7
Reserve distribution (%) 66.7
Top 3 producers
  • 1) Chile
  • 2) Japan
  • 3) USA
Top 3 reserve holders
  • 1) Chile
  • 2) Japan
  • 3) USA
Political stability of top producer 67.5
Political stability of top reserve holder 67.5


Specific heat capacity (J kg−1 K−1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus

A measure of the stiffness of a substance. 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

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A 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

Specific heat capacity
(J kg−1 K−1)
214 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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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 There's more information and other episodes of Chemistry in its element on our website at

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Visual Elements images and videos
© Murray Robertson 1998-2017.



W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J. J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions (version 4.1), 2015, National Institute of Standards and Technology, Gaithersburg, MD, accessed November 2016.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.


Uses and properties

John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.


Supply risk data

Derived in part from material provided by the British Geological Survey © NERC.


History text

Elements 1-112, 114, 116 and 117 © John Emsley 2012. Elements 113, 115, 117 and 118 © Royal Society of Chemistry 2017.



Produced by The Naked Scientists.


Periodic Table of Videos

Created by video journalist Brady Haran working with chemists at The University of Nottingham.