Iodine Element Facts

Iodine Vapor

Iodine vapor is purple. Photo by Matias Molnar.


Data Zone

Classification: Iodine is a halogen and a nonmetal
Color: bluish-black solid, purple vapor
Atomic weight: 126.9045
State: solid
Melting point: 113.5 oC, 386.6 K
Boiling point: 184 oC, 457 K
Electrons: 53
Protons: 53
Neutrons in most abundant isotope: 74
Electron shells: 2,8,18,18,7
Electron configuration: [Kr] 4d10 5s2 5p5
Density @ 20oC: 4.93 g/cm3
Show more, including: Heats, Energies, Oxidation, Reactions, Compounds, Radii, Conductivities
Atomic volume: 25.74 cm3/mol
Structure: layers of I2
Specific heat capacity 0.214 J g-1 K-1
Heat of fusion 15.52 kJ mol-1 of I2
Heat of atomization 107 kJ mol-1
Heat of vaporization 41.57 kJ mol-1 of I2
1st ionization energy 1008.4 kJ mol-1
2nd ionization energy 1845.8 kJ mol-1
3rd ionization energy 3184 kJ mol-1
Electron affinity 295.16 kJ mol-1
Minimum oxidation number -1
Min. common oxidation no. 0
Maximum oxidation number 7
Max. common oxidation no. 7
Electronegativity (Pauling Scale) 2.66
Polarizability volume 5 Å3
Reaction with air none
Reaction with 15 M HNO3 mild, ⇒ HIO3
Reaction with 6 M HCl none
Reaction with 6 M NaOH mild, ⇒ OI-, I-
Oxide(s) I2O5, I4O9, I2O4
Hydride(s) HI
Chloride(s) ICl, ICl3
Atomic radius 140 pm
Ionic radius (1+ ion)
Ionic radius (2+ ion)
Ionic radius (3+ ion)
Ionic radius (1- ion) 206 pm
Ionic radius (2- ion)
Ionic radius (3- ion)
Thermal conductivity 0.45 W m-1 K-1
Electrical conductivity 1.0 x 10-5 S m-1
Freezing/Melting point: 113.5 oC, 386.6 K

Discovery of Iodine

Iodine was discovered by Bernard Courtois in 1811 in France.

Courtois was trying to extract potassium chloride from seaweed. After crystallizing the potassium chloride, he added sulfuric acid to the remaining liquid.

This, rather surprisingly, produced a purple vapor, which condensed into dark crystals. These were the first crystals of iodine ever made.

Courtois studied this new substance and found that it combined well with phosphorous and hydrogen, but it did not form compounds easily with carbon or oxygen.

He also discovered that when mixed with ammonia it formed a brown colored solid (nitrogen triiodide) that exploded at the slightest touch.

Iodine’s name comes from the Greek work ‘iodes’ meaning violet.

Chemical clock reactions – amazing color changes.

Iodine crystals sublimate (turn from solid to gas without becoming liquid) and then freeze back to solid iodine.


Iodine Crystals. Photo by Ben Mills.

Appearance and Characteristics

Harmful effects:

In small doses, iodine is slightly toxic and it is highly poisonous in large amounts. Elemental iodine is an irritant which can cause sores on the skin. Iodine vapor causes extreme eye irritation.


Iodine is a bluish-black, lustrous solid. Although it is less reactive than the elements above it in group 17 (fluorine, chlorine and bromine) it still forms compounds with many other elements.

Although iodine is a non-metal, it displays some metallic properties.

When dissolved in chloroform, carbon tetrachloride or carbon disulfide, iodine yields purple colored solutions. It is barely soluble in water, giving a yellow solution.

Uses of Iodine

Iodine is important in medicine, in both radioactive and non-radioactive forms. Iodide and thyroxin, which contains iodine, are used inside the body.

A solution containing potassium iodide (KI) and iodine in alcohol is used to disinfect external wounds. Elemental iodine is also used as a disinfectant.

Silver iodide is used in photography.

Iodine is sometimes added to table salt to prevent thyroid disease.

Iodine’s other uses include catalysts, animal feeds and printing inks and dyes.

Abundance and Isotopes

Abundance earth’s crust: 450 parts per billion by weight, 73 parts per billion by moles

Abundance solar system: parts per billion by weight, parts per billion by moles

Cost, pure: $8.3 per 100g

Cost, bulk: $ per 100g

Source: In nature, iodine occurs in the form of iodide ions, mainly in seawater. It is introduced into the food chain via seaweed and other sea-plants. Iodine is found in some minerals and soils.

Commercially, iodine is obtained in several ways, such as taking iodine vapor from processed brine, by ion exchange of brine or by releasing iodine from iodate taken from nitrate ores.

Isotopes: 34 whose half-lives are known, with mass numbers 108 to 141. Naturally occurring iodine consists of the one stable isotope: 127I

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  1. Jessica Bickley says:

    Hey there! I noticed a small error on the site regarding the electron configuration of Iodine. It should read: [Kr] 5s2 4d10 5p5 because the 4d energy level is filled before the 5p level. Hope I helped!

    • Hi Jessica, thanks for your comment. The configuration we’ve shown is actually correct. :)

      The Convention

      What I should say to start with is that we’ve followed a convention for electron configurations so that, if there are electrons present in orbitals, lower principal quantum numbers are always shown preceding higher principal quantum numbers in the configuration. This means that it doesn’t matter whether in a real atom the 5s has lower or higher energy than the 4d orbitals. We always write 4d before 5s.

      In the case of iodine this convention actually does yield the correct electron configuration – although it may at first seem to disagree with the orbital energy levels shown in textbooks, which show the filling order for orbitals as:

      2s 2p
      3s 3p
      4s 3d 4p
      5s 4d 5p
      6s 4f 5d 6p
      7s 5f 6d 7p

      Why [Kr] 4d10 5s2 5p5 is Iodine’s Electron Configuration

      If we apply the Aufbau principal to the orbitals above, we would predict that the 5s orbital, because it has lower energy than the 4d orbitals, will fill with electrons before the 4d orbitals do – as you’ve said.

      In fact, when two electrons are present in the 5s orbital the energy of the 4d orbitals falls below the energy of 5s. Therefore, the correct configuration for iodine is [Kr] 4d10 5s2 5p5

      The reason we don’t always get the result we’d expect from applying the Aufbau Principal to orbitals is that when real electrons begin to interact with one another, some shifts in orbital energy levels can take place.

      Thanks for the interesting comment Jessica – it may be worth considering whether we continue to show configurations using the principal quantum number convention or whether we show the actual configuration, when it’s known. :)