Oxidizes or reduces HI

Name, symbol, atomic number Iodine, I, 53
Group, period, block17, 5, p
Look dark violet gray,
CAS number 7553-56-2
Mass fraction of the earth's envelope 6 · 10−6 %
Atomic mass 126.90447 u
Atomic radius (calculated) 140 (115) pm
Covalent radius 133 pm
Van der Waals radius 198 pm
Electron configuration [Kr] 4d10 5s2 5p5
Electrons per energy level 2, 8, 18, 18, 7
1. Ionization energy 1008.4 kJ / mol
2. Ionization energy 1845.9 kJ / mol
3. Ionization energy 3180 kJ / mol
Physical state firmly
Modifications 1
Crystal structure orthorhombic
density 4.94 g / cm3
Mohs hardness ?
magnetism diamagnetic
Melting point 386.85 K (113.70 ° C)
boiling point 457.4 K (184.20 ° C)
Molar volume 25,72 · 10−6 m3/ mol
Heat of evaporation 20.752 kJ / mol
Heat of fusion 7.824 kJ / mol
Vapor pressure

41 Pa at 298 K

Speed ​​of sound m / s
Specific heat capacity 214.5 J / (kg · K) at 298 K
Electric conductivity 8,0 · 10−8S / m
Thermal conductivity 0.449 W / (m K)
Oxidation states ±1, 3, 5, 7
Oxides (basicity) (very sour)
Normal potential 0,536
Electronegativity 2.66 (Pauling scale)
isotopeNHt1/2ZMZE MeVZP


59.408 d ε0,186125Te


13.11 d ε2,155126Te

100 %



24.99 min β-2,118128Xe


1,57 · 107aβ-0,194129Xe


12.365 h β-2,949130Xe


8.02070 dβ-0,971131Xe
safety instructions
Hazardous substance labeling
from RL 67/548 / EEC, Appendix I.
R and S phrases R: 20 / 21-50
S: (2-) 23-25-61
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions.

Iodine (also: iodine, Element iconI.) is a chemical element. It is in the 17th group of the periodic table of the elements (previously 7th main group, VIIA), the so-called halogens. The name is derived from the ancient Greek word "Iodes" (ιώο-ειδης) for "violet-colored": the vapors released when iodine is heated are purple.

While in German iodine represents the more commonly used spelling, prefers technical language Iodine; both spellings are correct regardless of the spelling reform. The element symbol J however, is out of date and was invariably by I. replaced.

Iodine is an essential trace element for humans as well as other living things.


The physiological importance of preparations containing iodine was already known in ancient times. For example, 1500 years before our era, goiter sufferers were prescribed the iodine-containing thyroid glands of sheep or the ashes of sea sponges.

Iodine was first extracted from the ashes of seaweed in 1811 by the Parisian saltpetre boiler Bernard Courtois in the manufacture of gunpowder. However, it was not until 1813 that the French scientists Nicolas Clément-Désormes and Joseph Louis Gay-Lussac researched its elementary character, who gave it its current name a year later.


Apart from astatine, iodine is much rarer than the other halogens. It is widespread in nature, but only in the form of its compounds, for example enriched (0.02–1%) in Chile's nitrate, mainly in the form of sodium iodate (NaIO3), but also sodium periodate (NaIO4) and Lautarit (Ca (IO3)2). It can be detected in small traces in soils and rocks. On average, 100 grams of anhydrous fine soil from southern Germany contains up to 34 milligrams of iodine [1]. The iodine content of the soil is essential for supplying the population with natural iodine. As hydrogen iodide, it occurs in very small quantities in volcanic gases.

Soluble iodine compounds such as alkali and alkaline earth iodides are released during the weathering of rocks by rainwater or disintegrate at higher temperatures. This is how they eventually end up in the oceans and groundwater. Some mineral waters contain iodine. The Woodhall Spa mineral spring in the USA produces water that is colored brown by iodine. The amount of iodine in seawater is 0.05 grams per ton.

Organic iodine compounds can be isolated from seaweed (19 grams of iodine per kilogram of dry matter), seaweed and sponges (up to 14 grams of iodine per kilogram of dry matter). The thyroid is an important store for organically bound iodine.

Naturally occurring iodine consists 100% of the isotope127I, so it is a pure element (anisotopic).

Extraction and presentation

In the past, iodine was extracted in the form of iodides and iodates by collecting the seaweed that was washed up on the beach by the tide and burning it. The ash obtained contained about 0.1-0.5% iodine. Today, however, this iodine production is only of local importance and accounts for around 2% of the world's annual production.

The technical production of iodine is closely linked to the production of saltpetre. The iodate contained in the mother liquor is converted into iodine by reduction. In the first reaction step, sulphurous acid is used to reduce iodic acid (iodate is the anion of this acid) to hydrogen iodide:

Iodic acid and sulphurous acid react to form hydrogen iodide and sulfuric acid

The hydrogen iodide is in turn oxidized to iodine in a second step by the iodic acid present in the solution. In this case, one speaks of a comproportionation, since iodine becomes elemental iodine with the oxidation state 0 in two different oxidation states (−1 in hydrogen iodide and +5 in iodic acid).

Iodic acid and hydrogen iodide comproportion to water and iodine

Alternatively, sulfur dioxide (SO2) are added to the final liquors from which the saltpetre has already crystallized.

In the production of oil and natural gas, significant amounts of brine are produced, which have an iodine content of between 30 and over 100 ppm. The iodine in the form of sodium iodide is released from the brine through oxidation with chlorine:

Sodium iodide and chlorine react to form sodium chloride and iodine

The iodine obtained is further purified by blowing it out with air, then reducing it again with sulfur dioxide in a sulfuric acid solution and finally oxidizing it back to iodine with gaseous chlorine.

Iodine is reduced to hydrogen iodide by sulfur dioxide
Hydrogen iodide and chlorine react to form hydrogen chloride and iodine

Chromatographically, iodine can be enriched on anion exchangers by adsorbing polyiodide. Potassium iodide and halogen-free copper sulfate are used for ultra-pure production.

On a laboratory scale, iodine can be produced by the action of sulfuric acid and manganese (IV) oxide on potassium iodide. It can also be extracted from the iodine-containing ashes of marine plants by treatment with chlorine.



Under normal conditions, iodine is a solid that forms gray-black, shiny metallic flakes with a density of 4940 kg / m³. They have the properties of a semiconductor. These properties are due to the presence of a layer lattice in which individual levels from I.2Molecules (bond length 2.715 Å) exist. The distance between the planes in an orthorhombic layer crystal is 4.412 Å and thus corresponds to the van der Waals distance between two iodine atoms (4.30 Å). The result of the measurement of the shortest distance between two iodine molecules is significantly lower at 3.496 Å.

Iodine goes on melting (melting point 113.70 ° C