Name:Iodine Symbol:I
Type:Halogen Atomic weight:126.9045
Density @ 293 K:4.93 g/cm3 Atomic volume: 25.74 cm3/mol

Iodine was discovered by Bernard Courtois in 1811. He isolated the element by adding sulfuric acid to seaweed ashes. This produced a purple vapor, which condensed to form dark crystals of iodine. Its name comes from the Greek work 'iodes', meaning violet.


State (s, l, g):solid
Melting point:386.6 K (113.5 °C) Boiling point:457 K (184 °C)


Specific heat capacity:0.214 J g-1 K-1 Heat of atomization:107 kJ mol-1
Heat of fusion: 15.52 kJ mol-1 of I2 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

Oxidation & Electrons

Shells:2,8,18,18,7 Electron configuration:[Kr] 4d10 5s2 5p5
Minimum oxidation number:-1 Maximum oxidation number:7
Min. common oxidation no.:0 Max. common oxidation no.:7
Electronegativity (Pauling Scale):2.66 Polarizability volume:5 Å3

Appearance & Characteristics

Structure:layers of I2 Color:bluish-black
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 disulphide, Iodine yields purple colored solutions. It is barely soluble in water, giving a yellow solution.


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.


Reaction with air:none Reaction with 6 M HCl:none
Reaction with 15 M HNO3:mild, ⇒ HIO3 Reaction with 6 M NaOH:mild, ⇒ OI-, I-


Oxide(s):I2O5, I4O9, I2O4 Chloride(s):ICl, ICl3
Hydride(s): HI


Atomic radius:140 pm Ionic radius (1+ ion):pm
Ionic radius (2+ ion):pm Ionic radius (3+ ion):pm
Ionic radius (2- ion):pm Ionic radius (1- ion): 206 pm


Thermal conductivity:0.45 W m-1 K-1 Electrical conductivity:1.0 x 10-7 S cm-1

Abundance & 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

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 vapour from processed brine, by ion exchange of brine or by releasing iodine from iodate taken from nitrate ores.


34 whose half-lives are known, with mass numbers 108 to 141. Of these, only one is stable: 127I




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