Iodine (I)

Stable Isotopes of Iodine

Isotope Z(p) N(n) Atomic Mass Natural Abundance Nuclear Spin
I-127  53  74  126.904468 100.00%  5/2+ 


Iodine was discovered in 1811 by Bernard Courtois. Its name comes from the Greek word iodes, meaning “violet.” It appears as bluish-black orthorhombic crystals.

Slightly soluble in water, iodine is also soluble in ethanol, carbon disulfide, benzene and chloroform (forming brown solutions). Sulfur, selenium, metal iodides and many organic compounds dissolve in liquid iodine. The chemical properties of iodine are quite similar to those of other halogens, especially chlorine and bromine, although it is less electronegative. All iodine reactions occur in vapor phase or aqueous media; vapor-phase reactions require elevated temperatures. Iodine vapors combine with metals, forming their iodides. In acid solutions, iodine reduces powerful oxidizing agents and is itself oxidized. Iodine combines with fluorine, chlorine and bromine, forming interhalogen compounds. Among organic compounds, alkenes readily react with iodine; substitution reactions occur at high temperatures.

Iodine is used in many dyes and as a colorant for foods and cosmetics. Its silver salt is used in photographic negative emulsions. Other industrial applications include the dehydrogenation of butane and butylenes, serving as a catalyst in organic reactions, in the treatment of naphtha to yield high-octane motor fuel, and in the preparation of many metals in high-purity grade. Radioactive isotopes of iodine are used for treating thyroid cancer and heart diseases (including tachycardia) and as a tracer for diagnosing certain diseases. An important application of iodine is in water purification and sanitation. It is used as a disinfectant in food-processing plants, dairies and restaurants. It is applied to disinfect municipal and other water supplies and swimming pools.

Iodine vapors are an irritant to eyes, nose and mucous membranes. Inhalation can cause headache, irritation and congestion of lungs. Oral intake can produce burning of the mouth, vomiting, diarrhea and abdominal cramps. Skin contact can cause rashes.

Properties of Iodine

Name Iodine 
Atomic number 53 
Atomic weight 126.905 
Standard state Solid at 298 °K 
CAS Registry ID 7553-56-2 
Group in periodic table 17 
Group name Halogen 
Period in periodic table
Block in periodic table p-block 
Color Violet-dark gray, lustrous 
Classification Non-metallic 
Melting point 113.7 °C
Boiling point 184.3 °C
Thermal conductivity 0.449 W/(m·K)
Electrical resistivity > 1015 x 10-8 Ω m 
Electronegativity 2.66 
Heat of vaporization 20.9 (per mole I atoms) kJ·mol-1
Heat of fusion 7.76 (per mole I atoms) kJ·mol-1
Density of vapor 6.75 g/L
Density of liquid 3.96 g/cm3 at 120 ºC
Density of solid 4.933 g/cm3 at 20 ºC 
Electron configuration [Kr]4d105s25p6
Atomic radius 1.33 Å 
Ionic (I-) radius  2.20 Å 
Common oxidation state -1
Other oxidation states  +1, +3, +5, +7 
Vapor pressure of solid 0.3075 torr at 25 ºC and 90.5 torr at 113.6 ºC
Critical temperature 545.8 ºC
Critical pressure 48.9 atm
Critical volume 155 cm3/mol