Periodic Table


Atomic number: 53
Atomic weight: 126.90447
Symbol: I
Group number: 17
Electronic configuration: [Kr].4d10.5s2.5p5


Iodine is a bluish-black, lustrous solid. It volatilises at ambient temperatures into a pretty blue-violet gas with an irritating odour.

It forms compounds with most elements, but is less reactive than the other halogens, which displace it from iodides. Iodine exhibits some metallic-like properties. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulphide to form beautiful purple solutions. It is only slightly soluble in water. Iodine compounds are important in organic chemistry and very useful in medicine and photography. Lack of iodine is the cause of goitre (Derbyshire neck). The deep blue colour with starch solution is characteristic of the free element. It is assimilated by seaweeds from which it may be recovered, and is found in Chilean saltpetre, caliche, old salt brines, and salt wells.

General information

Discoveror: Bernard Courtois
Date discovered: 1811
Discovered at: France
Meaning of name: From the Greek word "iodes" meaning "violet"

Physical data

Standard state: solid at 298 K
Colour: violet-dark grey, lustrous
Density of solid at ambient temperature/kg m-3: 4660
Molar volume/cm3: 25.72

Radii /pm

Atomic: 115
Covalent (single bond):
Pauling radius for the ion [I]-: 216

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius95.2109.241.1no data


Both values are quoted on the Pauling scale.

Pauling: 2.66
Allred Rochow: 2.21

Crystal Structure

structure: orthorhombic

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 386.85
boiling point: 457.4

Enthalpies /kJ mol-1

fusion: 15.52
vaporization: 41.6

single bond enthalpies:
I-F I-Cl I-Br I-I I-I
231 208 175 149 151.808

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance
127I126.904473 (5)100

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/I.html