ElementsWhat is iodine?
Iodine is a shiny, blue-black solid that turns straight into a violet vapour when heated. The thyroid gland needs it, which is why table salt is often iodised.
What is iodine used for?
Iodised salt, antiseptics, X-ray contrast agents, and making some plastics and screens.
Where the name comes from
From the Greek word iodes, violet, the colour of its vapour.
Key facts
- Symbol
- I
- Atomic number
- 53
- Atomic mass
- 126.9045 u
- Family
- Halogen
- Period
- 5
- Group
- 17
- Block
- p
- State at room temperature
- Solid
- Melting point
- 113.7 °C (386.85 K, 237 °F)
- Boiling point
- 184.4 °C (457.55 K, 364 °F)
- Density
- 4.93 g/cm³
- Electrons per shell
- 2, 8, 18, 18, 7
- Electron configuration
- [Kr]5s2 4d10 5p5
- Common oxidation states
- +7, +5, +1, -1
- Electronegativity (Pauling)
- 2.66
- First ionisation energy
- 10.451 eV
- Electron affinity
- 3.059 eV
- Atomic radius (van der Waals)
- 198 pm
- Discovered
- 1811
Place in the periodic table
Iodine is element number 53. It sits in period 5 (row 5) of the periodic table, in group 17 (column 17), and belongs to the p-block. Just before it comes tellurium (52), and just after it comes xenon (54). In the same column, bromine is above it and astatine is below it. Elements in one column tend to behave alike.
Family: Halogens.
Physical properties
At room temperature iodine is a solid. It melts at 113.7 °C and boils at 184.4 °C, so it stays liquid over a range of 71 degrees. Its density is 4.93 grams per cubic centimetre, so a one-litre block would weigh about 4.93 kilograms. By density it ranks 64th among the 96 elements with a measured value.
Inside the atom
An atom of iodine has 53 protons in its nucleus and, when neutral, 53 electrons around it. The electrons are arranged in 5 shells: 2, 8, 18, 18, 7. The outermost shell holds 7 electrons. Written in full, the electron configuration is [Kr]5s2 4d10 5p5.
Its electronegativity on the Pauling scale is 2.66. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 10.451 electronvolts (about 1,008 kilojoules per mole). One mole of iodine weighs 126.9045 grams, so a single gram contains about 4.75 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains iodine with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.