I
Iodine
Element 53 · Halogen
Iodine (I)
Iodine is a shiny, blue-black solid that turns into a violet vapour when heated. The thyroid gland needs it to make hormones.
- Halogen
- Period 5
- Group 17
- p-block
- solid at room temperature
Key properties
- Atomic mass
- 126.9045
- u
- Electron configuration
- [Kr] 4d10 5s2 5p5
- Melting point
- 113.7 °C
- 386.85 K
- Boiling point
- 184.4 °C
- 457.55 K
- Density
- 4.93 g/cm³
- Electronegativity
- 2.66
- Pauling
- Atomic radius
- 198 pm
- van der Waals
- 1st ionisation energy
- 10.451 eV
Compare with every element:Electronegativity chartAtomic radius chartIonisation energy chartDensity chart of the elementsMelting point chartElectron configuration chart
Did you know?
Notable facts about iodine
- 01
Courtois found iodine while extracting saltpeter from seaweed ash for Napoleon’s gunpowder.
- 02
Iodised salt has nearly eliminated goiter in many countries.
- 03
Iodine sublimes — it goes straight from solid to gas at gentle heat.
Applications
What is iodine used for?
- Antiseptics
- Iodised salt
- X-ray contrast agents
- Thyroid medicine
History
Discovery & name origin
Discovered by
Year
Where
France
Name origin
Greek iodes, “violet”, for the colour of its vapour.
Atomic structure
Atomic properties
Atomic structure
Atomic properties
- Atomic number
- 53
- Atomic mass
- 126.9045 u
- Group
- 17
- Period
- 5
- Block
- p-block
- Electron configuration
- [Kr] 4d10 5s2 5p5
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5
- Electrons per shell
- 2, 8, 18, 18, 7
- Electronegativity (Pauling)
- 2.66
- Electron affinity
- 3.059 eV
- Atomic radius (van der Waals)
- 198 pm
- Stable isotopes
- 1
Oxidation states
Highlighted: common oxidation states (+7, +5, +1, -1).
Successive ionisation energies
Energy to remove each electron in turn (kJ/mol). Big jumps show where a new, inner shell begins.
- 1st1,008.4
- 2nd1,845.9
- 3rd3,180
Physical
Physical properties & colour
Physical
Physical properties & colour
- State at 20 °C
- Solid
- Melting point
- 386.85 K · 113.7 °C · 236.7 °F
- Boiling point
- 457.55 K · 184.4 °C · 363.9 °F
- Density
- 4.93 g/cm³
- Molar heat capacity
- —
- Appearance
- Lustrous metallic grey, violet as a gas
- CPK model colour
- Copied
The CPK colour is the convention used for this element’s atoms in molecular models.
FAQ
Iodine questions
What is I on the periodic table?
I is the symbol for iodine, element 53 on the periodic table. It sits in group 17, period 5, in the halogen category, and has an atomic mass of 126.9045.
What is the atomic number of iodine?
Iodine has atomic number 53, so every iodine atom has 53 protons in its nucleus. Its symbol is I and its atomic mass is 126.9045.
Who discovered iodine?
Iodine was discovered by Bernard Courtois in 1811 (France). The name comes from Greek iodes, “violet”, for the colour of its vapour.
What is iodine used for?
Iodine is used for: antiseptics; iodised salt; X-ray contrast agents; thyroid medicine.
Is iodine a metal?
No. Iodine is classed as a halogen.
What is the electron configuration of iodine?
The electron configuration of iodine is [Kr] 4d10 5s2 5p5. Its 53 electrons are arranged in shells as 2, 8, 18, 18, 7.