Iodine (I)
Element 53 · Halogen · atomic mass 126.9 u · group 17, period 5
Electron shells: 2, 8, 18, 18, 7
Iodine sublimes straight from solid to a violet vapour when warmed. Your thyroid cannot make its hormones without it, which is why salt is iodised.
Iodising table salt eliminated the most common preventable cause of intellectual disability worldwide.
- Atomic number
- 53
- Atomic mass
- 126.9 u
- Electron configuration
- [Kr] 5s² 4d¹⁰ 5p⁵
- Group · period · block
- 17 · 5 · p
- Oxidation states
- −1, +1, +5, +7
- Electronegativity
- 2.66 (Pauling)
- Melting point
- 114 °C
- Boiling point
- 184 °C
- Density
- 4.93 g/cm³
- State at 20 °C
- Solid
- Appearance
- Lustrous purple-black solid
- Discovered
- Bernard Courtois, 1811
53 protons, and 53 electrons when neutral
IUPAC standard atomic weight
Halogens
What Iodine is used for
- Iodised salt and dietary supplements
- Antiseptics and disinfectants
- X-ray contrast agents
- LCD polarising film
Atomic structure of Iodine
A neutral iodine atom has 53 protons and 53 electrons. Its most common isotope carries about 74 neutrons, which is where the mass number 127 comes from. Change the neutron count and you have a different isotope of iodine; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 18, 7. In orbital notation that is [Kr] 5s² 4d¹⁰ 5p⁵, leaving 7 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Iodine's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Iodine sits in the periodic table
Group 17, one electron short of a full shell, which makes them aggressive oxidisers. They form salts directly with metals — "halogen" literally means salt-former.
Discovery and naming
Iodine was discovered by Bernard Courtois in 1811. Its name comes from Greek iodes, "violet".
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Iodine?
The standard atomic weight of Iodine is 126.9 u, as published by IUPAC's Commission on Isotopic Abundances and Atomic Weights.
How many protons, neutrons and electrons does Iodine have?
Iodine has 53 protons, because the atomic number is the proton count and that is what makes an atom Iodine at all. A neutral Iodine atom has the same number of electrons, 53. The neutron count varies by isotope; the most common one has about 74, since 127 − 53 = 74.
What is the electron configuration of Iodine?
The electron configuration of Iodine is [Kr] 5s² 4d¹⁰ 5p⁵. Its electrons are arranged 2, 8, 18, 18, 7 across 5 shells.
Is Iodine a metal?
No. Iodine is classified as a halogen, on the right-hand side of the table where elements gain or share electrons rather than losing them.
What group and period is Iodine in?
Iodine is in group 17 (the halogens), period 5, in the p-block. The group is its column and the period is its row.
What is the charge of Iodine?
Iodine most commonly forms a 1− ion. It is 1 electron short of a full outer shell, so it gains one rather than losing 7. Across all its compounds it shows oxidation states of −1, +1, +5, +7.
What is the melting point of Iodine?
Iodine melts at 114 °C and boils at 184 °C. That puts it in the solids at everyday temperatures.
What is Iodine used for?
Iodised salt and dietary supplements, Antiseptics and disinfectants, X-ray contrast agents, LCD polarising film. Iodine sublimes straight from solid to a violet vapour when warmed.
Who discovered Iodine, and where does its name come from?
Iodine (I) was discovered by Bernard Courtois in 1811. The name comes from Greek iodes, "violet".