Iodine (I)
Reactive nonmetal
Iodine is a shiny blue-black solid that skips the liquid phase at room pressure, subliming into violet vapour. Your thyroid gland cannot make its hormones without it — the reason table salt is iodised.
Position on the periodic table
Atomic properties
| Atomic number | 53 |
|---|---|
| Atomic mass | 126.904 amu |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5 |
| Noble gas shorthand | [Kr] 4d10 5s2 5p5 |
| Electrons per shell | 2, 8, 18, 18, 7 |
| Valence electrons (outer shell) | 7 |
| Common oxidation states | -1, +1, +5, +7 |
| Electronegativity (Pauling) | 2.66 |
| Covalent radius (approx.) | 139 pm |
| First ionization energy | 1008 kJ/mol |
| Electron affinity | 295.2 kJ/mol |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Density | 4.933 g/cm³ |
| Melting point | 113.7 °C (386.85 K) |
| Boiling point | 184.25 °C (457.4 K) |
| Appearance | lustrous metallic gray, violet as a gas |
| Radioactive | No |
| Origin | Occurs naturally |
Electron configuration of iodine
Iodine's ground-state electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5, usually shortened to [Kr] 4d10 5s2 5p5. Its electrons occupy 5 shells (2, 8, 18, 18, 7), placing it in period 5 of the p-block. The 7 outer-shell electrons drive its bonding behaviour. Explore it interactively in the electron configuration calculator.
Uses of iodine
- Iodised salt to prevent deficiency
- Antiseptics and surgical disinfectants
- X-ray contrast agents
- Animal feed supplements and catalysts
Biological role: Essential — thyroid hormones, which control metabolism and development, are built around iodine.
Occurrence: Concentrated in seaweed, Chilean nitrate deposits and brines.
Common compounds of iodine
KI
Potassium iodide
NaIO3
Sodium iodate
History and discovery
Discovered: 1811 — Bernard Courtois. Name origin: From Greek iodes, meaning violet-coloured.
Bernard Courtois discovered iodine in 1811 while processing seaweed ash for gunpowder production.
Safety notes
Dietary iodine is essential; concentrated iodine irritates skin and its vapour irritates lungs.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Iodine
1. What is the chemical symbol of Iodine?
2. What is the atomic number of Iodine?
3. Which category does Iodine belong to?
4. What is the state of Iodine at room temperature?
5. Which period of the periodic table is Iodine in?
Iodine FAQs
What is the atomic number of iodine?
Iodine's atomic number is 53 — every iodine atom has 53 protons in its nucleus.
What is the symbol for iodine?
I. From Greek iodes, meaning violet-coloured.
Is iodine a metal, nonmetal, or metalloid?
Iodine is classified as a reactive nonmetal.
What state is iodine at room temperature?
At about 25 °C, iodine is a solid. It melts at 113.7 °C.
How many valence electrons does iodine have?
Iodine has 7 electrons in its outer shell (shell pattern: 2, 8, 18, 18, 7).
What is iodine used for?
Key uses include: iodised salt to prevent deficiency; antiseptics and surgical disinfectants; x-ray contrast agents.
Related elements
Keep working with this element
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