Iodine (I): electron configuration, atomic mass and facts
Iodine (I) is element 53 of the periodic table, a halogen in period 5, group 17. Its electron configuration is [Kr] 4d10 5s2 5p5, with electrons arranged in shells 2, 8, 18, 18, 7, and its standard atomic weight is 126.9 u.
💡 Turns into a purple vapour when heated and turns starch blue-black.
▶ Watch Iodine’s electrons orbit in the interactive periodic tableIodine at a glance
| Symbol | I |
|---|---|
| Atomic number | 53 |
| Atomic mass | 126.9 u |
| Category | Halogen |
| Group | 17 |
| Period | 5 |
| Block | p-block |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5 |
| Noble-gas shorthand | [Kr] 4d10 5s2 5p5 |
| Electrons per shell | 2, 8, 18, 18, 7 |
| Outer-shell electrons | 7 |
| Electronegativity (Pauling) | 2.66 |
| State at 20 °C | solid |
Experiments with iodine
✨ Golden rain
Make lead(II) iodide, dissolve it hot, then watch glittering golden crystals form on cooling.
🐘 Elephant's toothpaste
Iodide-catalysed decomposition of 30% hydrogen peroxide produces a towering, steaming foam.
🟤 Halogen displacement
Chlorine water releases iodine from iodide; starch shows it as blue-black.
Where is Iodine on the periodic table?
Iodine is in period 5, group 17, in the p-block. The period number matches its 5 electron shells, and the block names the subshell its last electrons fill. In the same group, Bromine sits directly above it and Astatine sits directly below it. Elements in one group have the same number of outer-shell electrons (7 here), which is why they react in similar ways.
How to work out the electron configuration of Iodine
Put 53 electrons into subshells in order of rising energy, filling each before starting the next: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18), 4s² (20), 3d¹⁰ (30), 4p⁶ (36), 5s² (38), 4d¹⁰ (48), 5p⁵ (53). In shorthand, [Kr] stands for the 36 electrons of krypton, giving [Kr] 4d¹⁰ 5s² 5p⁵.
How electronegative is Iodine?
Iodine has a Pauling electronegativity of 2.66, ranking 7 of the 100 elements with a known value (fluorine is highest at 3.98). That is high: it pulls shared electrons strongly towards itself in a bond.
Iodine in the virtual lab
Faradium’s chemistry workbench includes these chemicals that contain iodine: Iodine crystals, Potassium iodide, Iodine solution. Open the Chemistry Lab to experiment with them.
Frequently asked questions
What is the electron configuration of Iodine?
The electron configuration of Iodine (I) is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5. In noble-gas shorthand it is written [Kr] 4d10 5s2 5p5.
How many electrons does Iodine have in each shell?
Iodine has 53 electrons arranged in shells as 2, 8, 18, 18, 7. It has 7 electrons in its outer shell.
What is the atomic mass of Iodine?
The standard atomic weight of Iodine is 126.9 u, so one mole of Iodine atoms has a mass of about 126.9 g.
Is Iodine a metal?
No. Iodine is a halogen.
Explore nearby elements
More halogens: Fluorine, Chlorine, Bromine, Astatine, Tennessine. See all elements on the periodic table.