Iodine has 53 electrons arranged as 2, 8, 18, 18, 7 across 5 shells, which is why it sits in period 5.
Start from the previous noble gas core, written in square brackets, then add the remaining electrons in filling order. For iodine that gives [Kr] 4d10 5s2 5p5.
| Atomic number | 53 |
| Symbol | I |
| Atomic mass | 126.90 u |
| Group | 17 |
| Period | 5 |
| Block | p-block |
| Category | Halogen |
| Valence electrons | 7 |
| Element | Configuration |
|---|---|
| Fluorine | [He] 2s2 2p5 |
| Chlorine | [Ne] 3s2 3p5 |
| Bromine | [Ar] 3d10 4s2 4p5 |
| Astatine | [Xe] 4f14 5d10 6s2 6p5 |
| Tennessine | [Rn] 5f14 6d10 7s2 7p5 |
Elements in the same group have the same number of outer electrons, which is why they behave alike.