Francium has 87 electrons arranged as 2, 8, 18, 32, 18, 8, 1 across 7 shells, which is why it sits in period 7.
Start from the previous noble gas core, written in square brackets, then add the remaining electrons in filling order. For francium that gives [Rn] 7s1.
| Atomic number | 87 |
| Symbol | Fr |
| Atomic mass | 223 u |
| Group | 1 |
| Period | 7 |
| Block | s-block |
| Category | Alkali metal |
| Valence electrons | 1 |
| Element | Configuration |
|---|---|
| Hydrogen | 1s1 |
| Lithium | [He] 2s1 |
| Sodium | [Ne] 3s1 |
| Potassium | [Ar] 4s1 |
| Rubidium | [Kr] 5s1 |
| Caesium | [Xe] 6s1 |
Elements in the same group have the same number of outer electrons, which is why they behave alike.