Rubidium vs Caesium
Rubidium and Caesium sit in the same column of the periodic table — same family, one period apart — so almost every difference between them comes from one thing: the extra electron shell Caesium wears.
Side by side
| Property | Rubidium | Caesium |
|---|---|---|
| Symbol | Rb | Cs |
| Atomic number (Z) | 37 | 55 |
| Atomic mass | 85.468 u | 132.91 u |
| Category | Alkali metal | Alkali metal |
| Group | 1 | 1 |
| Period | 5 | 6 |
| Phase at room temp | solid | solid |
| Electron configuration | [Kr] 5s^1 | [Xe] 6s^1 |
| Valence electrons | 1 | 1 |
| Atomic radius | 265 pm | 298 pm |
| Electronegativity | 0.82 | 0.79 |
| First ionisation energy | 403 kJ/mol | 376 kJ/mol |
The verdicts
- Bigger atom: Caesium — the lower an element sits in its group, the more shells it wears.
- Greedy electron-taker: Rubidium — higher electronegativity means it pulls shared electrons harder.
- Heavier: Caesium — compare the atomic masses in the table above.
Want the visual version with bars for radius, electronegativity and ionisation energy? Open the interactive compare tool with this pair already loaded.