Thallium has 81 electrons arranged as 2, 8, 18, 32, 18, 3 across 6 shells, which is why it sits in period 6.
Start from the previous noble gas core, written in square brackets, then add the remaining electrons in filling order. For thallium that gives [Xe] 4f14 5d10 6s2 6p1.
| Atomic number | 81 |
| Symbol | Tl |
| Atomic mass | 204.38 u |
| Group | 13 |
| Period | 6 |
| Block | p-block |
| Category | Post-transition metal |
| Valence electrons | 3 |
| Element | Configuration |
|---|---|
| Boron | [He] 2s2 2p1 |
| Aluminium | [Ne] 3s2 3p1 |
| Gallium | [Ar] 3d10 4s2 4p1 |
| Indium | [Kr] 4d10 5s2 5p1 |
| Nihonium | [Rn] 5f14 6d10 7s2 7p1 |
Elements in the same group have the same number of outer electrons, which is why they behave alike.