Thulium has 69 electrons arranged as 2, 8, 18, 31, 8, 2 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 thulium that gives [Xe] 4f13 6s2.
| Atomic number | 69 |
| Symbol | Tm |
| Atomic mass | 168.93 u |
| Group | f-block (no group number) |
| Period | 6 |
| Block | f-block |
| Category | Lanthanide |