Terbium has 65 electrons arranged as 2, 8, 18, 27, 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 terbium that gives [Xe] 4f9 6s2.
| Atomic number | 65 |
| Symbol | Tb |
| Atomic mass | 158.93 u |
| Group | f-block (no group number) |
| Period | 6 |
| Block | f-block |
| Category | Lanthanide |