Holmium has 67 electrons arranged as 2, 8, 18, 29, 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 holmium that gives [Xe] 4f11 6s2.
| Atomic number | 67 |
| Symbol | Ho |
| Atomic mass | 164.93 u |
| Group | f-block (no group number) |
| Period | 6 |
| Block | f-block |
| Category | Lanthanide |