Erbium has 68 electrons arranged as 2, 8, 18, 30, 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 erbium that gives [Xe] 4f12 6s2.
| Atomic number | 68 |
| Symbol | Er |
| Atomic mass | 167.26 u |
| Group | f-block (no group number) |
| Period | 6 |
| Block | f-block |
| Category | Lanthanide |