Actinium has 89 electrons arranged as 2, 8, 18, 32, 18, 9, 2 across 7 shells, which is why it sits in period 7.
Start from the previous noble gas core, written in square brackets, then add the remaining electrons in filling order. For actinium that gives [Rn] 6d1 7s2.
Exception to the Aufbau rule. Filling orbitals in order would give 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f1, but the real configuration is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d1 7s2 because the energies of the outer orbitals are very close, so the real ground state differs from the simple filling order.
| Atomic number | 89 |
| Symbol | Ac |
| Atomic mass | 227 u |
| Group | 3 |
| Period | 7 |
| Block | d-block |
| Category | Actinide |