Ruthenium has 44 electrons arranged as 2, 8, 18, 15, 1 across 5 shells, which is why it sits in period 5.
Start from the previous noble gas core, written in square brackets, then add the remaining electrons in filling order. For ruthenium that gives [Kr] 4d7 5s1.
Exception to the Aufbau rule. Filling orbitals in order would give 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d6, but the real configuration is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d7 5s1 because the energies of the outer orbitals are very close, so the real ground state differs from the simple filling order.
| Atomic number | 44 |
| Symbol | Ru |
| Atomic mass | 101.07 u |
| Group | 8 |
| Period | 5 |
| Block | d-block |
| Category | Transition metal |