According to Aufbau’s principle, the electron enters the orbital having lower n + l value. If both orbitals have same n + l values, electron enters the orbital with lower ‘n’ value.
Oprbital |
n + l value |
4d |
4 + 3 = 7 |
5p |
5 + 1 = 6 |
5s |
5 + 0 = 5 |
So, 3s has least n + l value. Therefore the electron enters 5s orbital first.