Electron Configuration [ENDORSED]

[Steven Garcia 1H]

I understand that the 4s- orbitals get filled before the 3d- orbitals because the 4s- orbitals have lower energy levels. However, I'm confused as to why some of the elements in the periodic tables (specifically the elements in period 5 of the d-block) have s^1 instead of s^2? Don't the 4^s-orbitals have to be completely filled before the electrons can start filling the d-orbitals? (please refer to periodic table)

-Steven

[Tarika Gujral 1K]

Atoms prefer to be in the lowest energy state possible. A half-filled or fully-filled d-subshell has a lower energy, which is why electron configurations that should be written as 3d4-4s2 or 3d9-4s2 are instead written 3d54s1 and 3d104s1