Electron configurations

[Marin_Rice_2D]

Hi!

Can someone explain why for Cr the ground-state electron configuration is [Ar] 3d^5 4s^1 instead of [Ar] 3d^4 4S^2? Also what other elements are irregular like this/ what is the rule for writing out their electron configurations?

[406135406]

Cr is an exception, as well as Co (copper) to this e-config rule, because they are one electron away from being more stable, either because they have 4 or 9 electrons in the d-orbital. Therefore, there is a concept where these elements will move their s electron to the d-orbital to stabilize.