Exceptions

[Ziyi Meng 2K]

Hi, can someone give me a full explanations of the exceptions during electron configuration?

I have in my notes:

"half full d5 and full d10 subshell have lower energy
chromium, Cr: [Ar] 3d54s1"

Could someone explain this to me? Thank you.

[905756606]

Hi! The reason why the electron configuration of chromium is, Cr: [Ar] 3d5 4s1 is because this configuration means one electron from the 4s can be used to fill the 3d orbitals so that they all contain 1 electron in with parallel spins (they are all spinning in the same direction). Half-filled orbitals with parallel spins minimise mutual repulsion which would be present if two electrons were in the same orbital, therefore, making this configuration more stable and energetically favourable. I hope this helps!

[Mark_Botros_1D]

Furthermore, to add to the discussion, chromium has an electron configuration of [Ar] 3D5 4S1 instead of [Ar] 3D4 4S2, which would usually be the case if the 4S orbital gets filled before the 3D orbital. Moreover, copper has an electron configuration of [Ar] 3D10 4S instead of an electron configuration of [Ar] 3D9 4S2, making it another exception.

[Jonathan Shyu 3L]

Just adding onto what has been stated before, simply stated, half and full filled d-orbital is more stable and thus the electron from say 4s2 would jump to the d-orbital to make 3d5 4d1 for Chromium or 3d10 4d1 for Copper. It is just a general rule/principle that happens and needs to be memorized. As Professor Lavelle explained it, electrons have feelings too and are more happy with d5 and d10.

[Saebean Yi 3E]

Also remember, that this is a general rule, so chromium and copper are not the only elements that do this.
I'm not completely sure why "half full d5 and full d10 subshell have lower energy" but I think that's referring to the minimization of repulsion (or increased "stability" since each orbital has equally one electron spinning same way)