Electron Config Exceptions

[Margia Adriano 2A]

Hi!

The two exceptions to the typical electron configuration rules are chromium and copper. I know that with their configurations, instead of filling the 4s orbital, it instead goes to the 3d orbital where it becomes the 3d⁵ for chromium and 3d¹⁰ for copper. I don't understand the reason for that which was that it does this for "symmetry purposes." Can someone explain the reason the configurations are that way or clarify what it means that it does this for symmetry?

Thank you!

[Alex Mele 2A]

These exceptions occur because half- and fully filled d sublevels are more stable than other partially filled d sublevels. Electrons will generally want to be in the lowest (most stable) energy state, so for Cu or Cr they will fill up a d sublevel halfway or fully instead of the s sublevel.

[Jiwon_Chae_3L]

I think that in regards to "symmetry" it means that it is more beneficial for the electrons to be distributed along the 3d orbital due to the d orbital having 5 sub-orbitals, each allowing room for 2 electrons of opposite spins to occupy it. Because chromium and copper have 5 and 10 electrons to fill their orbitals, it's more stable for those electrons to occupy the spaces in the 3d orbital instead of filling the 4s orbital. Sorry if I am being confusing, it's hard to explain in words.

[Jaclyn Dang 3B]

Filling the d sub orbitals first gives it a lower energy level. And electrons want to be in the lowest energy state because that is when they are most stable, therefore they fill those sub orbitals before the s.