d5 and d10 having lower energy

[Vicky Tong 3E]

Professor mentioned that there are two exceptions to the energy levels: half-full d5 and full d10 subshell have lower energy. He also showed that period 5 shows this trend because 4d state has lower energy than 5s state.

Can someone please explain why?

[Sevde Coban 2J]

For Cr, it has a half full d5 subshell because it gives the atom more stability. The half filled d sublevel is more stable than having the d sublevel partially filled. For Cu, instead of having 4s2 and 3d9, the configuration has 4s1 and 3d10 because the completely filled d sublevel is more stable.

[Rohita Thammineni 2D]

Hi! A half-full 3d subshell is more favorable because it allows for five unpaired electrons with parallel spins in the 3d subshell, with each orbital containing one electron. This is much more stable than having one empty orbital in the 3d subshell and a full 4s subshell. The same concept applies to having a completely full 3d subshell for copper because having all the electrons in each orbital in the 3d subshell be paired is more stable than having one half-full orbital in the 3d subshell with a full 4s subshell. In regards to period 5, it follows the same logic of stability as in period 4.

[Jelix Tsan 2H]

Normally, only after fully inhabiting the 4s subshell will electrons begin to occupy the 3d subshell one at a time. However, a 4s^1 3d^5 electron configuration is lower energy than a 4s^2 3d^4 configuration, and similarly a 4s^1 3d^10 electron configuration is lower energy than a 4s^2 3d^9 configuration. Therefore, whenever electrons are put in this scenario they will leave the 4s orbital only partially filled and choose to fully occupy the 3d orbital first.