4s vs 3d Orbitals

[Mona Reddy Kurra 1J]

Hi! In my notes, I wrote that after the 4s state is occupied and electrons enter the 3d state, the 3d state is considered lower in energy than 4s. Why is 3d lower in energy than 4s when 4s is completely occupied and 3d has at least 1 electron? Why are chromium and copper exceptions? Thanks!

[Edwin Montalvo 1G]

Normally, the 4s orbital is lower in energy than the 3d orbital due to the screening of electrons accounted for in the Aufbau system. However, a phenomenon exists where half-filled orbitals have extra stability. In the case of chromium, if we were to follow Aufbau's principle with [Ar] 4s2 3d4, we would have a filled 4s orbital but an almost half-filled d orbital, implying that the 4s has a lower energy state here which is not true due to the not half-filled d orbital. Instead, if we make the 3d orbital half-filled with parallel spin (3d5) getting [Ar] 3d5 4s1, we correctly account for the extra stability during half-filled orbitals where the 3d orbital would be occupied first before 4s.The same can be said with copper except instead of a half-filled d orbital it is the fully-filled d orbital providing the extra stability ([Ar]d104s1) since the d orbital can hold 10 electrons max. Hope this helps!

[Preston Pham 2H]

3d is lower in energy than 4s because it is in a lower subshell. Also in today's lecture he wrote how low energy goes from 1s 2s 2p 3s 3p 3d 4s 4p to high energy. The reason why chromium and copper are exceptions is because in the case of 3d5 and 3d10 the atom is more stable in that shape instead of being in lets say 3d4 and 3d9. That's why they have to take an electron from the 4s orbital to make the atom more stable. Thats basically all we need to know because in today's lecture he explained that we just need to memorize/know these exceptions.

[Marie Khijniak 2E]

Hi!
The 3d subshell is considered lower in energy than the 4s subshell because the 3d subshell is on a lower energy level of 3, rather than 4. In the case of chromium and copper, this is just a matter of creating more stable and the lowest possible ground state for these atoms. A half-full or completely full subshell are the most energy efficient, so instead of chromium being [Ar](3d^4)(4s^2), where there is one electron needed to complete the last d orbital, its electron configuration becomes [Ar](3d^5)(4s^1), giving it two half-0full orbitals and as a result putting it into a lower energy ground state. For copper, instead of [Ar](3d^9)(4s^2), where once again, only one electron is needed to complete the last d orbital, the actual electron configuration is [Ar](3d^10)(4s^1), giving it one half-full orbital and a full subshell, making its ground state energy lower and the atom more stable, as a result.
Hope this helps! :)