Electron Configuration Rules

[Martina]

Why is the electron configuration for Chromium [Ar] 3d5 4s1? Can someone please explain the exceptions to electron configuration rules?

[Christine Honda 2I]

A completely full or half full d sub-level is more stable than a partially filled d sub-level, so an electron from the 4s orbital is excited and rises to a 3d orbital. It is easier for them to remove a 4s electron and bring it to the 3d subshell, which will give them a half-filled or completely filled subshell, creating more stability.

[Julie Park 1G]

For Cr, instead of writing:
Cr: [Ar] 3d^4 4s^2
You would write:
Cr: [Ar] 3d^5 4s^1

The second version presents a more stable e- configuration (d subshell becomes half full, instead of being "almost half full")

For Cu, instead of writing:
Cu: [Ar] 3d^9 4s^2
You would write:
Cu: [Ar] 3d^10 4s^1

The second version presents a more stable electron configuration (d subshell becomes completely full, instead of being "almost" full)

[JonathanS 1H]

The way I understand the exceptions is that it takes more energy to have 2 e- in the 4d orbital than to have one in each sublevel evenly distributed (hence Cr: [Ar] 3d^5 4s^1). This also applies for copper in which the 3d shell is full.