Aufbau Princple

[Leslie Romo 1C]

Hi
Can anyone explain the "exceptions" to the Aufbau Principle?
I know that Silver is one of them but I don't understand why

[Michelle Dela Rosa 1D]

Silver is in the same group as Copper, which is an exception to the Aufbau Principle (the building-up principle), the other being Chromium. In these two cases, half full d^5 and full d^10 subshells have lower energy. For Chromium, the electron configuration would be [Ar] 3d^5 4s^1 instead of [Ar] 3d^4 4s^2; the half full d^5 subshell is lower in energy. Similarly, the electron configuration for Copper (Silver is in the same group) is [Ar] 3d^10 4s^1 instead of [Ar] 3d^9 4s^2; the full d^10 subshell is lower in energy. In these cases, lower total energy can be achieved if the electron enters a d orbital instead of the s orbital since the configurations complete a half subshell or full subshell.

[MattDavid1d]

A half filled subshell in the d orbital is more stable than if there was a 6th electron. Additionally having a half filled d orbital provides more stability than a full s orbital and a d-orbital that is not full or half filled.