Question

Why there are some exceptions for writing electronic configuration for copper and chromium ?

Answer 1

Electrostatic potential energy with low potential energies are generally more stable, and thus favored over those with higher potential energies.

Explanation:

A `3d` orbital can hold up to `10` electrons and a `4s` holds up to two.

One might expect a copper atom to have four electrons in its `3d` orbital and two in the `4s` orbital. By the Hund's rule and the exclusion principle, the two electrons in the `4s` orbital would have opposite spins and the four electrons in `3d` to share the same spin.

Atoms would prefer electron configuration with half-filled or fully-filled orbitals so as to minimizes potential energies. Also, orbitals with opposite spins tend to be unstable due to electrostatic repulsions between electrons and are therefore generally not as favored as orbitals with identical electron spins.

As a result, copper atoms would assign one of the electrons initially belonging to the `4s` orbital to the `3d` orbital, rendering both orbitals half-filled.

Similarly, electrons fill chromium orbitals in a way such that the `4s` is half-filled and `3d` fully filled, which would possess lower potential energy than the configuration with a filled `4s` and a `3d` orbital one electron from full.