The electronic configuration of valence shell of Cu is `3d^(10)4s^(1)` and not `3d^(9)4s^(2)`. How is this configuration explained?

Explanation

Configuration either exactly half-filled or fully filled orbitals are more stable due to symmetrical distribution of electrons and maximum exchange energy in `3d^(10)4s^(1)` d-orbitals are completely filled and s-orbital is half-filled. Hence it is more stable configuration.