Account for the following:
a. The expected electronic configuration of copper is '[Ar] 3d^9 4s^2' but actually it is '[Ar] 3 d^(10) 4 s^1'
b. In building up of atoms, the filling of 4s orbitals occurs before '3 d' - orbitals.

Explanation

a. It is because half filled and completely filled orbitals are more stable.

b. 4s orbital has lower.energy than .3 d. orbital, therefore, it is filled before .3 d. orbital.