(1) The predicted electronic configuration of Cu (Z=29) is 1s2 2s2 2p6 3s2 3p6 4s2 3d9 .
(2) But experimentally it has been found that the actual configuration is 1s2 2s2 2p6 3s2 3p6 4s1 3d10.
(3) Here Aufbau principle is violated.According to Aufbau principle, the electron occupies the orbital having least energy first.
(4) That means in the ground state the electronic configuration can be built up by placing electrons in the lowest available orbitals until the total number of electrons added is equal to the atomic number.
(5) But the atoms with half filled or fully filled orbitals in their outermost orbit are more stable when compared with their neighbouring elements.
(6) In order to get fully filled orbitals in the outermost orbit in copper the electrons in 4s and 3d redistribute their energies to attain stability.
(7) Hence, the actual electronic configuration of copper is Cu (Z=29) [Ar] 4s1 3d10.