As 3d and 4s orbital – these 2 levels are very close in energy there are the frequent exchange of electrons occurs between them and this gives rise to the more stable configuration 3d¹⁰4s¹ for Cu than the alternative 3d⁹4s²

Two major factors are responsible for this :

1. The n+l value of 4s is 4+0 = 4 and also for 3d is 3 + 2 = 5.

• Hence, the 3d orbital is higher energy than 4s so the electron should enter 4s first and after filling up then should enter 3d but this is not the actual case (Aufbau principle)

• . as the screening effect of outside the argon, the core is not sufficient to shield the 3d orbital electrons, therefore it lacks in energy and the electrons first enter the 3d instead of 4s and 4s become the outermost valence shell and even loss of electrons occur from it.

2. Another important factor is the extra stability of a full filed (d¹⁰) configuration. There are two major reasons behind this :

i. The symmetrical distribution of the electrons paired in the d orbital.

ii. And the exchange energy of the electrons is also maximum while holding the full-filled d orbital.