Stability of orbitals
According to Hund's rule atoms having half-filled or completely-filled orbitals are comparatively more stable and hence more energy is needed to remove an electron from such atoms. The ionization potential or ionization enthalpy of such atom is, therefore, relatively higher than expected normally from their position in the periodic table.
The extraordinary stability of half-filled and completely filled electron configuration can be explained in terms of symmetry and exchange energy. The half-filled and completely filled electron configurations have symmetrical distribution of electrons and this symmetry leads to stability. Moreover, in such configuration electron can exchange their positions among themselves to maximum extent. This exchange leads to stabilization for example, half-filled 2p orbital is Nitrogen and completely filled orbitals in Neon are given as follows.
Thus the p3,p6,d5,d10,f7 and f14 configuration which are either completely filled or exactly half-filled are more stable.
Further, it may be noted that chromium and copper have five and ten electrons in 3d orbitals rather than four and nine electrons respectively as expected. Therefore, to acquire more stability one of the 4s electron goes into 3d orbitals so that 3d orbitals get half-filled or completely filled in chromium and copper respectively.
Expected configuration : 1s2,2s2,2p6,3s2,3p6,3d4,4s2
Actual configuration : 1s2,2s2,2p6,3s2,3p6,3d5,4s1
Expected configuration : 1s2,2s2,2p6,3s2,3p6,3d9,4s2
Actual configuration : 1s2,2s2,2p6,3s2,3p6,3d10,4s1