How would you account for the irregular variation of ionisation enthalpies (first and second) in the first series of the transition elements?

Explanation

First ionisation enthalpy : As we move from left to right, it is expected in general that the first ionisation enthalpy should show an increasing trend. However, the trend is irregular because removal of the electron alters the relative energies of 4s and 3d orbitals. Thus, there is a reorgani-sation energy accompanying ionisation. This results into the release of exchange energy which increases as the number of electrons increases in the d-orbitals and also from the transference of s-electrons into d-orbitals. Cr has low first ionisation energy because loss of one electron gives stable electronic configuration `(3d^(5))`. Zn has very high ionisation energy because electron is removed from 4s orbital of the stable configuration `(3d^(10)4s^(2))`. Half-filled and fully filled configurations are more stable than others.
Second ionisation enthalpy : After the loss of one electron, the removel of second electron becomes much more difficult. Hence, second ionisation enthalpies are much higher and in general increase from left to right. However, Cr and Cu show much higher values because the second electron has to be removed from the stable configurations of `Cr^(+)(3d^(5))` and `Cu^(+)(3d^(10))`.