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 : On moving from left to right along the first transition series, the first ionisation enthalpy `(IE_(1))` is expected to increase. In practice the trend is irregular, because removal of one electron alters the relative energies of 4s and 3d orbitals. So the unipositive ions have `3d^(n)` configuration (instead of `3d^(n)4s^(1)`) with no 4s - electron. Thus there is a reorganisation energy accompanying ionisation with the transference of 4s electrons into 3d - orbitals. Cr and Cu has relatively low first ionisatio enthalpy, because loss of one electron gives stable electronic configurations `3d^(5)` and `3d^(10)`, respectively. Zn has very high ionisation enthalpy because one 4s electron is to be removed from stable configuration `3d^(10)4s^(2)`.
Second ionisation enthalpy : Second ionisation enthalpies are much higher because removal of second electron from positively charged species becomes difficult. In general, 2nd ionisation enthalpy increases from left to right. However, Cr and Cu have much higher values because 2nd electron is to be removed from the stable configuration of `Cr^(+)(3d^(5))` and `Cu^(+(3d^(10))`. The value for Zn is sufficiently low because the ionisation involves attainment of stable electronic configuration `(3d^(10)4s^(1)to 3d^(10)4s^(0))`.