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

Explanation

In the first transition series, there is irregular variation of ionisation enthalpies because the stability of 3d configuration differs to some extent. Generally the ionisation enthalpy increses with increase in effective nuclear charge, Thus, it is lower for Cr due to absence of any change in the d-configuration while high for Zn as it represents an jonisation from the 4s level. Second ionisation energy, shows the removal of second. The configurations like `d^(5) and d^(10)` are exceptionally stable. So, they have high ionisation enthalpies.