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

Explanation

The removal of one electron from the metal atom alters the relative energies of 4s and 3d orbitals. there will be a reorganisation of energy.after ionisation. It is not found to be regular for 3d elements. It is mainly due to the varying degree of stability ofilifferent 3d configuration. For example, .if ions formed by ionisation has `d^0,d^5ord^10` .configuration, the ionisation enthalpy will be lower than expected,