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

Answer

For first five elements , the effective nuclear charge predominates over shielding effect and thus ionisation energy increase . In next Five element , Shielding effect increase more rapidly due to dense packing of electron in d-orbitals (pairing take place ) and the effective nuclear charge and shield effect counter balance each other to show a net result constant ot irregular but closer trend in ionisation energy values.