The variation in ionisation enthalpy along
a series of transition elements is much less in comparison to the variation
along a period of non-transition elements The first ionisation enthalpy,
in general, increases, but the magnitude of the increase in the second
and third ionisation enthalpies for the successive elements, is much
higher along a series The irregular trend in the first ionisation enthalpy of the metals of
3d series, though of little chemical significance, can be accounted for
by considering that the removal of one electron alters the relative energies
of 4s and 3d orbitals You have learnt that when d-block elements form
ions, ns electrons are lost before (n – 1) d electrons