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 As we move along
the period in 3d series, we see that nuclear charge increases from
scandium to zinc but electrons are added to the orbital of inner subshell,
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