Example 4.2

There is an increase in ionisation enthalpy along each series of the transition elements from left to right due to an increase in nuclear charge which accompanies the filling of the inner d orbitals. Table . gives the values of the first three ionisation enthalpies of the first series of transition elements. These values show that the successive enthalpies of these elements do not increase as steeply as in the case of non-transition elements.

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 d series, though of little chemical significance, can be accounted for by considering that the removal of one electron alters the relative energies of s and d orbitals. You have learnt that when d -block elements form ions, ns electrons are lost before ( n – ) d electrons.

As we move along the period in d series, we see that nuclear charge increases from scandium to zinc but electrons are added to the orbital of inner subshell, i.e., d orbitals. These d electrons shield the s electrons from the increasing nuclear charge somewhat more effectively than the outer shell electrons can shield one another. Therefore, the atomic radii decrease less rapidly. Thus, ionization energies increase only slightly along the d series.

The doubly or more highly charged ions have d n configurations with no s electrons. A general trend of increasing values of second ionisation enthalpy is expected as the effective nuclear charge increases because one d electron does not shield another electron from the influence of nuclear charge because d -orbitals differ in direction. However, the trend of steady increase in second and third ionisation enthalpy breaks for the formation of Mn + and Fe + respectively. In both the cases, ions have d configuration.

Similar breaks occur at corresponding elements in the later transition series. The interpretation of variation in ionisation enthalpy for an electronic configuration d n is as follows: The three terms responsible for the value of ionisation enthalpy are attraction of each electron towards nucleus, repulsion between the