Question: 68: Gadolinium has a low value of third ionisation enthalpy because of
(1) high electronegativity
(2) high basic character
(3) small size
(4) high exchange enthalpy
Answer: Option (4)
Explanation:
Gadolinium has the electronic configuration [\mathrm{Xe}]\,4f^{7}5d^{1}6s^{2}.
On losing three electrons, gadolinium forms the \mathrm{Gd}^{3+}
ion with configuration [\mathrm{Xe}]\,4f^{7}.
The 4f^{7} configuration is a half-filled subshell,
which is highly stable due to maximum exchange energy.
Because the \mathrm{Gd}^{3+} ion attains a particularly stable half-filled
4f configuration, the removal of the third electron is easier.
This extra stability arises from high exchange enthalpy associated with
the half-filled 4f subshell.
Therefore, gadolinium has a comparatively low value of third ionisation enthalpy due to high exchange enthalpy.