Question

La, Gd and Lu have abnormally low third ionisation enthalpy values. Why? ​

Answer 1

La, Gd and Lu have abnormally low third ionisation enthalpy values Because of extra stability of $f^0,f^7$ and $f^{14}$ orbitals​

$\underset{\left[Xe\right]4f^{0}5d^1}{L{a}^{2+}}\to \underset{\left[Xe\right]4f^0}{L{a}^{3+}}$

$\underset{\left[Xe\right]4f^{7}5d^1}{G{d}^{2+}}\to \underset{\left[Xe\right]4f^7}{G{d}^{3+}}$

$\underset{\left[Xe\right]4f^{14}5d^1}{L{u}^{2+}}\to \underset{\left[Xe\right]4f^{14}}{L{u}^{3+}}$