Question

Give reasons:

First ionisation energy of 5d-series elements are higher than those of 3d-and 4d-transition elements.

Give a plausible reason for the fact that the first ionisation energies of 5d-transition metals are higher than those of 3d- and 4d-transition metals.

Answer 1

1. The 5d-series elements come after the filling of the 4f orbitals (lanthanoid series).
2. The 4f orbitals poorly shield the increasing nuclear charge because of their shapes.
3. This poor shielding causes the valence electrons in 5d elements to experience a stronger attraction from the nucleus.
4. As a result, the atomic size of 5d elements is smaller or comparable to 4d elements despite having more electrons.
5. Hence, more energy (higher ionisation energy) is required to remove the outermost electron from 5d elements compared to 3d and 4d elements.

This effect is called the lanthanoid contraction and explains the higher first ionisation energies of the 5d transition metals.