Question

Explain the causes of the lanthanoid contraction.

Answer 1

The causes of the lanthanoid contraction are as follows :

• As the atomic number of lanthanoids or 4f-block elements increases the positive nuclear charge increases and correspondingly electrons are added to the prepenultimate 4f sub-shell. 
• The attraction of nucleus on 4f-electrons increases with the increase in atomic number.
• The outer eleven electrons namely, 5s², 5p⁶, 5d³ and 6s² do not shield inner 4 felectrons from the nucleus.
• There is imperfect shielding of each 4f-electron from other 4f-electrons. 
• As compared to d sub-shell, the extent of shielding for 4f-electrons is less. 
• Due to these cumulative effects, 4f-electrons experience greater nuclear attraction and hence valence shell is pulled towards the nucleus to the greater extent decreasing atomic and ionic radii appreciably.
• From ₅₇La to ₅₈Ce, there is a sudden contraction in atomic radius from 187 pm to 183 pm but the further decrease up to the last 4f-element, ₇₁Lu is comparatively low (about 10 pm).