ChemistryMediumClass 12

Lanthanoid Contraction — Cause, consequences

d & f-block Elements

JEE Qs

Hard

min

Thoroughly understand the cause (poor 4f shielding) and the two major consequences: similar radii/properties of 4d/5d transition elements and the trend in basicity of lanthanoid hydroxides.

✅ Key Points for JEE

1Lanthanoid Contraction refers to the gradual decrease in the atomic and ionic radii (M³⁺ ions) of the lanthanoids with increasing atomic number (from La to Lu).
2The primary cause is the poor shielding effect of the 4f electrons. As nuclear charge increases across the series, the added 4f electrons do not effectively shield the outer electrons from the increased nuclear attraction, leading to a greater effective nuclear charge and a shrinkage in size.
3The most significant consequence is that the elements of the second (4d) and third (5d) transition series that follow the lanthanoids exhibit very similar atomic and ionic radii, leading to similar chemical properties (e.g., Zr and Hf, Nb and Ta, Mo and W).
4Another important consequence is the progressive decrease in the basicity of the lanthanoid hydroxides (Ln(OH)₃) from La(OH)₃ to Lu(OH)₃. As the size of Ln³⁺ decreases, the covalent character of the Ln-OH bond increases, and the basicity decreases.
5Lanthanoid contraction also leads to a slight increase in ionization enthalpy and electronegativity for the elements immediately following the lanthanoids in the periodic table.

⚠️ Common Mistakes

✕Confusing the cause with other shielding effects (e.g., d-block contraction, which is less significant). The poor shielding of 4f electrons is specific and critical.
✕Forgetting the direct impact on the similarity of 4d and 5d transition metal properties, which is a frequently tested consequence.
✕Misinterpreting the trend in basicity of lanthanoid hydroxides (often students confuse it with trends in main group elements).

NCERT Chapters

Class 12 Chemistry Part 1 Ch 8: The d- and f-Block Elements