Question

Discuss the consequences of lanthanide contraction.

Answer 1

Definition: The size of $M^{2+}$ ions decreases as we move through the lanthanides from lathanum to lutetium. This steady decrease in ionic radii of $M^3$ cations in the lanthanides. series is called lanthanide contraction.
Causes of lanthanide contraction: The lanthanide contraction is due to the imperfect shielding of one $4f$ electron by another in the same sub shell. As we move along the lanthanide series, the nuclear charge and the number of $4f$ electrons increase by one unit at each step. However, due to imperfect shielding, the effective nuclear charge increases causing a contraction in electron cloud of $4f$-subshell.
Consequences of lanthanide contraction:
(i) Basicity of ions: Due to lanthanide contraction, the size of ${Ln}^{3+}$ ions decreases regularly with increase in atomic number. According to Fajan's rule, decrease in size of ${Ln}^{3+}$ ions increase the covalent character and decrease the basic character between ${Ln}^{P3+}$ and ${OH}^{-}$ ion in $Ln{\left(OH\right)}_3$. Since the order of size of ${Ln}^{3+}$ ions are
${La}^{3+}>{Ce}^{3+}...>{Lu}^{3+}$
(ii) There is regular decrease in their ionic radii.
(iii) Regular decrease in their tendency to act as reducing agent, with increase in atomic number.
(iv) Due to lanthanide contraction, second and third rows of d-block elements (transition elements) are quite close in their properties.
(v) Due to lanthanide contraction, these elements occur together in natural minerals and are difficult to separate.