Question

What is meant by lanthanoid contraction? The actinoid contraction is greater than that of lanthanoid contraction. Why?

Answer 1

Lanthanoid contraction is the steady decrease in the atomic and ionic sizes of lanthanide elements with increasing atomic number. This decrease is not much and shows some irregularities.

Atomic radii decrease from La (165 pm) to Lu (156 pm).

Ionic radii (+3 oxidation state) decreases from $C{e}^{3+}$ (103 pm) to $L{u}^{3+}$ (85 pm). The consequences of lanthanoid contraction are as follows:

(1) Basicity decreases from $Ce(OH{)}_3$ to $Lu(OH{)}_3$.

(2) Covalent nature increases from $Ce(OH{)}_3$ to $Lu(OH{)}_3$.

(3) Electronegativity for $M^{3+}$ ion increases.
(4) There is slight increase in $E^o$ values.

In actinoids, 5f orbitals are filled. These 5f orbitals have a poorer shielding effect than 4f orbitals (in lanthanoids). Thus, the effective nuclear charge experienced by electrons in valence shells in case of actinoids is much more that than experienced by lanthanoids. Hence, the size contraction in actinoids is greater as compared to that in lanthanoids.