Question

Separation of lanthanides from their mixture is not easy because:
(A) Shielding effect
(B) Penetrating effect
(C) Consequences of lanthanide contraction
(D) Inert pair effect

Answer 1

Hint: Lanthanides are the elements in f-block of the periodic table. All lanthanides are metals. They are soft, ductile and malleable. Their general electronic configuration can be written as $(n - 2){f^{1 - 14}}(n - 1){d^{0,1}}n{s^2}$ where $n$ is the number of shell.

Complete answer: f-block consists of two series of elements known as lanthanides and actinides. The fourteen members of the lanthanide series have been placed along with lanthanum in the third group of the periodic table. In lanthanide series $4f$ orbitals are gradually filled up. The first member of this series is cerium and the last member is lutetium.
On moving from cerium to lutetium, the decrease in atomic radii occurs from $165pm$ to $156pm$ i.e., the decrease is only $9pm$. Thus for an increase of $14$ in the atomic number, the decrease in atomic radii or ionic radii are very small in comparison to the elements of other groups or periods.
In the lanthanide series as we proceed from one element to the next element in the lanthanide series, the nuclear charge i.e. atomic number increases by one unit and the addition of one electron occurs at the same time in the $4f$ energy shell. On the account of the very diffused shapes of $f - $ orbitals, the $4f$ electrons shield each other quite poorly from the nuclear charge. Thus the effect of nuclear charge increase is somewhat more than that of changed shielding effect. This brings valence shells nearer to the nucleus and hence the size of atom or ion goes on decreasing as we move in the series. This decrease in the size of atoms or ions is known as lanthanide contraction.
Since the change in the ionic radii in lanthanide series is very small, their chemical properties are small. Thus it is very difficult to separate these elements in the pure state.
Thus separation of lanthanides is difficult due to the consequences of lanthanide contraction.

The correct option is (C).

Note: i.$f - $ block elements are also called as inner transition elements.
ii. All lanthanides are strong reducing agents. The reducing power decreases from lanthanum to lutetium.
iii.Lanthanides are poor conductors of heat and electricity.