Question

Lanthanide contraction is caused due to:
(a) the same effective nuclear charge from Ce to Lu
(b) the imperfect shielding on outer electrons by 4f electrons from the nuclear charge
(c) the appreciable shielding on outer electrons by 4f electrons from the nuclear charge
(d) the appreciable shielding on outer electrons by 5d electrons from the nuclear charge

Answer 1

The lanthanide contraction is the greater-than-expected decrease in ionic radii of the elements in the lanthanide series from atomic number 57, lanthanum, to 71, lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements starting with 72, hafnium.

Complete step by step answer:
We have been asked about the lanthanide contraction:
The lanthanide contraction is the greater-than-expected decrease in ionic radii of the elements in the lanthanide series from atomic number 57, lanthanum, to 71, lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements starting with 72, hafnium.
for example:
- Due to lanthanide contraction the size for Zr and Hf is the same. Lanthanide contraction can be explained on the basis of shielding effect.In multi-electron atoms the electrons are added in outer shells. The electrons already present in inner shells, shield the outer electrons from nuclear charge, making them experience a lower effective charge of the nucleus. The shielding effect exerted by the inner electrons decreases in the order s > p > d > f . This f subshell poorly shields the outer electron from nuclear attraction which results in more attractive pull of nucleus on outer electron and smaller size.In case of post lanthanide elements like Hf, 4f subshell is filled and it is not very effective at shielding the outer shell (n = 5 and n = 6) electrons. it is similar to Zr.
- Now, we need to tell how is it caused:
The Lanthanide Contraction is caused by a poor shielding effect of the 4f electrons. Gd because as atomic number increases, the atomic radius decreases. Because the elements in Row 3 have 4f electrons. These electrons do not shield well, causing a greater nuclear charge. The correct answer is option “C” .

Note: Due to lanthanide contraction, the ionic size from La 3+ to Lu 3+ along the lanthanide series gradually decreases. As a result, the covalent properties of lanthanide metal hydroxide compounds regularly increase [according to Fajan's rule,] from lanthanum to lutetium.