Question

What is lanthanide contraction? Mention the causes for it.

Answer 1

In order to know what a lanthanide contraction is, we must be knowing about the lanthanides. The lanthanides are the chemical elements that belong to the f-block. It consists of elements from the lanthanum to lutetium (with atomic number 58-71).

Complete step by step answer:
Let us first learn what lanthanides are. Lanthanides are the 15 elements that are present in the periodic table and these elements belong to the f-block elements. Lanthanides will be having atomic numbers from $58-71$ i.e. from lanthanum to lutetium. It will be having a general electronic configuration of $[Xe]4{f^{1 - 14}}5{d^1}6{s^2}$.
Let us now see what a lanthanide contraction is. When we move down the lanthanide series i.e. from Lanthanum (58) to Lutetium (71). The atomic number will increase but the ionic radius will shrink. The steady decrease in the ionic radius along the series is called lanthanide contraction.
The cause for lanthanide contraction is the poor shielding effect of the f-block element. The f- block element’s shielding effect is very less compared to the d-block element. When the nuclear charge increases the valence shell will be pulled towards the nucleus.
Some of the consequences are it is hard to separate the lanthanides due to the strong attachment with one another. The ionic radii is inversely proportional to the electronegativity and ionisation energy.

Note: Due to decrease in the ionic radii, it is observed that the basicity of the elements in the lanthanide series will also decrease. This is due to the increase in the covalent bond between the atoms. As the ionic radii decreases, the electronegativity and the ionisation energy increases.