129. What is lanthanoid contraction? What are the consequences of lanthanoid contraction?

A group of fourteen elements following lanthanum i.e. from ₅₈Ce to ₇₁Lu placed in 6th period of long form of periodic table is known as lanthanoids (or lanthanide series). These fourteen elements are represented by common general symbol ‘Ln’. In these elements, the last electron enters the 4f-subshells (pre pen ultimate shell). It may be noted that atoms of these elements have electronic configuration with 6s² common but with variable occupancy of 4f level. However, the electronic configuration of all the tripositive ions (the most stable oxidation state of all lanthanoids) are of the form 4f ⁿ(n = 1 to 14 with increasing atomic number). These elements constitute one of the two series of inner transition elements or f-block.
Lanthanoid contraction: In the lanthanoide series with the increase in atomic number, atomic radii and ionic radii decrease from one element to the other, but this decrease is very small. The regular small decrease in atomic radii and ionic radii of lanthanides with increasing atomic number along the series is called lanthanoid contraction.
Cause of lanthanoid contraction: When one moves from ₅₈Ce to ₇₁Lu along the lanthanide series nuclear charge goes on increasing by one unit every time. Simultaneously an electron is also added which enters to the inner f subshell. The shielding effect of f-orbitals in very poor due to their diffused shape. It results in the stronger force of nuclear attraction of the 4f electrons and the outer electrons causing decrease in size.
Consequences of lanthanoid contraction:
(i)    Similarly in the properties of elements of second and third transition series e.g. Sr and Hf; Nb and Ta; Mo and W. This resemblance is due to the similarity in size due to the presence of lanthanoids in berween.
(ii)    Similarity among lanthanoids: Due to the very small change in sizes, all the lanthanoids resemble one another in chemical properties.
(iii)    Decrease in basicity: With the decrease in ionic radii, covalent character of their hydroxides goes on increasing from Ce(OH)₃ to Lu(OH)₃ and so base strength goes on decreasing.