CONSEQUENCES OF LANTHANIDE CONTRACTION AND CAUSES

Lanthanide contraction is important factor in allowing the separation of lanthanide from one another. Not only that, the lanthanide contraction has highly significant affect on the relative properties of the elements which precede and follow the lanthanide in this periodic table. The radius of the La³+ ion, For example is 0.21 A larger than that of Y³+ ion which lies immediately on the basis, 14th lanthanide had not occurred between Zr and Hf, the atomic radius of Hf⁴+ ion should have been greater than Zr⁴+ ion which lies immediately above it by about 0.2 A⁰. But occurrence of the lanthanide contraction of about the same order almost exactly cancels the expected increase. The result is that Hf⁴+ and Zr⁴+ ions have almost identical radii. Zirconium (Z=40) and hafnium ( Z=72) have almost identical chemical properties.

                                  Similarly, as a result of lanthanide contraction, the atomic covalent radii of the elements which follow the lanthanide. Eg, Hf, Ta, W, etc. are very similar to those of the elements of the previous row. 

                       It's seen that the normal increase in size from Sc→Y→La disappear after that the lanthanide and pairs of elements, Zr-Hf, Nb-Ta, Mo-W, etc. Posses almost the same size. The properties of these pairs of elements are also very similar.

         It's thus a direct consequence of lanthanide contraction that elements of the second and third transition series resemble each other much more closely than do the elements of the first and second transition series.

Causes of Lanthanide contraction :

            The lanthanide contraction, although more pronounced, is similar to the contraction that was encountered in (d) block transition elements. The cause is also the same in both cases. As we move through the lanthanide series. 4f electrons are being added one at all each step. The mutual shielding effect of (f) electrons is very little, being even similar than that of (d) electrons. This is due to shape of the (f) orbitals. The nuclear charge, however increase by one at each step. Hence the inward pull experienced by the 4 (f) electrons increases. This causes of a reduction in the size of the entire 4f(n) shell. The sum of the successive gives the total lanthanide contraction. It may be noted that the decrease in size, although continuous, is not regular. The decrease is much more in the case of the first 6 elements than in the case of subsequent elements. The properties of lanthanide compounds, accordingly, show some divergence from regularity.