Lanthanoids

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ce + Pr + Tb + Yb Tm Sm Eu Ionic radii/pm Atomic number + . . Electronic Configurations . .

Atomic and Ionic Sizes It may be noted that atoms of these elements have electronic configuration with s common but with variable occupancy of f level (Table . ). However, the electronic configurations of all the tripositive ions (the most stable oxidation state of all the lanthanoids) are of the form f n (n = to with increasing atomic number). The overall decrease in atomic and ionic radii from lanthanum to lutetium (the lanthanoid contraction ) is a unique feature in the chemistry of the lanthanoids.

It has far reaching consequences in the chemistry of the third transition series of the elements. The decrease in atomic radii (derived from the structures of metals) is not quite regular as it is regular in + ions (Fig. . ).

This contraction is, of course, similar to that observed in an ordinary transition series and is attributed to the same cause, the imperfect shielding of one electron by another in the same sub-shell. However, the shielding of one f electron by another is less than one d electron by another with the increase in nuclear charge along the series. There is fairly regular decrease in the sizes with increasing atomic number. The cumulative effect of the contraction of the lanthanoid series, known as lanthanoid contraction , causes the radii of the members of the third transition series to be very similar to those of the corresponding members of the second series.

The almost identical radii of Zr ( pm) and Hf ( pm), a consequence of the lanthanoid contraction, account for their occurrence together in nature and for the difficulty faced in their separation. In the lanthanoids, La(II) and Ln(III) compounds are predominant species. However, occasionally + and + ions in solution or in solid compounds are also obtained. This irregularity (as in ionisation enthalpies) arises mainly from the extra stability of empty, half-filled or filled f subshell.

Thus, the formation of Ce IV is favoured by its noble gas configuration, but it is a strong oxidant reverting to the common + state. The E o value for Ce + / Ce + is + . V which suggests that it can oxidise water. However, the reaction rate is very slow and hence Ce(IV) is a good analytical reagent.

Pr, Nd, Tb and Dy also exhibit + state but only in oxides, MO . Eu + is formed by losing the two s electrons and its f configuration accounts for the formation of this ion. However, Eu + is a strong reducing agent changing to the common + state. Similarly Yb + which has f configuration is a reductant.

Tb IV has half-filled f -orbitals and is an oxidant. The behaviour of samarium is very much like europium, exhibiting both + and + oxidation states.