The Lanthanoid Contraction

The lanthanoid contraction is the steady decrease in size along the series of elements La—Lu. 

The elements La to Lu are characterized by the + 3 oxidation state, and the chemistry is mostly that of the Ln  

The overall decrease in atomic and ionic radii (Table 24.1) from La to Lu has major consequences for the chemistry of the third row of J-block metals (see Section 22.3). The contraction is similar to that observed in a period of d-block metals and is attributed to the same effect the imperfect shielding of one electron by another in the same sub-shell. However, the shielding of one 4/ electron by another is less than for one d electron by another, and as the nuclear charge increases from La to Lu, there is a fairly regular decrease in the size of the Af sub-shell. 

The ionic radii for the lanthanoids in Table 24.1 refer to 8-coordinate ions, and those for the actinoids to 6-coordina-tion. The values should only be used as a guide they increase with increasing coordination number and are by no means absolute values. 

In Section 19.7, we looked at coordination numbers up to 9. The large size of the lanthanoid and actinoid metals means that in their complexes, high coordination numbers ( 6) are common. The splitting of the degenerate set of/ orbitals in crystal fields in small (A gt 1 kJmoH ) and crystal field stabilization considerations are of minor importance in lanthanoid and actinoid chemistry. Preferences between different coordination numbers and geometries tend to be controlled by steric effects. 

Lawrence Livermore National Laboratory Science Photo Library 

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It was also proposed that the significant s and p orbital contraction at row four (Cu) is caused by the post-transition metal effect (d contraction), caused by an increase of the effective nuclear charge for the 4s electrons due to filling the first d shell (3d). A similar interpretation is possible for the row six (Au). This effect is commonly called lanthanoid contraction due to the effect of filling the 4f shell. The traditional explanation for the smaller size of gold (compared to Ag) is the lanthanoid contraction. However, this effect is only sufficient to cancel the shell-structure expansion, to make Au (nonrelativistic) similar to Ag (nonrelativistic). 

Since much of the chemistries of Cd and Hg are distinct, we shall deal with the two metals separately. In making this decision, we are effectively saying that the consequences of the lanthanoid contraction are of minor significance for the heavier metals of the last group of the J-block. 

Atom and ion sizes and the lanthanoid contraction Spectroscopic and magnetic properties Sources of the lanthanoids and actinoids... 

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