Properties of the Lanthanide Metals; Correlations and Discontinuities

The article “Characterization and Classification of Lanthanides by Multivariate-Analysis Methods” is very interesting, but it strikes me that correlations somehow miss those very important discontinuities that give a valuable guide to the chemical properties. For example, the very low values of density and melting point for europium and ytterbium compared with those of their immediate neighbors indicate a 2+ oxidation state. Conversely the link of the large value of IE3 to the low value of ∆H of atomization for Eu and Yb is well explained by their electron configurations. These peculiar effects are not fortuitous.

If Eu and Yb have the “most stable” electron configurations, one might expect their first two ionization energies to be much larger than those of their neighbors. They are not. It is the third ionization energy that shows the relative inertness of the 7 and 14 f-electron shells in the 2+ cations. The sums of IE1 and IE2 are similar for the pairs Sm/Eu and Tm/Yb. It is the high melting points of Gd and Lu which correlate with the low values of IE3 [as shown in Table 4 on p 476 and Figure 5 on p 478 of ref and as explained in Johnson’s book].

What is always fascinating is how macroscopic properties like melting point and boiling point can be meaningfully correlated with atomic properties like ionization energy, and how there is still a place for the old relationships of Trouton, and Dulong and Petit.