Rare earth elements and their place in the Periodic Table

2 Rare earth elements and their place in the Periodic Table 

The ionic radii decrease by ca. 20% from La to Lu (Table 1.1, values for 8-coordination ). It arises through inadequate screening of the nuclear charge by the f electrorrs. As the nrunber of f electrons increases, there is an increase in the effective nuclear charge leading to a progressive reduction in the Ln size. The effect is so pronotmced that with Ho and Er , the ionic raditts has decreased to that of Y , the rare earth of the second transition metal series. Consequently, the 

Rare earth metals are highly electropositive and hence they are readily oxidized by air. In general for Ln +3e — Ln , E°=ca. -2.3V comparable with reduction of Mg to Mg, and considerably more negative than for reduction of AE. Thus, smelting of rare earth oxides/salts to the metal is thermodynamically veiy disfavoured. 

Up until the last 25 years, the known oxidation states for molecular rare earth compounds were those of the final colunrn of Table 1.1 not in parentheses. Thus, all rare earths form compounds in very stable -i-III state, three Sm, Eu, Yb give +II compounds and Ce, Pr, Tb have compounds in the +IV state. As shown by the data in parentheses in the final column of Table 1.1, new oxidation state molecular compounds have been a recent research highlight, and the divalent state is now known for all rare earth elements except Pm. On the other hand, ionic divalent halides LnX (X=Cl, Br, I) have been known for elements (Ln=Nd, Dy, Tm) other than Ln= Sm, Eu, Yb for many years from the work of soUd state chemists. However, the diiodides, Lnl (Ln=La, Ce, Pr, Gd) do not contain Ln but Ln and are to be formulated La (e )(T)j. They show metallic properties.  

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