How would you account for the following :
(i) Among lanthanoids , Ln(III) compounds are predominant , However, occasionally in solutions or in solid compounds , +2 and +4 ions are also obtained .
(ii) The `E_(M2+//M)^(@)` for copper is positive (0.34V ) . Copper is the only metal in the first series of transition elements showing this behaviour .
(iii) The metallic radii of the third (5d) series of transition metals are nearly the same as those of the corresponding members of the second series .

Explanation

Lanthanoid metals show +2 and +4 oxidation states in solution or in solid compounds to attain extra stable `f^0` and `f^7` and `f^(14)` configurations. For example, Eu and `Y_b` exhibit +2 oxidation states due to extra stability of half-filled and completely filled 4f-subshell respectively. Ce and Tb exhibit + 4 oxidation states due to the extra stability of empty and half-filled 4f-subshell respectively.