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_(M^(2+)//M)^(0)` for copper is positive (0.34 V). 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

(i) Lanthanoids exhibit occasionally +2, +4 ions because of extra stability of empty, half-filled and completely filled 4f-subshell respectively i.e., `4f^(0), 4f^(7), 4f^(14)` configuration
(ii) The `E_(M^(2+)//M)^(0)` for copper is +ve. This is because high energy is required to transform `Cu "to" Cu^(2+)` which is not balanced by its hydration enthalpy.
(iii) In moving down a group, there is increase in number of shells leading to increase in size. So, size of elements of 4d-series is larger than those of 3d-series. The similar atomic radii of second and third transition series is due to phenomenon called lanthanold contraction, associated with intervention of 4f-orbitals which must be filled before the 5d-series of elements begin.