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

Yes, the above fact is correct. Actually, in lanthanoids, the oxidation states of +2 and +4 are shown only by those elements which can gain a stable configuration of `f^(0), f^(7) or f^(14)` by losing 2 or 4 electrons. E.g. `Eu^(2+)` has configuration `[Xe]4f^(7) Ce^(4+)` has configuration `[Xe]4f^(0) Yb^(2+)` has a configuration `[Xe]4 f^(14)` etc.
These stable configurations may be achieved in aqueous solution, because in aqueous state the Lu atom forms hydroxide with water and as they have the tendency to make stable configuration like , `f^(0),f^(7)` for `f^(14)`, they find `H^(+)` ions easily available in water. So, they lose their additional electrons to get stabilized.
In aqueous solution Ln-atom are surrounded by nine water molecules and their coordination number is 8. So, it becomes easier to get protons in the solution.