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

Yes, the above fact is correct Actually, in lanthanoids, the oxidation states of +2 and +4 only by those elements which can gain a stable configuration of `f^(0) , f^(7) or f^(14)` loosing 2 or 4 electrons Ex : `Eu^(+2)` has configuration `[Xe]_(54) f^(7), Ce^(+4)` has configuration `[Xe]_(54)f^(0), Y_(b + 2)` has a configuration `[Xe]_(54) F^(14)` etc.
These stable configuration may be achieved in aqueous solution, because in aq, state the Lu-atom forms hydroxide with water and as they have the tendency to make stable configuration like.
`f^(0), f^(7) or f^(14)` they find `H^(+)` ions easily available in water and so they loose their additional electrons to get stabilized.
Actually in aqueous solution Ln-atom are surrounded by nine water molecules and their co-ordination no. is 8 So it becomes easier to get protons in the solution.