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 .

Answer

(i) Lanthanoids show +3 oxidation state mostly as `2` electrons from outer `6s` orbital and one electron from `5d` orbital take part in bond formation. Some show `+2` and `4` oxidation state due to stability of half filled and completely filled `4f` orbitals.
(ii) It is because of high `Delta_(a)H^(@)` (enthalpy of atomisation) and and low hydration enthalpy `Delta_(hyd)H^(@)`.
(iii) it is due to lanthanoids contraction, effective nuclear charge remains almost same therefore , metallic radii are nearly same .