How would you account for the following:
(i) Among lanthanoids, Ln (III) compounds are predominant. However, occasionally in solution or in solid compounds, +2 and +4 ions are also obtained.
(ii) The E⁰M²/M for copper is positive (0.34 V). Copper is the only metal in the first series of transition elements showing this behavior.
(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.

(i) Some lanthanoids show +2 and +4 oxidation states in ionic solutions or solid components due to the extra stability that arises because of empty, half-filled, or fully filled 4f-subshell. This is why occasionally in solution or in solid compounds, +2 and +4 ions are also obtained, even though Ln (III) compounds are predominant.
(ii) This is because copper has high enthalpy of atomization and low enthalpy of hydration. Hence, E⁰Cu²⁺/Cu is positive.
(iii) This is because of lanthanoid contraction; 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. Lanthanide contraction is a term used in chemistry to describe the greater-than-expected decrease in ionic radii of the elements in the lanthanide series from atomic number 57, lanthanum, to 71, lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements starting with 72, hafnium.