Question

How would you account for the following:
i) Among lanthanoids, Ln (III) compounds are predominant. However, occasionally in a solution or in solid compounds, +2 and +4 ions are also obtained.
ii) The $\text{E}_{{{{\text{M}}^{\text{+2}}}}/{\text{M}}\;}^{\text{0}}$ 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 metal are nearly the same as those of the corresponding members of the second series.

Answer 1

(i) Variable oxidation states are seen in lanthanides, out of this +3 is most stable. However, due to some other favourable factors +2 and +3 ions are also seen.
(ii) Outer electronic configuration of Copper in $\text{C}{{\text{u}}^{^{+2}}}$ is $\text{3}{{\text{d}}^{9}}$ which is unstable. Hence, it tries to achieve stable configuration by reducing to Cu.
(iii) The atomic radii of lanthanide ions decrease steadily from La to Lu due to increasing nuclear charge and electrons entering inner (n-2) f orbital. This is called Lanthanide contraction.

Complete answer:
i) The energy difference between 4f and 6s subshells is extremely high. This makes it difficult to remove electrons from the 4f subshell. Hence, most lanthanides loose tend to lose two 6s electrons and 1 is lost from 4f. This causes predominance of +3 Oxidation state in lanthanide. However, in ionic or aqueous solutions extra stability occurs due to half-filled orbitals or completely filled 4f subshell. Hence, even though Ln (III) compounds are predominant, occasionally in a solution or in solid compounds, +2 and +4 ions are also obtained.
ii) The change in enthalpy that occurs when all the bonds in one mole of a given substance, are broken to get atoms in gaseous phase, is called as enthalpy of atomisation. When one mole of an ion, in gaseous state under 1 bar pressure and standard conditions, dissolves in water the change in enthalpy that occurs is called as enthalpy of hydration. Copper has a high value of enthalpy of atomisation and low value of enthalpy of hydration. Hence, $\text{E}_{{\text{C}{{\text{u}}^{\text{+2}}}}/{\text{Cu}}\;}^{\text{0}}$value of copper is positive (0.34 V).
iii) From La to Hf, total 14 elements are present between 4d and 5d series. These elements are involved in lanthanoid contraction. It occurs due to increasing nuclear charge and electrons entering the inner (n-2) f orbital. Hence, the metallic radii of the third (5d) series of transition metal are nearly the same as those of the corresponding members of the second series.

Note:
(i) The elements with atomic numbers from 58 to 71 following the element Lanthanum are also called rare earth metals. These are present at the bottom of the periodic table.
(ii) Atomic number of copper is 29 and its electronic configuration is $\text{ }\\!\\![\\!\\!\text{ Ar }\\!\\!]\\!\\!\text{ 3}{{\text{d}}^{10}}\text{4}{{\text{s}}^{1}}$. It usually shows +1 and +2 oxidation states.
(iii) Due to similar atomic radii, lanthanide elements show similar chemical properties and hence their separation becomes difficult.