How would you account for the following : (i) Among lanthanoids,Ln(III) compounds are predominant. However,occasionally in solution or in solid compound,`+2` and `+4` ions are also obtained. (ii) The `E_(M^(2)//M)^(@)` 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 member of the second series.

How would you account for the following : (i) Among lanthanoids,Ln(III

1.	How would you account for the following : (i) Among lanthanoids,Ln(III) compounds are predominant. However,occasionally in solution or in solid compound,`+2` and `+4` ions are also obtained. (ii) The `E_(M^(2)//M)^(@)` 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 member of the second series.
Answer» Correct 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 .

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