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(a) What is meant by the term lanthanoid contraction? What is it due to and what consequences does it have on the chemistry of elements following lanthanoids in the periodic table?(b) Explain the following observations:(i) Cu+ ion is unstable in aqueous solutions.(ii) Although Co2+ ion appears to be stable, it is easily oxidised to Co3+ ion in the presence of a strong ligand.(iii) The Eo Mn 2+ |Mn value for manganese is much more than expected from the trend for other elements in the series. |
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Answer» (a) Lanthanoid contraction: The steady decrease in the atomic and ionic radii of lanthanoids with increase in atomic number is known as lanthanoid contraction Cause of lanthanoid contraction: As we move along the lanthanoid series, for every additional proton in the nucleus, the corresponding electron goes into 4f -subshell, there is poor shielding of one electron by another in this subshell due to the shapes of these f-orbitals. The imperfect shielding is not able to counterbalance the effect of the increased nuclear charge. Thus the net result is decrease in size with increase in atomic number. Consequences: (ii) The basic strength of hydroxides decreases from La(OH) 3 to Lu(OH)3 . (b) (i) Because the high hydration enthalpy of Cu2+ easily compensates the second ionization enthalpy of Cu. (ii) Because strong ligand cause spin pairing giving rise to diamagnetic octahedral complex which are very stable and have very large crystalfield stabilization energy. This splitting energy overcomes the ionization enthalpy. (iii) This is due to stability of Mn 2+ as it has half filled d5 configuration. |
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