(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 `Co^(2+)` ion appears to be stable , it is easily oxidised to `Co^(3+)` ion in the presence of a strong ligand.
(iii) The `E_(Mn^(2+)//Mn)^(@)` value for manganese is much more than expected from the trend for other elements in the series.

Explanation

(a) Steady decreases in the ionic radius from `La^(3+)` to `Lu^(3+)` is termed as lanthanoid contraction .
Cause : Effective nuclear charge increase an addition of `e^(-)` and electrons added inf-subshell causes imperfect shielding which is unable to counter balance the effect of increased nuclear charge .
Consequences : (1) Chemical properties are similar (2) size of group 2nd and 3rd elements of transition elements of same group are similar to lanthanoids .
(b) (i) Since `Cu^(+)` in aqueous solution undergoes disproportionation .i.e.,
`2Cu^(+) (aq) to Cu^(2+) (aq) + Cu(s)`
The `E^(@)` value for this is favourable .
(iii) Co(III) is stabilised because of higher crystal field splitting energy [or strong ligand causes pairing of electrons to give more stable Co (III) ions ].
(iii) The comparatively high value for Mn shows that `Mn^(2+) (d^(5))` is particularly stable.