(a) Complete the following equations :
(i) `Cr_(2)O_(7)^(2-)+2OH^(-)to`
(ii) `MnO_(4)^(-)+14H^(+)+3e^(-)to`
(b) Account for the following:
(i) Zn is not considered a transition element.
(ii) Transition metals form a large number of complexes.
(iii) The `E^(@)` value for the `Mn^(3+)//Mn^(2+)` couple is much more positive than that for `Cr^(3+)//Cr^(2+)` couple.

Explanation

(a) (i) `Cr_(2)O_(7)^(2-)+2OH^(-)to 2CrO_(4)^(2-)+H_(2)O`
(ii) `MnO_(4)^(-)+4H^(+)+3e^(-)to MnO_(2)+2H_(2)O`
(b) (i) Zn is not considered as a transition element.
Zn atom has completely filled d-orbitals `(3d^(10))` in its ground as well as in its oxidised state.
Therefore, it is not regarded as a transition element.
(ii) Transition metals form a large number of complexes because of the following:
They have comparatively smaller sizes of the metal ions.
They have high ionic charges.
d-orbitals are available for bond formation.
(iii) `E^(@)` value for `Mn^(3+)//Mn^(2+)` couple is much more positive i.e., this reduction takes place with greater tendency because `Mn^(2+)` has `d^(5)`, half-filled stable configuration. In the `Cr^(3+)//Cr^(2+)` couple the change is from `d^(3)` to `d^(4)` which takes place with smaller tendency, hence `E^(@)` value is less positive (in fact, it is negative).