(a) (i) `8Mn O_(4(aq))^(2-) + 3S_(2)O_(3(aq))^(2-) + 8H_(2)O(l) to 8MnO_(2(aq)) + 6SO_(4(aq))^(2-) + 2OH_((aq))^(-)`
(ii) `Cr_(2)O_(7 (aq))^(2-) + 3H_(2)S_((g)) + 8 H_((aq))^(+) to 2Cr_((aq))^(3+) + 7H_(2)O(l) + 3S(s)`
(b) (i) The actinoid contraction is similar to lanthanoid contraction , this contraction is due to poor shielding by 5 f electrons in the actinoids than that by 4 f electrons in the lanthanoids.
(ii) The highest oxidation in the middle of the transition series is due to the use of all 4s and 3d electrons.
(iii) `Cr^(2+)` has the configuration `d^(4)` and easily changes to `d^(3)` has half filled orbitals and hence stable . Therefore , `Cr^(2+)` is reducing . On the other hand , `Mn^(2+)` is more stable due to half filled `d^(5)` configuration and `Mn^(3+)` easily changes to `Mn^(2+)` and hence is oxidising .