(a) (i) The variability in oxidation state of transition metal is due to the incomplete filling of d-orbitals in such a way, that their oxidation states differ from each other by unity, e.g., `Fe^(2+) and Fe^(3+)` etc. In case of P-block elements, the oxidation state differ by units by two, e.g. `+3 and +5`. Moreover, in transition elements, the higher oxidation states are more stable for heavier elements in a group, e.g., `Mo^(+6)` is more stable than `Cr^(4+)`. In P-block elements, the lower oxidation states are more stable for heavier members due to inert pair effect, e.g., `Pb^(2+)` is more stable than `Pb^(4+)`.
(ii) `Cu^(+)` is unstable in aqueous solution than `Cu^(2+)`. This is because, although `2^(nd) I.E` of copper is large but hydration enthalpy for `Cu^(2+)` is much more negative than that or `Cu^(+)` and compounds are unstable in aqueous solution and undergo disproportionation.
`2Cu^(+) rarr Cu^(2+) + Cu`
(iii) Orange colour of `Cr_(2)O_(7)^(2-)` ion changes to yellow when treated with an alkali because of the formation of chromate ions
`{:(Cr_(2) O7^(2-) + 2OH^(-),rarr,2CrO_(4)^(2-) + H_(2)O),("Dichromate ion",,"Chromate ion"),("(orange)",,"(yeloow)"):}`
(b) Chemistry of actinoids is complicated as compared to lanthanoids because:
(i) actinoids show wide range of oxidation states i.e., `+3, +4, +5, +6` due to small energy difference between `5f, 6d and 7s` subshells of actinoids.
(ii) actinoids are radioactive due to that, chemistry of actinoids is complicated.
