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(a) (i) How is the variability in oxidat...

(a) (i) How is the variability in oxidation states of transition metals different from that of the p-block elements ?
(ii) Out of `Cu^(+)` and `Cu^(2+)` which ion is unstable in aqueous solution and why ?
(iii) Orange colour of `Cr_(2)O_(7)^(2-)` ion changes to yellow when treated with an alkali. Why ?
(b) Chemistry of actinoids is complicated as compared to lanthanoids. Give two reasons.

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(a) (i) In the p-block, the lower oxidation states are favoured by heavier elements due to inert pair effect, the opposite is true in the case of d-block. For example, in group 6, Mn(VI) and W(VI) are found to be more stable than Cr(VI).
(ii) `Cu^(+)` is unstable in aqueous solution due to disproportionation.
`2Cu^(+)toCu^(2+)+Cu`
Greater stability of `Cu^(2+)` (aq) rather than `Cu^(+)` (aq) is due to much more negative `Delta_(hyd)H^(0)` of `Cu^(2+)` (aq) than `Cu^(+)`, which more than compensates for the second ionisation enthalpy of Cu.
(iii) Orange colour of `Cr_(2)O_(7)^(2-)` ion changes to yellow when treated with an alkali because of the following reaction :
`Cr_(2)O_(7)^(2-)+2OH^(-)to2CrO_(4)^(2-)+H_(2)O`
(b) Chemistry of actinoids is complicated as compared to lanthanoids. The reasons are :
(i) Actinoids exist in different oxidation states.
(ii) Many of the actinoids are radioactive.
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