(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.

(a) (i) How is the variability in oxidation states of transition metal

1.	(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.
Answer» Solution :(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.