Assign reasons for the following :
(i) Cu(I) ion is not known to exist in aqueous solutions.
(ii) Transition metals are much harder than the alkali metals.
(iii) From element to element, actinoid contraction is greater than the lanthanoid contraction.

Explanation

(i) an aqueous medium , `Cu^(2+)` is more stable than `Cu^(+)` . This is because although energy is required to remove one electron from `Cu^(+)` to `Cu^(2+)` , high hydration energy of `Cu^(2+)` compensates for it. Therefore, `Cu^(+)` ion is an aqueous solution is unstable. It disproportiontes to give `Cu^(2+)` and Cu.
`" "Cu^(+)(aq) to Cu^(2+)(aq) + Cu(s)`
(ii) Transition metals behave differently than other metals. These elements are used for much more than just coins, and many other, but copper alloys with such elements as zinc and tin to form harder brasses and bronzes. Iron oxides are mildly alkaline, so iron resists attacks by alkalis, acids attack it. The transition metals react vigorously with water, producing hydrogen gas and but are harder, more dense, and melt at higher temperatures than the alkali metals. Some references take them out fo the transition metals and consider them.
(iii) Actionoid contraction is greater than lanthanoid contration due to poor shielding by 5f electrons than by 4f electron.