(a) Account for the following :
(i) Transition metals form large number of complex compounds
(ii) The lowest oxide of transition metal is basic whereas the highest oxide is amphoteric or acidic
(iii) `E^(@)` value for the `Mn^(3+)//Mn^(2+)` couple is highly positive `(+1.57V)` as compare to `Cr^(3+)//Cr^(2+)`
(b) Write one similarity and one difference between the chemistry of lanthanoid and actinoid elements.

Explanation

(i) Transition metals form large no. of complex compounds because of their smaller size, high ionic charges and availability of vacant d-orbitals
(ii) The lowest oxide of transition metal is basic because the metal has low oxidation state whereas, the highest oxide is acidic or amphoteric due to highest oxidation state. In the low oxidation state, some of the valence `e^(-)s` of metal ion are not involved in bonding. Hence, it can donate electrons and behave as base. In higher oxidation state, valence `e^(-)s` are involved in bonding and are not available, Instead, effective nuclear charge is high. Hence, it can accept `e^(-)s` and hence behave as acid. [`e^(-)s` means electrons]
(iii) The large value of `E^(0)` value for `Mn^(3+)//Mn^(2+)` shown than `Mn^(2+)` is much more stable than `Mn^(3+)`. This is due to the fact, that `Mn^(2+)` has a `3d^(5)` configuration. It can lose `e^(-)` to form `Cr^(3+)` which has stable `3d^(3)` configuration (half filled `t_(2)g`). Hence, less `E^(0) Cr^(3+)//Cr^(2+)`
(b) Similarity between the chemistry of lanthanoid and actinoid elements: Both show mainly an oxidation state of `+3` and both are electro positive and very reactive.
Difference between the chemistry of lanthanoid and actinoids: Lanthanoids have less tendency to form complex whereas, actinoids have great tendency to form complex. Lanthanoids are colourless whereas, actinoids are coloured.