Assign reasons for the following :
(i) The enthalpies of atomisation of transition elements are high.
(ii) The transition metals and many of their compounds act as good catalyst.
(iii) From element to element the actinoid contraction is greater than the lanthanoid contraction.
(iv) The `E^(@)` value for the `Mn^(3+)//Mn^(2+)` couple is much more positive than that for `Cr^(3+)//Cr^(2+)`.
(v) Scandium (Z = 21) does not exhibit variable oxidation states and yet it is regarded as transition element.

Explanation

(i) The trasition elements exhibit high enthalpies of atomisation because they have large number of unpaired electrons in their atoms. Due to it they have stronger interatomic interaction and hence stronger bonding between atoms.
(ii) Many transition metals and their compounds acts as catalysts. The catalytic activity is due to their ability to exhibit multiple oxidation states. For example, `V_(2)O_(5)` in contact process and finely divided iron in Haber Process.
(iii) Is due to poor shielding by 5 f electrons in the actinoids than that by 4f electrons in the lanthanoids.
(iv) Much larger third ionisation energy of Mn (where the required change is `d^(5) to d^(4)` is mainly responsible for this. This also explains that +3 state of Mn is of little importance.
(v) Scandium (Z = 21) has incompletely filled 3d orbitals in the ground state `(3d^(1))`. Hence, it is considered as a transition element.