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.

Answer

(i) The tranisition elements exhibit high enthalpies of atomisation because they have large number of unpaired electrons in their atoms and because of their smaller size . Due to the presence of these electrons, they have stronger inter-atomic interaction. Hence , they have strong bonding between their atoms.
(ii) Many transition metals and their compounds act as catalyst. The catalytic activity is due to their ability to exhibit multiple oxidation states. they form unstable intermediate compound and provide a new process . (iii) Actinoids constraction is similar to lanthanoid contraction. Actinoid contraction take place of the poor shielding by `5f` electrons and lanthanoid contraction take place because of the poor shielding bu `4f` electrons . As the shielding provided by `4f` electrons is slightly better than that provided by `5f` electrons, more contraction is observed in the case of actinoids.
(iv) The fully and half -filled orbitals are more stable . In `Mn`, 2 electrons are present in the `4s` orbitals and 5 electrons are present in the 3d orbital . After losing 2s electrons. Mn has electrons left. As the d orbital is exactly half-filled , it become difficult to take out the extra electron, Therefore, much greater energy (third ionisation energy ) is required for this . On the other hand, after lsoing two electrons, Cr has only 4 electrons in the d orbital. Thus, the energy required to take out one more electron in this case is not as high as in the previous case.
(v) An element is called a transition elements electrons in the d orbital . Scandium `(Z=21)` is a transition metal as it has an incompletely-filled 3d orbital in the ground state `(3d^(1))`
OR
(a) In a transition series , those oxidation state are stable which lead to exactly half-filled or completely -filled d orbitals.
`3d^(3)4s^(2)`: Oxidation state : `+2,+3,+4,+5` : state oxidation state : `+4,+5`
`3d^(5)4s^(2)`:Oxidation state : `+2,+3,+4,+5,+6,+7`: State oxidation state: `+2,+7`
`3d^(6)4s6(2)`: Oxidation state: `+2,+3,+4,+6` : Stable oxidation state : `+2,+3`
(b) (I) The powered chromite ore is heated with an alkali and quick lime in the presence of air to get yellow- Coloured sodium chromate.
`underset"Chromite ore"(4FeCr_(2)O_(4))+16 NaOH+7O_(2) to underset"yellow"underset"Sodium chromate"(8Na_(2)CrO_(4)+2Fe_(2)CrO_(4)+2Fe_(2)O_(3)+8H_(2)O`
(ii) Finely -powdered pyrolusite ore is fused with `KOH` in the presence of air to give green -coloured potassium manganate.
`underset"pyrolusite"(2MnO_(2))+KOH+O_(2) to underset("Green")underset"Potassium manganate"(2K_(2)MnO_(4))+2H_(2)O`