Assign reasons for the following:(i) The enthalpies of atomisation of

1.	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 catalysts.(iii) From element to element the actinoid contraction is greater than the lanthanoid contraction.(iv) The E° value for the Mn3+ / Mn2+ couple is much more positive than that of Cr 3+ / Cr 2+.(v) Scandium (Z = 21) does not exhibit variable oxidation states and yet it is regarded as a transition element.
Answer» (i) This is because transition metals have strong metallic bonds as they have a large number of unpaired electrons. (ii) The catalytic activity of transition metals is attributed to the following reasons: (a) Because of their variable oxidation states transition metals form unstable intermediate compounds and provide a new path with lower activation energy for the reaction. (b) In some cases, the transition metal provides a suitable large surface area with free valencies on which reactants are adsorbed. (iii) This is due to poorer shielding by 5f electrons in actinoids than that by 4f electron in the lanthanoids. (iv) This is because half filled d-subshell (3d⁵ ) in Mn ²⁺ is more stable. (v) This is because scandium has partially filled d orbitals in the ground state (3d¹ 4s ² ).

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 catalysts.(iii) From element to element the actinoid contraction is greater than the lanthanoid contraction.(iv) The E° value for the Mn3+ / Mn2+ couple is much more positive than that of Cr 3+ / Cr 2+.(v) Scandium (Z = 21) does not exhibit variable oxidation states and yet it is regarded as a transition element.

Answer»

(i) This is because transition metals have strong metallic bonds as they have a large number of unpaired electrons.
(ii) The catalytic activity of transition metals is attributed to the following reasons:
(a) Because of their variable oxidation states transition metals form unstable intermediate compounds and provide a new path with lower activation energy for the reaction.
(b) In some cases, the transition metal provides a suitable large surface area with free valencies on which reactants are adsorbed.
(iii) This is due to poorer shielding by 5f electrons in actinoids than that by 4f electron in the lanthanoids.

(iv) This is because half filled d-subshell (3d⁵ ) in Mn ²⁺ is more stable.
(v) This is because scandium has partially filled d orbitals in the ground state (3d¹ 4s ² ).

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