Question

Explain giving reasons: 
 (i) Transition metals and many of their compounds show paramagnetic behaviour. 
 (ii) The enthalpies of atomisation of the transition metals are high. 
 (iii) The transition metals generally form coloured compounds. 
 (iv) Transition metals and their many compounds act as good catalyst.

Answer 1

(i) Transition metals show paramagnetic behaviour. Paramagnetism arises due to the presence of unpaired electrons with each electron having a magnetic moment associated with its spin angular momentum and orbital angular momentum. However, in the first transition series, the orbital angular momentum is quenched. Therefore, the resulting paramagnetism is only because of the unpaired electron.

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(ii) Transition elements have high effective nuclear charge and a large number of valence electrons. Therefore, they form very strong metallic bonds. As a result, the enthalpy of atomization of transition metals is high.

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(iii) Most of the complexes of transition metals are coloured. This is because of the absorption of radiation from visible light region to promote an electron from one of the d-orbitals to another. In the presence of ligands, the d-orbitals split up into two sets of orbitals having different energies. Therefore, the transition of electrons can take place from one set toanother. The energy required for these transitions is quite small and falls in the visible region of radiation. The ions of transition metals absorb the radiation of a particular wavelength and the rest is reflected, 
imparting colour to the solution.

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(iv) The catalytic activity of the transition elements can be explained by two basic facts. 
(a) Owing to their ability to show variable oxidation states and form complexes, transition metals form unstable intermediate compounds. Thus, they provide a new path with lower activation energy, E_a, for the reaction. 
(b) Transition metals also provide a suitable surface for the reactions to occur