8.11 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.
(i) Transition metals show paramagnetic behaviour. Paramagnetism arises from the presence of unpaired electrons, each such electron having a magnetic moment associated with its spin angular momentum and orbital angular momentum. For the compounds of the first series of transition metals, the contribution of the orbital angular momentum is effectively quenched and hence is of no significance. Hence, paramagnetism is totally generated by an unpaired electron in the d subshell.
(ii) Transition elements have a 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.
(iii) Most of the complexes of transition metals are coloured.
When an electron from a lower energy d orbital is excited to a higher energy d orbital and this is known as d-d transition, the energy of excitation corresponds to the frequency of light absorbed.
This frequency generally lies in the visible region. The colour observed corresponds to the complementary colour of the light absorbed.
(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, Ea, for the reaction.
(b) Transition metals also provide a suitable surface for the reactions to occur.