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 catalysts.

(N/A) $(i)$ 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. In the first transition series,the orbital angular momentum is quenched,so the resulting paramagnetism is primarily due to the unpaired electrons.
$(ii)$ Transition elements have high effective nuclear charge and a large number of valence electrons,which leads to the formation of very strong metallic bonds. Consequently,the enthalpy of atomisation of transition metals is high.
$(iii)$ Most transition metal complexes are coloured due to $d-d$ transitions. In the presence of ligands,the $d$-orbitals split into two sets of different energies. Electrons absorb radiation from the visible region to transition between these sets,and the reflected light imparts colour to the solution.
$(iv)$ The catalytic activity of transition elements is due to:
$(a)$ Their ability to exhibit variable oxidation states and form complexes,creating unstable intermediate compounds that provide a reaction path with lower activation energy,$E_a$.
$(b)$ Their ability to provide a suitable surface area for reactant molecules to adsorb and react.