Explain the following observations:
(i) Generally there is an increase in density of elements from titanium (Z = 22) to copper (Z=29) in the first series of transition elements.
(ii) Transition elements and their compounds are generally found to be good catalysts in chemical reactions.

Explanation

(i) Density `=("Mass")/("Volume")=("Mass")/((4)/(3)pi r^(3))`, where r is the radius of the atom.
From titanium to copper in the first series of transition elements, there is successive decrease in radius and there is successive increase in mass.
Thus, the density shows an increase from Ti to Cu.
(ii) This is due to their ability to adopt multiple oxidation states and to form complexes. Catalyst at the solid surface involve the formation of bonds between reactant molecules and the atoms of the surface of the catalyst. The metals of first transition series involve 3d and 4s electrons for bonding. This results in increase in concentration of the reactants at the surface and also weakening of bonds in the reacting molecules.