Explain the following observation:
(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 catalysts in chemical reactions.

Explanation

(i) Due to decrease in size and incresing mass.
(ii) Beacause of variable oxidation states exhibited by them.