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.

Answer

(i) it is because atomic mass increase more than atomic volume , therefore density increases from titanium `(Z=22)` to copper `(Z=29)`.
`(ii)` it is because they show variable oxidation state and have vacant d-orbital forming unstable intermediates which readily change into products.