Explain the following observations: (i) Many of the transition elements are known to form interstitial compounds. (ii) There is a general increase in density from titanium (Z = 22) to copper (Z = 29). (iii) The members of the actinoid series exhibit a larger number of oxidation states than the corresponding members of the lanthanoid series.

I) Most of the transition metals exist as CCP and HCP in which we can observe many vacant places (holes), these holes are occupied by elements like oxygen, nitrogen, hydrogen, carbon, etc., leading to the formation of interstitial compounds.
II) As we move from titanium to copper, the size decreases so the volume decreases and mass increases so density increases (because density is directly proportional to mass and inversely proportional to volume).
III) Actinides exhibit a large number of oxidation states but not lanthanides because the energy difference between orbitals of actinides is very less, so that they can exhibit different large numbers of oxidation states.