Transition elements (except Zn, Cd and Hg) are much harder and less volatile. They exhibit high enthalpies of atomization. The maximum value lies at about middle of each series indicating the interatomic interaction increases with the number of unpaired d-electrons. The metals of 4d and 5d transition series have greater enthalpies of atomization than the corresponding elements of the first (3d) series. This is due to much more frequent metal-metal bonding in their compounds.
