why are enthalpies of atomisation of 4d and 5d series are greater than 3d series?

The enthalpies of atomisation of 4d and 5d series are greater than 3d series due to poor shielding of 4d and 5d electrons in comparision to 3d electrons.The outer electrons ion 4d and 5d series are more strongly held and there is more metallic bonding in 4d and 5d series.