2nd nd 3rd transition series have higher enthalpies of atomisation compared to 1 series.y??

Dear student!

The enthalpy of atomization depends on no. of unpaired electrons because of large number of unpaired electrons in their atoms they have stronger interatomic interaction and hence stronger bonding between atoms resulting in higher enthalpies of atomization i,e higher melting point.

But, down he group, as the f-orbitals enter in second and third transition series the atomic size decreases due to lanthanoid and actinoid contraction and so they have more density and it becomes more difficult to atomize the atoms and they have higher enthalpy of atomization and higher melting point too.