why does copper have a high enthalpy of atomistaion while zinc has lowest enthalpy of atomisation in 3d series despite the fact that both have completely filled d orbitals ?

Dear Student,

Copper has high enthalpy of atomisation whereas Zinc has least enthalpy of atomisation due to the absence of any unpaired electron in Zn. Cu however contains an unpaired electron in the 4s orbital. 
Enthalpy of atomisation depends on the metallic bonding and higher the number of unpaired electrons, higher will be the metal-metal bond. 
Hence, Zn has least enthalpy of atomisation whereas Cu has high enthalpy of atomisation.