Enthalpy of atomization depends on the strength of metallic bonding, which is determined by the number of unpaired electrons available for delocalization. Zinc ($Zn$) has a fully filled d-subshell ($3d^{10} 4s^{2}$) with no unpaired electrons for metallic bonding. Consequently, its metallic bonds are weak, leading to the lowest enthalpy of atomization in the series.