In the series $\operatorname{Sc}(Z=21)$ to $\operatorname{Zn}(Z=30)$, the enthalpy of atomisation of zinc is the lowest, i.e.. $126\, kJ\, mol ^{-1}$. Why?

The extent of metallic bonding an element undergoes decides the enthalpy of atomization. The more extensive the metallic bonding of an element, the more will be its enthalpy of ato mization. In all transition metals (except $Zn$, electronic configuration: $3 d^{10} 4 s ^{2}$ ), there are some unpaired electrons that account for their stronger metallic bonding. Due to the absence of these unpaired electrons, the inter-atomic electronic bonding is the weakest in $Zn$ and as a result, it has the least enthalpy of atomization.