What is enthalpy of atomisation? Explain. $OR$ Write a note on enthalpy of atomisation.

(N/A) The energy required to form $1 \ mol$ of gaseous atoms from its element in its standard state is called standard enthalpy of atomisation $(\Delta_a H^{\ominus})$.
The transition elements have high enthalpies of atomisation because of the presence of strong inter-atomic interactions (strong metallic bonds) between the atoms.
The enthalpy of atomisation generally increases with the increase in the number of unpaired electrons,which results in the formation of stronger metallic bonds.
The elements of $4d$ and $5d$ series have greater enthalpy of atomisation than the corresponding elements of the first transition series ($3d$ series),which results in much more metal-metal bonding in compounds of the heavy transition elements.
Trends in enthalpies of atomisation of transition elements:
In the first transition series,$Zn$ has the minimum enthalpy of atomisation because of the absence of unpaired electrons in its $d$-orbitals ($3d^{10} 4s^2$ configuration).