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

The energy required to form a $1$ mole of gaseous atoms from its element in its standard state is called standard enthalpy of atomisation.

The transition elements have high enthalpies of atomisation because of presence of strong bonds between the atoms. The enthalpy of atomisation increases with the increase in number of unpaired electrons which results in formation of strong bonds due to strong inter-atomic interactions.

The elements of $4 d$ and $5 d$ series have greater enthalpy of atomisation than corresponding 

elements of first transition 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 first transition series, $\mathrm{Zn}$ has minimum enthalpy atomisation because of absence of unpaired electrons.