Q.Why do transition metals have high enthalpies of atomization ?

In transition metals, both the $ns$ and the $(n-1)d$ electrons participate in metallic (interatomic) bonding, giving much stronger metal–metal bonds than in s- or p-block metals — so a large amount of energy (a high enthalpy of atomisation) is needed to break the lattice apart into free gaseous atoms.

Enthalpy of atomisation is the energy required to convert one mole of a solid metal into free gaseous atoms — it is a direct measure of the strength of metallic bonding holding the lattice together.

In transition elements, in addition to the two $4s$ (or $ns$) electrons, unpaired $(n-1)d$ electrons also overlap and delocalise between neighbouring atoms, contributing extra 'covalent-like' character to the metallic bond. The greater the number of unpaired d electrons, the more extensive this d-orbital overlap, and the stronger the interatomic bonding — hence the higher the enthalpy of atomisation.

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