Question

Why the lattice enthalpy decreases when the size of ions increases ?

Answer 1

Lattice enthalpy is defined as heat evolved when one mole of a crystalline ionic compound forms from its gaseous ions, measured under standard conditions.

As we go down the group in the periodic table, the atomic radius increases so the force of attraction between electrons and nucleus decreases hence lattice enthalpy decreases as we move down the group in a periodic table.

The magnitude of the lattice enthalpy depends on how closely the ions pack together in the crystal structure. The more closely they pack, the more stable is the compound and hence the more negative is the value of lattice enthalpy.