Question

whydoes lattice enthalpy increase down the group

Answer 1

Lattice enthalpy is directly proportional to the charge on the ions and inversely proportional to the size of the ions, as we move down the group the charge on the ions remains same, so the dominant factor will be the size of the ions. On moving down, the group the size of the ions increases, thus the force of attraction will be less, leading to release of less amount of lattice enthalpy. Thus, lattice enthalpy decreases on moving down the group.