In an ionic compound, the size of the ions affects the internuclear distance (the distance between the centers of adjacent ions), which affects lattice energy (a measure of the attractive force holding those ions together). Based on ion sizes, rank these compounds of their expected lattice energy..
Note: Many sources define lattice energies as negative values. Please rank by magnitude and ignore the sign. |Lattice energy| = absolute value of the lattice energy.
RbCl ,RbBr ,Rbl ,RbF

The lattice energy is an indicator of the strength of an ionic bond. It is also a rough indicator of the probability that an ionic substance will dissolve in water. The higher the lattice energy, the more difficult it is for the substance to dissolve in water.

Lattice energy depends on the relative sizes of ions present in the substance. As already known, the order of increasing sizes of halogen atoms is F<Cl<Br<I. The lesser the size, the higher the lattice energy.

Since the cation size is constant, lattice energy is only affected by increasing anion size and follows the pattern highlighted in the paragraph above. Hence RbF has the highest lattice energy and RbI has the least lattice energy.