Answer:
MgO> NaCl > KBr >RbI
Step-by-step explanation:
Lattice energy refers to a measure of the energy contained in the crystal lattice of a compound. It is equal to the energy that would be released if the component ions were brought together from infinity. It is also defined as a measure of the energy released when ions are combined to make a compound or a measure of the cohesive forces that bind ions together in an ionic solid.
Melting points of ionic substances depend on their relative lattice energies. Similarly, the lattice energy depends on the size of the ions in the ionic solid. The smaller the sizes of the component ions, the greater the lattice energy. It can be seen that the sizes of cations and anions decrease steadily from RbI to MgO hence the lattice energy and melting point of the ionic solids increases in the order shown in the answer. The smaller the component ions in the lattice the greater the melting point of that ionic solid.