Question

why is the lattice energy of potassium bromide more exothermic than the lattice energy of rubidium iodide in 510 words

Answer 1

You are comparing two chemicals (e.g. KBr and RbI) which are known as IONIC COMPOUNDs, this means that THERE ARE IONs WHICH FORM Polyatomic Cells of KBr and RbI.
Now, you get two clues treating about the BOND's ENERGY ABLE TO CREATE DEBATED Polyatomic Cells STARTING FROM CONSTITUTING UNITs.
Indeed, you may imagine that KBr AND/OR RbI STAY INSIDE A CALORIMETER WHICH MEASURES THE HEAT SPENT BY A FLAME TO VOLATILIZE IT AS FREE IONS

KBr(s) ---> K+(spread in flame) + Br-(spread in flame)
RbI(s) ---> Rb+(spread in flame) + I-(spread in flame)

the required energies result equal than Lattice energies
Why do different compounds exhibit different Lattice energies

ANSWER
The LATTICE ENERGY FOR an IONIC SOLID COMES FROM THE ELECTRICAL ATTRACTIVE FORCE INTERACTING AMONG CONSTITUTING IONS. It is known that those electrical forces depend on the basis of electrical potential exerted by each ions, so THE SMALLER IONIC RADIUS THEN THE STRONGER THE ACTING POTENTIAL.
IT IS EVIDENT THAT "KBr" INVOLVES IONS SMALLER THAN "RbI" DOES.