The melting points RbBr is `682^(@)C`, while that of NaF is `988^(@)`. The principal reason of this fact is :

Explanation

Both are crystalline solids and the melting points are directly linked to the respective lattice energies. Now, the magnitude of lattice energy is directly proporational to charge on ions and inversely to the interionic distance. In the above case both the cation shave +1 unit charge while anions have -1 unit charge. Since `Br^(-)` ions is bigger than `F^(-)` ion, the lattice energy for NaF crystals is more and therefore, the melting point is higher than of RbBr crystals.