The melting point of RbBr is , while that of NaF is
. The principal reason that melting point of NaF is much higher than that of RbBr is that:
The internuclear distance is greater for RbBr than that for NaF
These two crystals are not isomorphous
The bond in RbBr has more covalent character than the bond in NaF
The molar mass of NaF is smaller than that of RbBr
The internuclear distance is lesser for NaF than RbBr due to which there is a strong electrostatic force of attraction in NaF. Thus, the melting point of NaF is greater than RbBr.
Therefore, Option(1) is correct