The melting point of RbBr is 682°C while that of NaF is 988°C. The principal reason for this fact is — Chemical Bonding Chemistry Question

💡 Solution & Explanation

Step 1: The melting point of ionic compounds is primarily determined by their lattice energy, which represents the strength of electrostatic forces holding the ionic crystal lattice together. Step 2: Lattice energy is directly proportional to the product of the ionic charges and inversely proportional to the sum of the ionic radii (internuclear distance, r_c + r_a). Since NaF and RbBr both have +1 and -1 charges, their lattice energies depend on the internuclear distance. Because Rb^+ and Br^- are much larger than Na^+ and F^-, the internuclear distance is significantly greater for RbBr than for NaF. Step 3: This larger internuclear distance in RbBr results in a much lower lattice energy and consequently a lower melting point (682°C) compared to NaF (988°C). Therefore, the correct option is (d).