The melting point of `RbBr` is `682^(@)C`, while that of `NaF` is `988^(@)C` . The principle reason that melting point of `NaF` is much higher than that of `RbBr` is that `:`

Options

A. The molar mass of NaF is smaller than that of RbBr
B. The bond of RbBr has more covalent character than the bond in NaF
C. The difference in electronegativity between Rb and Br is smaller than the difference between Na and F
D. The inter-nuclear distance, `r_(c)+r_(a)` is greater for RbBr than for NaF

Answer

To determine why the melting point of NaF is much higher than that of RbBr, we can analyze the factors that influence the melting points of ionic compounds.

### Step-by-Step Solution:

1. **Understanding Melting Points**: The melting point of an ionic compound is influenced by the strength of the ionic bonds between the cations and anions. Stronger ionic bonds result in higher melting points.

2. **Comparing Ionic Radii**:
- RbBr consists of rubidium ions (Rb⁺) and bromide ions (Br⁻).
- NaF consists of sodium ions (Na⁺) and fluoride ions (F⁻).
- The ionic radius of Na⁺ is smaller than that of Rb⁺, and the ionic radius of F⁻ is smaller than that of Br⁻.

3. **Analyzing Ionic Distances**:
- The distance between the cation and anion in NaF is shorter compared to that in RbBr. This is because smaller ions can get closer together, leading to stronger electrostatic forces of attraction.
- The internuclear distance (the distance between the centers of the cation and anion) is crucial. For NaF, this distance is less than that for RbBr.

4. **Conclusion on Melting Points**:
- Since the ionic bonds in NaF are stronger due to the shorter internuclear distance, NaF has a higher melting point (988°C) compared to RbBr (682°C).
- Therefore, the principal reason that the melting point of NaF is much higher than that of RbBr is that the internuclear distance (Rc + Ra) is greater for RbBr than for NaF.

### Final Answer:
The principal reason that the melting point of NaF is much higher than that of RbBr is that the internuclear distance (Rc + Ra) is greater for RbBr than for NaF.

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