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

Options

A. the two crystals are not isomorphous
B. the molar mass of NaF is smaller than that of RbBr
C. the bond in RbBr has more covalent character than the bond in NaF
D. the internuclear distance `(r_(c)+r_(a))` is greater for RbBr than for NaF.

Explanation

To determine why the melting point of NaF (988°C) is much higher than that of RbBr (682°C), we need to analyze the factors affecting the melting points of ionic compounds.

### Step-by-Step Solution:

1. **Understanding Ionic Compounds**:
Both RbBr and NaF are ionic compounds. The melting point of an ionic compound is influenced by the strength of the electrostatic forces between the cations and anions.

**Hint**: Recall that ionic compounds consist of positively charged cations and negatively charged anions, which are held together by electrostatic forces.

2. **Comparing Cation and Anion Sizes**:
- Rb+ (Rubidium ion) is larger than Na+ (Sodium ion).
- Br- (Bromide ion) is larger than F- (Fluoride ion).
- The larger the ions, the greater the distance between them in the crystal lattice.

**Hint**: Consider how ionic size affects the distance between ions and the strength of the ionic bond.

3. **Calculating Inter-nuclear Distance**:
The inter-nuclear distance in an ionic compound can be approximated as the sum of the radii of the cation and anion. For RbBr, the inter-nuclear distance (Rb+ + Br-) is greater than that of NaF (Na+ + F-).

**Hint**: Use the formula: Inter-nuclear distance = radius of cation + radius of anion.

4. **Effect of Inter-nuclear Distance on Melting Point**:
A larger inter-nuclear distance results in weaker electrostatic forces between the ions. Since RbBr has a greater inter-nuclear distance than NaF, the forces holding the RbBr lattice together are weaker compared to those in NaF.

**Hint**: Remember that the strength of ionic bonds decreases with increasing distance between the ions.

5. **Conclusion**:
Therefore, the principal reason that the melting point of NaF is much higher than that of RbBr is due to the greater inter-nuclear distance in RbBr, leading to weaker ionic bonds compared to NaF.

**Final Answer**: The inter-nuclear distance (cation radius + anion radius) is greater for RbBr than that of NaF, resulting in a lower melting point for RbBr.