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

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 maller the difference between Na and F
D. the intermoleculear distance, `r_(c)+r_(a)` is greater for RbBr than for NaF

Answer

To determine the principal reason for the difference in melting points between RbBr (682°C) and NaF (988°C), we can analyze the ionic bonds and the factors affecting their strengths.

### Step-by-Step Solution:

1. **Identify the Ionic Compounds**:
- RbBr consists of rubidium (Rb) and bromine (Br).
- NaF consists of sodium (Na) and fluorine (F).

2. **Understand the Nature of Ionic Bonds**:
- Both RbBr and NaF are ionic compounds formed by the electrostatic attraction between positively charged cations (Rb⁺ and Na⁺) and negatively charged anions (Br⁻ and F⁻).

3. **Compare the Electropositivity and Electronegativities**:
- Sodium (Na) is more electropositive than rubidium (Rb).
- Fluorine (F) is more electronegative than bromine (Br).
- This means that NaF has a stronger ionic bond due to the greater difference in electronegativity between Na and F compared to Rb and Br.

4. **Consider the Ionic Sizes**:
- Rb⁺ is larger than Na⁺, and Br⁻ is larger than F⁻.
- The larger size of Rb⁺ and Br⁻ leads to a greater distance between the ions in RbBr compared to NaF.

5. **Analyze the Effect of Ionic Size on Bond Strength**:
- The strength of an ionic bond is inversely related to the distance between the ions.
- In NaF, the smaller ionic sizes result in a shorter distance between ions, leading to a stronger ionic bond.
- In RbBr, the larger ionic sizes result in a longer distance between ions, leading to a weaker ionic bond.

6. **Conclude the Reason for Melting Point Differences**:
- The weaker ionic bond in RbBr (due to larger ionic sizes and lower charge density) results in a lower melting point (682°C).
- The stronger ionic bond in NaF (due to smaller ionic sizes and higher charge density) results in a higher melting point (988°C).

### Final Answer:
The principal reason for the difference in melting points is that the ionic bond in NaF is stronger than that in RbBr due to the smaller size of the ions in NaF, leading to a shorter distance between the ions and a stronger electrostatic attraction.