Which of the following has the lowest melting point ?

Options

A. NaCI
B. NaF
C. NaBr
D. NaI

Answer

To determine which of the given salts has the lowest melting point, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Salts**: First, we need to identify the salts provided in the question. Let's assume the salts are sodium fluoride (NaF), sodium chloride (NaCl), sodium bromide (NaBr), and sodium iodide (NaI).

2. **Understand the Cation and Anion**: In all these salts, the cation is the same (sodium ion, Na⁺), while the anions are different halides (F⁻, Cl⁻, Br⁻, I⁻).

3. **Analyze the Anions**: The anions belong to Group 17 (the halogens). As we move down the group from fluorine to iodine, the size of the anions increases, and their electronegativity decreases.

4. **Electrostatic Forces of Attraction**: The melting point of ionic compounds is influenced by the strength of the electrostatic forces of attraction between the cation and anion. Stronger forces result in higher melting points.

5. **Comparison of Electrostatic Forces**:
- Sodium fluoride (NaF) has the smallest anion (F⁻), resulting in strong electrostatic attraction and a high melting point.
- Sodium chloride (NaCl) has Cl⁻, which is larger than F⁻ but smaller than Br⁻ and I⁻, resulting in a lower melting point than NaF.
- Sodium bromide (NaBr) has Br⁻, which is larger than Cl⁻, leading to a further decrease in melting point.
- Sodium iodide (NaI) has the largest anion (I⁻), resulting in the weakest electrostatic attraction and the lowest melting point among these salts.

6. **Conclusion**: Based on the analysis, sodium iodide (NaI) has the lowest melting point due to the weakest electrostatic forces of attraction between the sodium cation and the iodide anion.

### Final Answer:
The salt with the lowest melting point is **sodium iodide (NaI)**.