The correct set from the following in which both pairs are in correct order of melting point is :

Options

A. `LiF gt LiCl , MgO gt NaCl`
B. `LiCl gt LiF , NaCl gt MgO`
C. `LiF gt LiCl , NaCl gt MgO`
D. `LiCl gt LiF , MgO gt NaCl`

Answer

To determine the correct set in which both pairs are in the correct order of melting points, we need to analyze the melting points of the given ionic compounds based on their ionic character and lattice energy.

### Step-by-Step Solution:

1. **Identify the Compounds**:
We need to compare the melting points of the given ionic compounds. Let's assume we have the following pairs:
- Pair 1: LiF and LiCl
- Pair 2: MgO and NaCl

2. **Understand Ionic Compounds**:
Ionic compounds generally have high melting points due to strong electrostatic forces of attraction between the positively charged cations and negatively charged anions.

3. **Analyze Pair 1 (LiF and LiCl)**:
- **Cation**: Li⁺ (same for both)
- **Anions**: F⁻ (smaller) and Cl⁻ (larger)
- According to Fajan's rule, the smaller the anion, the stronger the ionic character due to higher polarizing power of the cation. Thus, LiF will have a stronger ionic character than LiCl.
- **Conclusion**: LiF has a higher melting point than LiCl.

4. **Analyze Pair 2 (MgO and NaCl)**:
- **Cations**: Mg²⁺ (charge +2) and Na⁺ (charge +1)
- **Anions**: O²⁻ (charge -2) and Cl⁻ (charge -1)
- The lattice energy is directly proportional to the product of the charges of the ions. MgO has higher charges (Mg²⁺ and O²⁻) compared to NaCl (Na⁺ and Cl⁻).
- Therefore, MgO will have a significantly higher lattice energy and thus a higher melting point than NaCl.
- **Conclusion**: MgO has a higher melting point than NaCl.

5. **Final Comparison**:
- From Pair 1: LiF > LiCl
- From Pair 2: MgO > NaCl
- Therefore, both pairs are in the correct order of melting points.

### Final Answer:
The correct set in which both pairs are in the correct order of melting point is:
- Pair 1: LiF > LiCl
- Pair 2: MgO > NaCl