The correct set from the following in which both pairs are in correct order of melting point is:
Chemical Bonding · Class 11 · JEE Main Previous Year Question
The correct set from the following in which both pairs are in correct order of melting point is:
- a
LiF > LiCl; NaCl > MgO
- b
LiCl > LiF; MgO > NaCl
- c
LiCl > LiF; NaCl > MgO
- d✓
LiF > LiCl; MgO > NaCl
LiF > LiCl; MgO > NaCl
🧠 Melting point of ionic compounds reflects lattice energy: higher lattice energy → higher melting point. Lattice energy increases with higher ionic charges and smaller ionic radii.
🗺️ Compare the pairs:
LiF vs LiCl (same cation Li⁺):
- F⁻ (radius 133 pm) < Cl⁻ (radius 181 pm)
- Smaller F⁻ → shorter interionic distance → higher lattice energy
- LiF > LiCl ✓ (m.p. LiF ≈ 848 °C, LiCl ≈ 610 °C)
MgO vs NaCl:
- MgO: Mg²⁺ (charge +2) and O²⁻ (charge −2) → doubly-charged ions
- NaCl: Na⁺ (+1) and Cl⁻ (−1) → singly charged ions
- Higher charge in MgO gives enormously higher lattice energy
- MgO > NaCl ✓ (m.p. MgO ≈ 2852 °C, NaCl ≈ 801 °C)
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