Question

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

• A. LiCl>LiF ; MgO>NaCl
• B. LiF>LiCl ; MgO>NaCl
• C. LiCl>LiF ; NaCl>MgO
• D. LiF>LiCl ; NaCl>MgO

Hint:

Lattice energy (and thus Melting Point) is influenced more by the charge of the ions than by their size.

Answer 1

The Correct Option is B

Step 1: Melting point of ionic compounds depends on Lattice Energy ($U \propto \frac{q_1 q_2}{r}$).
Step 2: For $LiF$ vs $LiCl$: Both have +1/-1 charges, but $F^-$ is smaller than $Cl^-$. Smaller internuclear distance means higher lattice energy. Thus, $LiF>LiCl$.
Step 3: For $MgO$ vs $NaCl$: $MgO$ has charges +2/-2, whereas $NaCl$ has +1/-1. Higher product of charges leads to significantly higher lattice energy. Thus, $MgO>NaCl$.