Question

Which of the following substances has the highest melting point?

• A. $NaCl$
• B. $KCl$
• C. $MgO$
• D. $BaO$

Answer 1

Answer: (C)

Melting points, among other things, depend upon lattice energy.
Now $NaCl$ and $KCl$ have unit charge on their ions while $MgO$ and $BaO$ have two units of charge,
therefore, lattice energies of $MgO$ and $BaO$ are expected to be larger than those of $NaCl$ and $KCl$.
Further since $M{g}^{2+}$ is smaller than $B{a}^{2+}$,
therefore, $MgO$ has the highest lattice energy and hence has the highest melting point.