Question

Which of the following substance has the highest melting point?

• A. BaO
• B. MgO
• C. KCl
• D. NaCl

Answer 1

The correct option is B

MgO

The explanation for the correct option:

B) $\mathrm{MgO}$

• The melting point of an ionic compound depends on lattice energy. As lattice energy increases melting point also increases. Lattice energy is directly proportional to the charge of the ions and inversely proportional to the size of the ions.
• The given compounds, Magnesium oxide $\left(\mathrm{MgO}\right)$ and Barium oxide$\left(\mathrm{BaO}\right)$, both have a common anion, Oxide $\left({\mathrm{O}}^{2-}\right)$. So in such a case, consider the charge and size of the cation.
• Both Magnesium $\left(\mathrm{Mg}\right)$ and Barium $\left(\mathrm{Ba}\right)$ have a charge of +2. But considering the size of the cation, ${\mathrm{Mg}}^{2+}$is smaller than ${\text{Ba}}^{2+}$. As the ${\mathrm{Mg}}^{2+}$ is smaller in size, lattice energy is more, and thus melting point will be high.

The explanation for the incorrect options:

A) $\mathrm{BaO}$

• Given compounds, Magnesium oxide $\left(\mathrm{MgO}\right)$ and Barium oxide$\left(\mathrm{BaO}\right)$, both have a common anion, Oxide $\left({\mathrm{O}}^{2-}\right)$.
• So in such a case, consider the charge and size of the cation. Both Magnesium $\left(\mathrm{Mg}\right)$ and Barium $\left(\mathrm{Ba}\right)$ have a charge of +2. But considering the size of the cation, ${\text{Ba}}^{2+}$ is large in size compared to ${\mathrm{Mg}}^{2+}$.
• As lattice energy is inversely proportional to the size of the ion, ${\text{Ba}}^{2+}$ have more lattice energy. Since lattice energy is directly proportional to the melting point, the melting point of $\mathrm{BaO}$ is less compared to $\mathrm{MgO}$.

C) $\mathrm{KCl}$

• The charge of Potassium $\left(\mathrm{K}\right)$ in Potassium chloride $\mathrm{\left(}\mathrm{KCl}\mathrm{\right)}$ is +1. So the charge of Potassium is less compared to Magnesium. So it has a low melting point.

D) $\mathrm{NaCl}$

• The charge of Sodium $\left(\mathrm{Na}\right)$ in Sodium chloride $\mathrm{\left(}\mathrm{NaCl}\mathrm{\right)}$ is +1. So the charge of Sodium is less compared to Magnesium. So it has a low melting point.

Hence, option (B) is correct.