Concept: The melting point of a compound is influenced by the nature of its chemical bonding (ionic vs. covalent) and the strength of the forces holding its constituent particles together. Ionic compounds generally have higher melting points than covalent compounds due to strong electrostatic forces. For covalent compounds, factors like molecular size and intermolecular forces play a role. For ionic compounds, lattice energy is a key factor.

Why (B) is correct:

1. ${\text{BeCl}}_2$ (Beryllium chloride): Beryllium is a small, highly polarizing cation. Due to its high charge density and small size, ${\text{BeCl}}_2$ exhibits significant covalent character, even though it is formed between a metal and a non-metal. It forms a polymeric chain structure in the solid state, but its overall bonding is more covalent than ionic compared to ${\text{CaCl}}_2$. Its melting point is relatively low (approx. 405 °C).

2. ${\text{CaCl}}_2$ (Calcium chloride): Calcium is an alkaline earth metal, and ${\text{CaCl}}_2$ is a typical ionic compound. It has a high lattice energy due to the strong electrostatic attraction between ${\text{Ca}}^{2+}$ and ${\text{Cl}}^{-}$ ions. Ionic compounds generally have high melting points. Its melting point is very high (approx. 772 °C).

3. ${\text{HgCl}}_2$ (Mercuric chloride): Mercury is a transition metal, and ${\text{HgCl}}_2$ is predominantly a covalent compound. This is due to the high polarizing power of ${\text{Hg}}^{2+}$ (pseudo noble gas configuration) and the relatively large size of the anion. It exists as discrete molecules held together by weak intermolecular forces (van der Waals forces). Covalent compounds with weak intermolecular forces have low melting points. Its melting point is the lowest among the three (approx. 277 °C).

Therefore, the order of melting points is ${\text{HgCl}}_2$ < ${\text{BeCl}}_2$ < ${\text{CaCl}}_2$, which corresponds to $\text{iii}$ < $\text{i}$ < $\text{ii}$.

Option Analysis:

• A) $\text{i}$ < $\text{ii}$ < $\text{iii}$: Incorrect. This order suggests ${\text{BeCl}}_2$ has the lowest melting point and ${\text{HgCl}}_2$ the highest, which is contrary to their bonding characteristics.
• B) $\text{iii}$ < $\text{i}$ < $\text{ii}$: Correct. This order correctly reflects the increasing ionic character and strength of forces: ${\text{HgCl}}_2$ (covalent, weak intermolecular forces) < ${\text{BeCl}}_2$ (significant covalent character, polymeric) < ${\text{CaCl}}_2$ (ionic, strong lattice energy).
• C) $\text{i}$ < $\text{iii}$ < $\text{ii}$: Incorrect. This places ${\text{HgCl}}_2$ between ${\text{BeCl}}_2$ and ${\text{CaCl}}_2$, which is wrong as ${\text{HgCl}}_2$ has the lowest melting point.
• D) $\text{ii}$ < $\text{i}$ < $\text{iii}$: Incorrect. This suggests ${\text{CaCl}}_2$ has the lowest melting point, which is incorrect as it is a highly ionic compound.

Correct Answer: (B)