Question

The melting point of $RbBr$ is 683$^oC$ , while that of $NaF$ is 988$^oC$ . The principal reason that melting point of $NaF$ is much higher than that of $RbBr$ is that:
A.The molar mass of $NaF$is smaller than that of $RbBr$.
B.The bond in $RbBr$ has more covalent character than the bond in $NaF$.
C.The difference in electronegativity between $Rb$ and $Br$ is smaller than the difference between $Na$ and $F$.
D.The internuclear distance, ${r_c} + {r_a}$ is greater for $RbBr$ than for $NaF$.

Answer 1

This question is based on the concept of reasoning of exceptional cases that include the characteristics such as the internuclear distance and the effect of internuclear distance on the properties of compounds. This concept can also be identified through the trends of the periodic table.

Complete answer:
Intermolecular forces are forces that exist between molecules and are collectively referred to as such. The physical properties of the material are mostly determined by intermolecular forces. The condensed states of matter are caused by intermolecular forces. Intermolecular forces hold the particles that make up solids and liquids together, and these forces influence a variety of physical properties of matter in these two states.
We can compare the intensities of intermolecular forces by comparing the boiling points of different substances. Because the heat received by the substance at its boiling point is used to break these intermolecular interactions and convert the liquid to vapour, this is the case.
The melting point of ionic compounds falls as the internuclear distance rises, hence the melting point lowers as we proceed down the group. As ${r_c} + {r_a}$ is greater for $RbBr$than for $NaF$, the melting point of $NaF$ is higher.
Hence, Option (D) is correct.

Note:
This force is responsible for a substance's various physical and chemical features. The strength of a substance's intermolecular forces determines its boiling point; the greater the intermolecular interactions, the higher the boiling point.