Question

Why is the melting point of sodium chloride so high as compared to that of phosphorus trichloride?

Answer 1

Why? Because God wanted it that way? Other than this reason, sodium chloride is a non-molecular species, whereas phosphorus trichloride is decidedly molecular.

Explanation:

Sodium chloride is a non-molecular species, whose interparticle interaction extends throughout the lattice. Each sodium ion in the lattice is attracted electrostatically to EVERY negatively charged chloride ion in the lattice. And each chloride ion in the lattice is mutually attracted to EVERY positively charged sodium ion.

Of course, there is likewise electrostatic repulsion between like charges, but if you sum up the attractive interactions versus the repulsive interactions, which can certainly be done in a Born Haber type process, attractive interaction wins. The result? An infinite array of charged ions within an electrostatic lattice. This non-molecularity gives rise to hardness and rigidity, and exceedingly high melting points, as well as non-conductivity in the solid state.

Contrast this with phosphorus trichloride. Phosphorous trichloride is a volatile, molecular species. The individual `P-Cl` bonds may be strong. Nevertheless the interaction between molecules are of the (weak) dipole-dipole/dispersion type. This is orders of magnitude below the electrostatic interaction between ions in the sodium chloride lattice.

So let's consider the metrics (and as physical scientists we should certainly do so!). The melting point of sodium chlorde is `801` `""^@C`, versus a melting point for `PCl_3` at `-95` `""^@C`, and a normal boiling point of `76` `""^@C`. These physical properties certainly point to the molecularity of salt versus `PCl_3`.