Question #58486

1 Answer
Nov 22, 2016

If the compound is molecular it should have intrinsically lower melting points, and boiling points, as compared to an ionic compound..........


Its electrical conductivity should also be negligible, but so too should be that of the SOLID phase of the solid ionic compound. (In the liquid phase, or in solution, the ions should be free to be move, but this is an exceptional circumstance.)

An ionic compound is intrinsically NON-molecular: each ion in the lattice should be attracted to every other counterion in the entire lattice structure. Of course, it should be electrostatically repelled by every other ion with like charge in the lattice, but if you sum up all these forces of attraction versus forces of repulsion over the entire lattice, (which can certainly be done), a net attractive force operates.

This attractive force, this electrostatic force, is held to be responsible for the high melting point, and high boiling point of ionic materials, and also their tendency to be brittle, and to fracture when sufficient force is supplied to a macroscopic ionic crystal.

On the other hand, a molecular material should have intrinsically low boiling points, and melting points, based on the reduced forces of intermolecular attraction. Hydrogen bonding can operate between molecules, so can dispersion forces. But these INTERMOLECULAR forces of attraction tend to be moderate, and of much lower magnitude than the INTRAMOLECULAR ones.

In summary, one of the best indicators of the physical properties of a material is whether it is molecular or non-molecular. This distinction also accounts for the similarly high melting and boiling points of network covalent solids such as diamond, and silicon oxide, in which (strong) covalent bonding persists thru the entire lattice, where molecular boundaries do not exist.