On what factors does the magnitude of London dispersion forces depend?

Covalent molecules are usually held in a crystal structure by

Which one of the following pairs is an example of polar molecular solids?

Particles of quartz are packed by

Explain the repulsive forces between two molecules.

The lattice enthalpy of an ionic compound is the enthalpy change when one mole of an ionic compound in its solid state dissociates into its ions in the gaseous state. It is impossible to determine it directly by experiment. Suggest and explain an indirect method to measure the lattice enthalpy of $NaCl_{(s)}$.