What factors affect the lattice enthalpy of an ionic solid?

We must remember that negative energies (in any context) generally represent energetically stable configurations and as the energy of a system increases, it will become more unstable.

Firstly, ionic size affects the lattice enthalpy of a ionic crystal. In an ionic crystal, smaller ions can pack more closely together and exert a stronger electrostatic force of attraction on each other, making the compound more stable and the lattice enthalpy more negative. Secondly, as the magnitude of the product of the ionic charges increases, the electrostatic force of attraction will increase in magnitude, again resulting in a more stable lattice and more negative lattice enthalpy.