What is the lattice energy of an ionic compound?

Lattice energy is a measure of the strength of the forces between the ions in an ionic solid. The greater the lattice energy [enthalpy], the stronger the forces.

Those forces are only completely broken when the ions are present as gaseous ions, scattered so far apart that there is negligible attraction between them.

Lattice energycould be described in either of two ways.

You could describe it as the energy change when 1 mole of sodium chloride (or whatever) was formed from its scattered gaseous ions. In other words, you are looking at a downward arrow on the diagram. see reference

In the sodium chloride case, that would be -787 kJ mol-1.

Or, you could describe it as the energy change when 1 mole of sodium chloride (or whatever) is broken up to form its scattered gaseous ions. In other words, you are looking at an upward arrow on the diagram in the link diagram

In the sodium chloride case, that would be +787 kJ mol-1.

http://www.chemguide.co.uk/physical/energetics/lat...