Lattice Energy and Its Effect on Properties

The negative potential energy of an ionic compound is sometimes referred to as its lattice energy, which can also be defined as the amount of work required to separate the ions from the solid state into the gaseous state. Thus, for NaF, the lattice energy is the energy required for the process 

To make sure that you see that the lattice energy and the negative of the potential energy are the same, consider what happens to the potential energy when we move the ions to a large separation (such as occurs in the gas state). 

The potential energy is close to zero. From the equation 

Which has the greater hardness—cuprite (CuaO) or chalcocite (CuaS) Hardness is the relative resistance to abrasion and is measured on a scale devised by the German mineralogist, Friedrich Mohs, in 1812. On this scale, various 

Cuprite is harder because of the smaller size of the oxide ion and consequent higher lattice energy. The hardness of cuprite is 3.5-4, whereas chalcocite has a hardness of 2.5-3. 

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