The
lattice energy of a crystalline
solid is usually defined as the
energy of formation of the crystal from infinitely-separated
ions and as such is invariably negative. The concept of lattice energy was originally developed for
rocksalt-structured and
sphalerite-structured compounds like NaCl and ZnS, where the ions occupy high-symmetry crystal lattice sites. In the case of NaCl, the lattice energy is the energy released by the reaction